OVERCOMING REDUCED R H I S A -L T

OBSERVING AND OVERCOMING REDUCED

RIGHT HEMISPHERE INVOLVEMENT IN

SPATIAL ATTENTION IN ADULTS WITH

AUTISTIC-LIKE TRAITS

Michael Charles Woodfield English

Bachelor of Science (Honours)

This thesis is presented for the degree of Doctor of Philosophy of The University of Western Australia

School of Psychological Science

September 2017

I

THESIS DECLARATION

I, Michael Charles Woodfield English, certify that:

This thesis has been substantially accomplished during enrolment in the degree.

This thesis does not contain material which has been accepted for the award of any

other degree or diploma in my name, in any university or other tertiary institution.

No part of this work will, in the future, be used in a submission in my name, for any

other degree or diploma in any university or other tertiary institution without the prior

approval of The University of Western Australia and where applicable, any partner

institution responsible for the joint-award of this degree.

This thesis does not contain any material previously published or written by another

person, except where due reference has been made in the text.

The work(s) are not in any way a violation or infringement of any copyright,

trademark, patent, or other rights whatsoever of any person.

The research involving human data reported in this thesis was assessed and approved

by The University of Western Australia Human Research Ethics Committee. Approval

number(s): RA/4/1/6140, RA/4/1/7383 and RA/4/1/5247.

The work described in this thesis was funded by Australia Research Council Discovery

Project grants to TAWV (DP120102313) and MTM (DP120104713). Funding was also

provided to MCWE by the University of Western Australia.

This thesis contains published work and/or work prepared for publication, some of

which has been co-authored.

Signature: Date: _07 April 2017_

III

ABSTRACT

Individuals with autism spectrum conditions (ASC) show a reduced preference

for coherent, globally-organized visual stimuli, as demonstrated by superior

performance on tasks where attention must be directed towards local details, such as

locating a simple shape (e.g. a triangle) hidden within a complex structure (e.g. a

grandfather clock) (Muth et al., 2014). This behaviour is also shown by neurotypical

individuals with high levels of autistic-like traits as assessed by the Autism-Spectrum

Quotient (High AQ) relative to those with low levels of such traits (Low AQ) (Cribb et

al., 2016). Given that global and local processing are generally attributed to right (RH)

and left hemisphere activation respectively (Ivry & Robertson, 1998), a reduced drive

for global processing associated with autism may be symptomatic of relatively reduced

RH activation for spatial attention.

Pseudoneglect – over-attention to the left-side of visual space by neurotypical

individuals – is thought to be driven by RH specialization for spatial attention, and

therefore may be reduced in individuals with ASC or High AQ, similar to how left

neglect arises following RH damage (Heilman et al., 2003). Several studies suggest that

leftward attentional bias is also reduced for individuals with ASC, but these studies

have used face stimuli in their designs (e.g. Dundas et al., 2012), and reduced activation

of face-specific regions might account for these effects. Whether attentional biases for

non-face stimuli are similarly affected is relatively unknown, and the possibility

remains that pseudoneglect may be reduced for visual stimuli more broadly, consistent

with a wider-ranging link between ASC and relatively reduced RH activation for spatial

attention.

The present thesis comprises four experimental studies that aim to present

new evidence on whether reduced RH activation for spatial attention characterises

individuals with High AQ relative to their Low AQ counterparts, and to explore whether

increasing RH activation can influence distribution of spatial attention. Given

similarities in attentional profiles between individuals with ASC and those with High

AQ, investigating the spatial performance of individuals differing in levels of autistic-

like traits is a useful means of providing insight into ASC with the benefit of access to

larger samples, while avoiding complexities associated with clinical ASC, like comorbid

diagnoses (Landry & Chouinard, 2016).

Study 1 recruited a large sample (n=277) of university students, and found that

High AQ participants, relative to Low AQ participants, showed reduced pseudoneglect

IV

on a greyscales task, suggesting that RH activation of spatial attention was reduced for

these individuals. Study 2 replicated this finding in another large student sample

(n=104), and observed similar effects for a landmark task. However, pseudoneglect

was found to be intact for High AQ individuals on a mental number line bisection task.

Thus, the mechanism responsible for reduced pseudoneglect in the other tasks is likely

to be linked to mechanisms related specifically to the visual system.

In Study 3 (n=200), a continuous performance attentional task (CPT) that

specifically engages the RH (Degutis & Van Vleet, 2010) was successful at: 1) increasing

attention directed towards global aspects of hierarchical Navon figures in neurotypical

individuals even when participants were tasked with categorizing the local aspects,

replicating earlier results (Van Vleet et al., 2011), and 2) extending these training

benefits to individuals with High AQ. This training effect is a desirable result that

encourages further investigations of whether CPT can improve processing of other

stimuli amenable to global processing, like faces. In Study 4 (n=38), non-invasive,

excitatory transcranial direct current stimulation over the right posterior parietal

cortex (PPC) increased pseudoneglect on the greyscales task relative to a sham

condition that delivered no stimulation for a High AQ group. Identical stimulation on a

Low AQ group produced no such changes. These findings provide direct evidence that

reduced right PPC activation contributes to reduced pseudoneglect in High AQ.

In conclusion, Study 1 and 2 provide novel evidence for reduced RH activation

of spatial attention for individuals with High AQ, and that reduced pseudoneglect may

be representative of a potentially new phenotypical expression of autism. The findings

of Study 3 and 4 are perhaps even more important, highlighting that some atypical

aspects of attention associated with autism are not necessarily fixed. Overall, the

results of the reported studies further our understanding of attention in autism, and

hopefully encourage further investigation into spatial attention and the specific

involvement of the RH in the disorder.

References

Cribb, S. J., Olaithe, M., Di Lorenzo, R., Dunlop, P. D., & Maybery, M. T. (2016). Embedded Figures

Test Performance in the Broader Autism Phenotype: A Meta-analysis. Journal of Autism

and Developmental Disorders. http://doi.org/10.1007/s10803-016-2832-3

Degutis, J. M., & Van Vleet, T. M. (2010). Tonic and phasic alertness training: a novel behavioral

therapy to improve spatial and non-spatial attention in patients with hemispatial

V

neglect. Frontiers in Human Neuroscience, 4, 1–17.

http://doi.org/10.3389/fnhum.2010.00060

Dundas, E. M., Best, C. A., Minshew, N. J., & Strauss, M. S. (2012). A lack of left visual field bias

when individuals with autism process faces. Journal of Autism and Developmental

Disorders, 42(6), 1104–11. http://doi.org/10.1007/s10803-011-1354-2

Heilman, K. M., Watson, R. T., & Valenstein, E. (2003). Neglect and related disorders. In K. M.

Heilman & E. Valenstein (Eds.), Clinical Neuropsychology (pp. 279–336). New York:

Oxford University Press.

Ivry, R. B., & Robertson, L. C. (1998). The two sides of perception. Cambridge, MA: MIT Press.

Landry, O., & Chouinard, P. A. (2016). Why We Should Study the Broader Autism Phenotype in

Typically Developing Populations. Journal of Cognition and Development, 17(4), 584–

595. http://doi.org/10.1080/15248372.2016.1200046

Muth, A., Hönekopp, J., & Falter, C. M. (2014). Visuo-Spatial Performance in Autism: A Meta-

analysis. Journal of Autism and Developmental Disorders, 44(12), 3245–3263.

http://doi.org/10.1007/s10803-014-2188-5

Van Vleet, T. M., Hoang-duc, A. K., DeGutis, J., & Robertson, L. C. (2011). Modulation of non-

spatial attention and the global/local processing bias. Neuropsychologia, 49(3), 352–9.

http://doi.org/10.1016/j.neuropsychologia.2010.11.021

VII

TABLE OF CONTENTS

THESIS DECLARATION ........................................................................................ I

ABSTRACT ........................................................................................................ III

References .......................................................................................................................... iv

TABLE OF CONTENTS .......................................................................................VII

ACKNOWLEDGEMENTS ...................................................................................... XI

AUTHORSHIP DECLARATION: CO-AUTHORED PUBLICATIONS ........................ XIII

OTHER WORK GENERATED DURING THE CANDIDATURE .................................. XV

A NOTE ABOUT THE FORMAT OF THIS THESIS ............................................... XVII

CHAPTER ONE: GENERAL INTRODUCTION ........................................................ 1

Autism ................................................................................................................................... 1

Symptoms ............................................................................................................................... 1

Subtypes ................................................................................................................................. 2

Prevalence .............................................................................................................................. 3

ASC and the neurotypical population ..................................................................... 4

Genetic Origins of ASC ....................................................................................................... 4

Autistic-like traits................................................................................................................ 4

Weak Central Coherence Theory of Autism ......................................................... 7

Conceptualisation ............................................................................................................... 7

Mixed support for Weak Central Coherence ............................................................ 9

Perceptual Mechanisms Underlying ASC: Attention and the Right

Hemisphere ..................................................................................................................... 12

Global processing ............................................................................................................. 12

Visual Neglect and Pseudoneglect ............................................................................. 13

Representational Neglect and Pseudoneglect ....................................................... 16

Modulating attentional biases by increasing right hemisphere

activation .......................................................................................................................... 18

VIII

Continuous performance task training ................................................................... 18

Transcranial direct current stimulation ................................................................. 19

Summary............................................................................................................................ 20

Thesis overview ............................................................................................................. 21

References ......................................................................................................................... 24

CHAPTER TWO: INDIVIDUALS WITH AUTISTIC-LIKE TRAITS SHOW REDUCED

LATERALIZATION ON A GREYSCALES TASK ...................................................... 45

Methods.............................................................................................................................. 47

Participants ........................................................................................................................ 47

Materials and Procedure............................................................................................... 47

Greyscales ........................................................................................................................... 48

Results ................................................................................................................................ 49

Discussion ......................................................................................................................... 52

References ......................................................................................................................... 54

CHAPTER THREE: REDUCED PSEUDONEGLECT FOR PHYSICAL SPACE, BUT NOT

MENTAL REPRESENTATIONS OF SPACE, FOR ADULTS WITH AUTISTIC TRAITS 59

Methods.............................................................................................................................. 64

Participants ........................................................................................................................ 64

Materials ............................................................................................................................. 64

Procedure ........................................................................................................................... 67

Results ................................................................................................................................ 68

Data Screening .................................................................................................................. 68

Greyscales Task ................................................................................................................ 69

Landmark Task ................................................................................................................. 69

Number Line Task ........................................................................................................... 70

Discussion ......................................................................................................................... 71

References ......................................................................................................................... 74

CHAPTER FOUR: MODULATION OF GLOBAL AND LOCAL PROCESSING BIASES IN

ADULTS WITH AUTISTIC-LIKE TRAITS ............................................................. 81

Experiment 1 ................................................................................................................... 84

Method ................................................................................................................................. 85

Results .................................................................................................................................. 88

IX

Discussion ........................................................................................................................... 93

Experiment 2 ................................................................................................................... 93

Method ................................................................................................................................. 94

Results .................................................................................................................................. 95

Discussion ........................................................................................................................ 101

General Discussion ..................................................................................................... 101

References ...................................................................................................................... 106

CHAPTER FIVE: MODULATING ATTENTIONAL BIASES OF ADULTS WITH

AUTISTIC TRAITS USING TRANSCRANIAL DIRECT CURRENT STIMULATION ... 111

Method ............................................................................................................................. 113

Participants ..................................................................................................................... 113

Materials ........................................................................................................................... 113

Procedure ......................................................................................................................... 114

Results .............................................................................................................................. 115

Discussion....................................................................................................................... 117

References ...................................................................................................................... 119

CHAPTER SIX: GENERAL DISCUSSION .......................................................... 123

Central Aims of this Thesis ..................................................................................... 123

Part 1: New Evidence for Reduced Right Hemisphere Activation for

adults with autistic-like traits ............................................................................... 124

Individuals with autistic-like traits show reduced lateralization on a

greyscales task ............................................................................................................... 124

Beyond greyscales: reduced pseudoneglect for other tasks ........................ 125

Part 1: Discussion .......................................................................................................... 125

Part 2: Increased Right Hemisphere Activation ‘Corrects’ Attention in

Adults with Autistic-Like Traits ........................................................................... 131

Improving global processing using a RH-activating continuous

performance task .......................................................................................................... 131

Increasing pseudoneglect using cortical stimulation ...................................... 132

Part 2: Discussion .......................................................................................................... 133

Implications for ASC and Future Directions ................................................... 138

Final thoughts ............................................................................................................... 142

References ...................................................................................................................... 142

X

XI

ACKNOWLEDGEMENTS

There have been many occasions over the last few years when I have sat at the

computer, staring at the flashing cursor of a Word document while I wonder how to

begin the next paragraph or section. This is usually followed by ten or so attempts at

typing a first sentence in which I am satisfied that the weight of the words carry the

meaning and impact I wish to impart to the reader. The section you are reading now

has been by far one of the most difficult, because there are so many ways to begin, so

many people to recognise, and so much gratitude and thanks to convey.

First, I would like to offer my sincerest thanks and appreciation to my two

supervisors, Troy Visser and Murray Maybery. Without your constant guidance and

mentoring, navigating “the PhD” would have been an arduous task that has otherwise

been mostly enjoyable! There has rarely been an occasion during my candidature

where I have knocked on either of your office doors and haven’t been able to ramble

about my latest idea or show off some fresh data. I am grateful for your experience,

dedication to supervision, and unwavering faith in my abilities, and feel incredibly

fortunate to have had the two of you as my supervisors. I look forward to the

opportunity to work with each you further in the future.

I would also like to acknowledge the many participants who made the

experimental studies possible. Literally hundreds of students have endured the

experiments I have designed. While they may not have been the most exciting tasks in

the world to complete, I hope that most you have learned or taken away something

from the experience.

To all my friends who have supported me throughout the entire university

journey, from the early fresher years, through honours, and finally postgrad. Some of

you will have listened to my moans and complaints about university life for over eight

years – I apologize that, after all this time, I still can’t read your mind or know what you

are thinking. I must also confess that, despite the moaning, I am enjoying myself most

of the time (or have become chronically institutionalised!).

Serena, Diana, Simone, Ines and Michael – thank you for always making

yourself available for a crisis coffee when times have been tough. Having friends who

walk the same path and knowing that what we go through, we go all go through

together, has made the journey so much the better. I will always be around to support

those of you who are approaching the finish line still (you do can do it!).

XII

James, Ella and Katerina – I know you must have often been perplexed by my

explanations about the latest problem I had with data collection, analysis or Reviewer

2’s comments, but you would always listen attentively nonetheless. Thank you for

being there for me from the start. This thesis would have taken a lot longer to complete

if it were not for your constant support over the last few years. While the next year will

likely see many of us go our separate ways, I will always remember and appreciate the

friendship that pulled me through some of the most stressful points of my time spent at

university.

To Pam and Wayne – thank you so much for your endless generosity in giving

myself (and my friends) nice, affordable accommodation for the past several years.

While there has always been something to stress and worry myself over, I am fortunate

not to have had to concern myself with my housing situation. It has made everything

10x easier, and I am extremely grateful for your kindness, and hope that I can repay it

in some small way in the future.

To my wonderful and ever-supportive partner, Bronwyn – I must say that you

edge out reduced pseudoneglect for adults with high levels of autistic-like traits as the

best discovery that I made during my candidature. Even whilst completing your own

PhD, you maintained a passion for life and your constant energy and enthusiasm

enhanced the last few years like nothing else has. Thank you for continuing to remind

me daily that academia is just one part of life, and that there is also dancing and

cartoons and hiking and big breakfasts at nice cafes (because we deserve it). My sanity

and mental health is indebted to you, and I look forward to our post-PhD adventures

together.

Finally, to my family – especially, my parents. You have fostered in me a sense

of curiosity and thirst for knowledge throughout my years, and made sure that every

educational opportunity was made available to me. I cannot thank you enough for

everything you have done for me.

My candidature was supported by an Australian Government Research

Training Program (RTP) Scholarship and a UWA Completion Scholarship. Additional

funding was provided by the UWA Graduate Research School and School of

Psychological Science for conference-related travel.

XIII

AUTHORSHIP DECLARATION:

CO-AUTHORED PUBLICATIONS

This thesis contains work that has been published and/or prepared for publication.

Chapter Two

English, M. C. W., Maybery, M. T., & Visser, T. A. W. (2015). Individuals with Autistic-

Like Traits Show Reduced Lateralization on a Greyscales Task. Journal of Autism and

Developmental Disorders, 45(10), 3390–3395. http://doi.org/10.1007/s10803-015-

2493-7

The student designed the study, collected and analysed the data, and was the primary

contributor on the manuscript.

Chapter Three

English, M. C. W., Maybery, M. T., & Visser, T. A. W. (2017). Reduced pseudoneglect for

physical space, but not mental representations of space, for adults with autistic traits.

Journal of Autism and Developmental Disorders, 47(7), 1956–1965.

http://doi.org/10.1007/s10803-017-3113-5



Chapter Four

English, M. C. W., Maybery, M. T., & Visser, T. A. W. (2017). Modulation of Global and

Local Processing Biases in Adults with Autistic-like Traits. Journal of Autism and


3198-x



Chapter Five

English, M. C. W., Kitching, E. S., Maybery, M. T., & Visser, T. A. W. (under review).

Modulating attentional biases of adults with autistic traits using transcranial direct

current stimulation.

This manuscript is under review with Autism Research.

The student designed the study, shared data collection with ESK, analysed the data, and

was the primary contributor on the manuscript.

Date: _07 April 2017_

I, Troy Visser, certify statements regarding their contribution to each

of the works listed above are correct

Coordinating supervisor signature: __ Date: _07 April 2017_

http://doi.org/10.1007/s10803-015-2493-7

http://doi.org/10.1007/s10803-015-2493-7

http://doi.org/10.1007/s10803-017-3113-5

http://doi.org/10.1007/s10803-017-3198-x

http://doi.org/10.1007/s10803-017-3198-x

XV

OTHER WORK GENERATED DURING THE

CANDIDATURE

Research Articles

English, M. C. W., Maybery, M. T., & Visser, T. A. W. (2017). Threatening faces fail to

guide attention for adults with autistic-like traits. Autism Research, 10(2), 311–320.

http://doi.org/10.1002/aur.1658

Conference Presentations

Individuals with autistic-like traits show reduced lateralization on a greyscales

task (Chapter Two), presented at the Australasian Society for Autism Research 2014 and

the Australian Experimental Psychology Conference 2015.

Threatening faces fail to guide attention for adults with autistic-like traits,

presented at the Asia Pacific Conference on Vision 2016.

Atypical lateralization of attention in adults with autistic-like traits (Chapters

Two, Three and Five), presented at the Australasian Society for Autism Research 2016.

Prize winner for Best Student Presentation.


XVII

A NOTE ABOUT THE FORMAT OF THIS THESIS

This thesis was constructed using the “thesis as a series of papers”, or “thesis by

publication” format that is increasingly used by PhD students in Australia as opposed

to the traditional monograph. The thesis begins with an introductory chapter that

provides an overview of the key aims of the thesis. These aims are then tackled in the

following four chapters, each of which is a self-contained research article that

underwent peer-review and was published during the candidature. The thesis then

concludes with a broader discussion that synthesizes the findings of each of the prior

chapters together.

While each “experimental” chapter is logical progression of the prior chapter,

these pieces of work were also written to stand as independent articles. As such, the

thesis is akin to a scientific research volume that has received input from many

authors. Like these books, each chapter in the thesis begins with an overview of

research pertinent to the current chapter and while some of the literature may have

been raised earlier in the thesis the reader can be assured that the current chapter can

be comprehended without having read prior chapters.

Due to the different types of manuscripts that form each of these chapters there

is some variation in chapter length. The manuscripts that Chapter Two and Five are

drawn from are Brief Reports, whereas Chapter Three and Four are based on

traditional Articles, and are considerably longer.

Finally, to maintain consistency between the published and thesis versions of

each piece of work, the research articles presented in the thesis entirely unchanged

save for formatting changes. However, where examiners made suggestions for

additions or changes to the work, these comments are addressed in footnotes (which

are naturally absent in the published version).

CHAPTER ONE General Introduction Behavioural performance on attentional tasks can often provide insight into

underlying cortical processes. In this thesis, several studies are outlined that a)

provide novel evidence for reduced right hemisphere activation for visuospatial

attention in individuals with high levels of autistic-like traits and, b)

demonstrate that techniques that increase right hemisphere activation can

modulate aspects of attention in these same individuals such that they perform

more comparable to their low-trait peers. The current chapter is a review of the

literature pertinent to the present thesis, providing an overview of autism and

autistic-like traits, how visuospatial ability manifests in the context of autism,

and the role of the right hemisphere in visuospatial attention. Finally, the

chapter highlights how studies investigating the possibility of reduced right

hemisphere activation for visuospatial attention by individuals with high levels

of autistic-like traits may expand our current understanding of attention in

autism.

Autism

Symptoms

The term “autism” was first used by psychiatrist Leo Kanner (1943) in a paper

where he described his observations of 11 children whose specific patterns of

abnormal behaviour, interests and social ability did not fit any diagnosis available at

the time. The diagnostic criteria for autism have undergone several changes since

Kanner’s description, with the condition currently defined in the Diagnostic and

Statistical Manual of Mental Disorders – Fifth Edition (DSM-5) as a

2 GENERAL INTRODUCTION

neurodevelopmental disorder that is primarily characterized by repetitive and

restrictive patterns of behaviour and interests, and impairments in communication and

social functioning (American Psychiatric Association, 2013). However, while research

into autism has steadily increased, our understanding of its exact nature and causal

factors are still limited.

Repetitiveness may be demonstrated in several modalities; from simple motor

movements (e.g. hand flapping, body rocking) and speech (echoing others, repeating

simple phrases), to routines and rituals (walking the same route to school, sitting in the

same chair every day). Repetitive behaviours are often accompanied by a strong

instinct for sameness, and individuals are inflexible with change or deviation from their

desired patterns and rituals. Extreme distress is usually displayed when completing a

behaviour in the desired manner is blocked. Autistic individuals show restricted

interests and fixate on objects or ideas with uncommon intensity (e.g. needing to know

what caused a crack in the ceiling), and distress is again displayed when engaging in a

special interest is impeded. Socially, autistic individuals fail to develop many

fundamental skills, showing difficulty with conversational turn-taking, avoiding eye-

contact and making limited use of non-verbal communication. Individuals with the

condition also have deficits in the ability to share in others interests when they are not

personally held and difficulty with adapting to different situations to suit the context or

mood. These limitations typically result in failure to develop and maintain close

relationships.

Subtypes

As research into autism progressed, references to several different ‘sub-types’

based on variability and severity of the symptoms became more common in the

literature. Often referred to, but not present within current diagnostic manuals, is a

distinction between “low-functioning” and “high-functioning” autism1. Low-functioning

autism is generally characterized by the additional presence of an intellectual disorder,

and individuals with this designation demonstrate the most extreme levels of symptom

severity and are unlikely to gain the ability to live independently. In contrast, those

described with “high-functioning” autism exhibit largely intact cognitive abilities and

can function with a degree of independence. Under the previous incarnation of the

Diagnostic and Statistical Manual of Mental Disorders (DSM-IV), Autistic Disorder was

1 While the terms low- and high-functioning autism are commonly used in the literature, it should be noted that these terms are currently at odds with current practice in describing autism and the movement towards more neurodiverse language.

CHAPTER ONE 3

supplemented by two further categories – Asperger’s Disorder and Pervasive

Developmental Disorder-Not Otherwise Specified (PDD-NOS) (American Psychiatric

Association, 2000). Though in the current DSM-5 these diagnoses have since been

folded into a single diagnosis of Autism Spectrum Disorder (American Psychiatric

Association, 2013), the former terms are still commonly used today (and indeed

variants of them currently exist within the current, 10th revision of the International

Statistical Classification of Diseases and Related Health Problems (World Health

Organization, 1992)). Previously, some “high-functioning” individuals may have

received a diagnosis of Asperger’s Disorder if they met two of the three primary

diagnostic criteria for Autistic Disorder, but their language development was not

impaired or delayed (American Psychiatric Association, 2000). A diagnosis of PDD-NOS

was made if individuals exhibited several key symptoms associated with Autistic

Disorder, but did not show enough symptoms with the severity required for a specific

diagnosis (American Psychiatric Association, 2000).

In this thesis, I follow the example set by Simon Baron-Cohen (2009) and will

refer to the family of diagnostic categories as autism spectrum conditions (ASC). This

term serves two functions: first, ‘condition’ is less stigmatising than ‘disorder’ and,

second, the term recognises that while autistic individuals have disabilities, they are

also not without cognitive strengths, which will be discussed later.

Prevalence

A recent review estimated that, globally, ASC affects approximately 62

individuals in every 10,000, though minimal data regarding prevalence rates is

available from many low- and middle-income countries (Elsabbagh et al., 2012). In

Australia, the latest estimates indicate that ASC was reported in some form in 115,400

(0.5%) individuals (Australian Bureau of Statistics, 2012). While males are significantly

more likely to receive an ASC diagnosis, estimates of the exact male-to-female ratio

vary substantially. Across the full intelligence quotient range, ASC is more common in

males at a ratio of 4.3:1 (Fombonne, 2003, 2005), but among those without co-

occurring intellectual disabilities, the ratio ranges from 5.75:1 to 16:1 (Baird et al.,

2006; Fombonne, 2003, 2005; Scott, Baron-Cohen, Bolton, & Brayne, 2002). Further

complicating matters are suggestions that ASC is currently under-reported in females

(Attwood, 2006; Constantino, 2011; Dworzynski, Ronald, Bolton, & Happé, 2012;

Gillberg, 2005; Goldman, 2013). The large body of research using predominantly male

samples (Thompson, Caruso, & Ellerbeck, 2003), and clinical tools that focus on

externalized behaviours rather than the internalizing problems that are more common


in females (Giarelli et al., 2010; Mandy et al., 2012; Solomon, Miller, Taylor, Hinshaw, &

Carter, 2012) may contribute towards potentially inflated reporting of sex ratios (for

reviews, see: Kreiser & White, 2014; Lai, Lombardo, Auyeung, Chakrabarti, & Baron-

Cohen, 2015).

Although the reported prevalence of ASC diagnoses has increased over several

decades, it is unknown if actual prevalence of the disorder has changed since diagnostic

categories and criteria have been updated, screening methods have improved and

there is increased public awareness of autism (Hill, Zuckerman, & Frombonne, 2014;

Karapurkar, Lee, Curran, Newschaffer, & Yeargin-Allsopp, 2004; Newschaffer et al.,

2007; Wing & Potter, 2002). All of these changes increase the likelihood of an

individual receiving an ASC diagnosis today when she/he would have been left

undiagnosed in the past.

ASC and the neurotypical population

Genetic Origins of ASC

In his formative paper, Kanner (1943) noted that many of the parents of

children examined in his study provided very detailed diaries, painting a picture of

parental obsessiveness. He also observed that ‘warm-hearted’ mothers and fathers’

were few, and that many family members were more preoccupied with “abstractions of

a scientific, literary, or artistic nature, and limited in genuine interest in people” (p.

250). These observations, in conjunction with studies on the effects of social

deprivation in monkeys (Harlow & Harlow, 1962) and developments in mother-child

attachment theories (Bowlby, 1951), led to the concept of “refrigerator mothers”; the

notion that autism was caused by a lack of maternal warmth (Kanner, 1949). What

Kanner may have unknowingly touched upon was a genetic factor in autism, which

might have explained why many of the parents he interviewed showed milder forms of

some of the characteristics present in their children. Unfortunately, the “refrigerator

mother” account of autism persisted until Folstein and Rutter's (1977) seminal finding

that the concordance rate for autism was significantly higher between monozygotic

twins than between dizygotic twins. A recent meta-analysis of the twin studies that

have ensued since then indicates that heritability estimates are in the range of 64-91%

(Tick, Bolton, Happé, Rutter, & Rijsdijk, 2016).

Autistic-like traits

Since the genetic basis of ASC was established, many studies have compared

healthy individuals with immediate ASC relatives to those without as to their

CHAPTER ONE 5

personality traits and other characteristics. Features such as rigid personalities and

obsessiveness (Bolton, Pickles, Murphy, & Rutter, 1998; Demir, Demir, Albayrak,

Kayaalp, & Dogangun, 2009; Lainhart et al., 2002; Piven, Palmer, Landa, et al., 1997),

social reticence or aloof dispositions (Bolton et al., 1994; Murphy et al., 2000), fewer

and less reciprocal relationships (Lainhart et al., 2002; Piven, Palmer, Jacobi, Childress,

& Arndt, 1997), language abnormalities (Ben-Yizhak et al., 2011; Bishop et al., 2004;

Landa et al., 1992; Le Couteur et al., 1996; Pickles et al., 2000; Piven, Palmer, Jacobi, et

al., 1997; L. J. Taylor et al., 2013; Whitehouse, Coon, Miller, Salisbury, & Bishop, 2010),

and deficits in theory of mind (Nagar Shimoni, Weizman, Yoran, & Raviv, 2012) have all

been reported with higher rates in relatives of individuals with ASC compared to

control groups, and produce an overall profile that is qualitatively similar to ASC (for

review, see Pisula & Ziegart-Sadowska, 2015 and Sucksmith, Roth, & Hoekstra, 2011).

This pattern of milder autistic-like traits is commonly referred to as the Broader

Autism Phenotype (for review, see Ingersoll & Wainer, 2014).

Several attempts have been made to develop psychometric measures that can

quantify the degree of autistic-like traits that an individual possesses. One of the first

developed is the Autism Family History Interview (AFHI; Bolton et al., 1994), a

standardized clinical interview which asks questions of ASC individuals and their

immediate family members with respect to personality traits and behaviours in areas

that are relevant to ASC, such as circumscribed interests, rigidity, perfectionism,

obsessions and compulsions. Interviews are recorded, and blind raters determine the

extent to which interviewee’s responses are qualitatively similar to the ASC

behavioural profile.

While structured interviews such as the AFHI are effective tools for identifying

ASC-like behavioural profiles in immediate relatives of individuals with ASC (Piven,

Palmer, Jacobi, et al., 1997), the lengthy administration process led to the development

of more accessible measures. A commonly-used scale is the 50-item self-report Autism

Spectrum Quotient (AQ; Baron-Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001).

The AQ can be likened to the AFHI, in that both can be used to measure autistic-like

traits in healthy, neurotypical individuals, but the self-report nature and simple four-

point Likert scale format of the AQ has driven increasing use of the measure since its

inception. Similar to the AFHI, items on the AQ tap into key aspects of ASC (e.g. the item

“I would rather go to a library than a party” is related to social functioning). Responses

to statements are made using a four-point scale labelled “Strongly Disagree”, “Slightly

Disagree” “Slightly Agree” and “Strongly Agree”. Baron-Cohen et al. (2001)


recommended dichotomous (0-1) scoring of responses, leading to an overall index

ranging from 0-50, where higher scores indicate greater levels of autistic-like traits.

Since the dichotomous scoring method does not distinguish between “strongly” or

“slightly” agreeing/disagreeing with a statement, an alternative “1-4” method that

distinguishes these responses, resulting in an index of 50-200, is often used instead

(Austin, 2005).

Parents of children with ASC are reported to have higher AQ scores (more

autistic-like traits) than parents with typically developing children (Bishop et al., 2004;

Wheelwright, Auyeung, Allison, & Baron-Cohen, 2010; Woodbury-Smith, Robinson,

Wheelwright, & Baron-Cohen, 2005), and twin studies observing levels of autistic-like

traits in children show moderately similar heritability estimates to those reported for

autistic children, ranging between 36-87% (for a review, see Ronald & Hoekstra, 2011).

Furthermore, higher AQ scores are also associated with other measures of social

functioning, such as reduced explicit and implicit knowledge of nonverbal cues

(Ingersoll, 2010) and greater tendencies towards obsessional personality (Kunihira,

Senju, Dairoku, Wakabayashi, & Hasegawa, 2006). Observations of autistic-like traits

and behaviours in both individuals with and without immediate relatives with ASC has

led to the conceptualization of autism as a continuum that extends beyond those with

clinical diagnoses and into the neurotypical population (Constantino & Todd, 2003;

Lundström, 2012; Posserud, Lundervold, & Gillberg, 2006; Ruzich et al., 2015) where

AQ scores are reported to be normally distributed (Baron-Cohen et al., 2001; Hoekstra,

Bartels, Verweij, & Boomsma, 2007).

The autism-as-a-continuum notion has allowed for the expansion of autism-

related research into the broader neurotypical population, and a recent literature

review identified over 870 studies that had used the AQ (Ruzich et al., 2015),

suggesting that a substantial number of researchers believe that the examination of

individuals with high levels of autistic-like traits is a valuable method of furthering our

understanding of ASC. There are several benefits to conducting such studies (Landry &

Chouinard, 2016), such as the ease of recruiting neurotypical volunteers, making it

easier to test large sample sizes, and the fact that potential confounds, such as IQ, are

more easily controlled for with neurotypical samples. One method by which these

studies are conducted is by examining how task performance correlates with AQ scores

in an unselected sample, such as a young adult student sample (i.e. the AQ scores for

the sample would be expected to be normal in distribution). Alternatively, participants

may be selectively recruited, based on their AQ scores, to create distinct comparison

CHAPTER ONE 7

groups (i.e. “Low” vs “High” AQ groups). This latter method has the advantage of

paralleling the methodology of clinical studies, where group comparisons are similarly

made between groups differing substantially on the autism continuum, that is,

neurotypical and ASC samples.

However, the use of AQ-selected groups as a proxy for ASC/non-ASC groups is

not without its critics. Gregory and Plaisted-Grant (2013) argued that differences

between Low and High AQ groups may be in part due to undiagnosed, or otherwise

unidentified, individuals with ASC included in the High AQ group. Consequently, AQ-

based studies are ASC/non-ASC comparisons, and not true Low AQ/High AQ

comparisons. Whilst the relatively low prevalence of ASC means it is unlikely that any

High AQ group will be comprised of a significant proportion of unidentified individuals

with ASC (Australian Bureau of Statistics, 2012), it is possible to reduce the impact of

any ASC influence within High AQ groups. For example, recruiting particularly large

samples can minimise the impact of any individual participant, and analyses can be

conducted with and without the participants with the highest AQ scores (who are most

likely to be unidentified individuals with ASC). As a result, Gregory and Plaisted-Grant’s

(2013) concerns can be addressed and the usefulness of the AQ as a research tool is

retained.

Weak Central Coherence Theory of Autism

Conceptualisation

Though ASC is primarily characterised by a specific collection of impairments,

there exists several domains in which individuals with ASC excel. Kanner (1943) was,

again, the first to take note of some of these skills, observing that the children who

were verbal had “astounding” vocabulary, “excellent” memory for events that had

occurred in years past, and “phenomenal” rote memory for poems, names and other

complex patterns and sequences (p. 247). Kanner also noted that these children had an

inability to “experience wholes without full attention to the constituent parts” (p. 246).

This latter behaviour is in sharp contrast to the usual focus of their typically developing

peers on “wholes”.

The innate drive for coherence in neurotypical individuals is a notion that dates

back to the Gestalt movement, instigated by the suggestion that humans perceptually

organise stimuli in a top-down manner, thus interpreting ‘the whole’ before attending

to ‘the parts’ (Wertheimer, 1924/1938a). This ‘global coherence’ can be demonstrated

by gestalt grouping principles; neurotypical individuals tend to automatically group


objects according to principles such as proximity, similarity and/or closure

(Wertheimer, 1924/1938b). For example, grouping by proximity is shown in a

configuration such as ‘… … …’, which is generally interpreted as three ellipses, rather

than nine periods. The drive for coherence over detail is not limited to the visual

domain. In a series of experiments where participants were tasked with remembering

and recounting verbal material, Bartlett (1932) noted that participants tended to

provide the gist and “general impression of the whole” when recounting, and from this

they would “construct the probable detail” as a process secondary to recall of meaning

(p. 206).

Frith (1989) expanded upon Kanner’s observations and suggested that both the

deficits and “islets of ability” in ASC could be accounted for by a deficiency in global

processing and weaker drive for coherence. This account became known as the weak

central coherence (WCC) theory of autism, and is often generalised to suggest that a

global processing deficit exists in individuals with ASC. Frith argued that because of

impairments in the ability to construct a coherent whole, individuals with ASC showed

superior performance on tasks that benefited from greater attention to detail, such as

the Embedded Figures Test (EFT; Witkin, 1971) and Block Design subtest of the

Wechsler Intelligence Scales (Wechsler, 2014), compared to neurotypical individuals

(Kolinsky, Morais, Content, & Cary, 1987; Shah & Frith, 1983). In the EFT, participants

are required to locate a simple shape (e.g., a triangle) that is embedded in a larger,

complex picture (e.g. a grandfather clock). WCC suggests that performance is inversely

related to the drive for coherence; those with an automatic tendency for integrating the

details into a cohesive whole, which is most neurotypical individuals, have difficulty

accessing details due to interference from the larger configuration. However,

individuals with ASC do not exhibit the same drive for coherence and therefore have

greater access to the details, resulting in faster reaction times by this group.

Performance on the Block Design test operates similarly. Participants must

recreate a pattern using multiple blocks that have coloured squares or triangles on

their faces. The pattern is seen as a cohesive whole by neurotypical participants, rather

than multiple segments which would aid in locating the necessary blocks. Under the

WCC theory of autism, ASC participants are less influenced by the whole, and so are

able to locate the blocks required to reproduce the pattern faster (Shah & Frith, 1983).

Consistent with this position, if the pattern to-be-copied is pre-segmented,

neurotypical participants show marked improvement in reaction times, whilst ASC

participants are unaffected (Shah & Frith, 1993). Conversely, WCC also explains why

CHAPTER ONE 9

individuals with ASC perform worse on tasks that have a greater emphasis on

integrating multiple independent stimuli. For example, in a study in which children had

to remember a pattern of coloured counters (Frith, 1970), typically developing

children quickly learned the overall pattern and often exaggerated it. On the other

hand, children with ASC used only the most recent counters to guess the colour of the

next instead of using the overall pattern structure, and consequently performed worse

on the task.

Mixed support for Weak Central Coherence

Since Frith’s original account, subsequent research has tested the extent to

which the WCC theory holds. One of the most prevailing findings, the superior

performance on the EFT by individuals with ASC, has been replicated many times

(Brosnan, Gwilliam, & Walker, 2012; De Jonge, Kemner, & Van Engeland, 2006; Edgin &

Pennington, 2005; Falter, Plaisted, & Davis, 2008; Jarrold, Gilchrist, & Bender, 2005;

Jolliffe & Baron-Cohen, 1997; Keehn et al., 2009; Morgan, Maybery, & Durkin, 2003;

Pellicano, Gibson, Maybery, Durkin, & Badcock, 2005; Pellicano, Maybery, Durkin, &

Maley, 2006; Ropar & Mitchell, 2001; Schlooz & Hulstijn, 2014; Van Lang, Bouma,

Sytema, Kraijer, & Minderaa, 2006). A similar pattern is also seen when comparing

neurotypical individuals who have low- or high-levels of autistic trait as measured by

the AQ, with a recent meta-analysis (Cribb, Olaithe, Di Lorenzo, Dunlop, & Maybery,

2016) showing that High AQ groups reliably have superior performance on the EFT

relative to Low AQ groups. Not all studies have supported the WCC theory, with several

studies finding comparable EFT performance between neurotypical and ASC groups

(Bölte, Holtmann, Poustka, Scheurich, & Schmidt, 2007; Chen, Lemonnier, Lazartigues,

& Planche, 2008; Dillen, Steyaert, Op de Beeck, & Boets, 2015; Kaland, Mortensen, &

Smith, 2007; Ozonoff, Pennington, & Rogers, 1991; Pring, Ryder, Crane, & Hermelin,

2010; Schlooz et al., 2006; Spek, Scholte, & Van Berckelaer-Onnes, 2011; L. J. Taylor,

Maybery, Grayndler, & Whitehouse, 2014; White & Saldaña, 2011) and, occasionally,

superior performance in ASC groups (Burnette et al., 2005; Spek et al., 2011).

With respect to the Block Design test, several studies have also replicated Shah

and Frith's (1983) initial observations (Happe, 1994; Morgan et al., 2003; Pellicano et

al., 2006; Pring, Hermelin, & Heavey, 1995; Ropar & Mitchell, 2001; Soulières, Zeffiro,

Girard, & Mottron, 2011), and identical patterns have also been reported in studies

using neurotypical individuals with autistic-like traits, where individuals with High AQ

outperforming those with Low AQ (Grinter et al., 2009; Stewart, Watson, Allcock, &

Yaqoob, 2009). Despite some conflicting findings, a recent meta-analysis found that,


overall, individuals with ASC generally show superior performance on both the EFT

and Block Design test (Muth, Hönekopp, & Falter, 2014).

Moving beyond the initial tasks used by Shah and Frith , differences between

ASC and control individuals have also been noted using hierarchical Navon figures

(Navon, 1977), likely due to the similar properties that these figures share with stimuli

in the EFT. A Navon figure (illustrated in Figure 1.1) consists of multiple, small

characters (local level) that make up the shape of a single, larger character (global

level). Most variations of tasks that use Navon figures require participants to attend

and respond to either the global or local level of the figure, whilst ignoring the other

level (e.g. categorize the global level character as a number or a letter). Optimum task

performance is generally achieved when the two levels are congruent (e.g. the first two

examples of Figure 1.1 share the same global and local level category). However,

performance can be impaired when the two levels are incongruent (e.g. the last two

examples of Figure 1.1 do not share the same global and local level category), and the

amount of interference from the to-be-ignored level provides insight into participant’s

attentional preferences.

When neurotypical individuals are directed to categorize the global level as a letter or

number, task reaction times are typically not significantly impaired when the local

level is incongruent to the global level relative to reaction times for stimuli with

congruent global and local features. However, when categorizing the local level, the

presence of global features that are incongruent significantly increases reaction times

relative to occasions when the global features are congruent, demonstrating that the

drive for coherence is present even when it is irrelevant to the current task and results

in sub-optimal performance. In contrast, individuals with ASC do not demonstrate as

great a level of global interference as neurotypical individuals, instead showing greater

interference from local features (Behrmann et al., 2006; Gross, 2005; Mottron &

Belleville, 1993; Plaisted, Swettenham, & Rees, 1999; Rinehart, Bradshaw, Moss,

Brereton, & Tonge, 2000; L. Wang, Mottron, Peng, Berthiaume, & Dawson, 2007).

CHAPTER ONE 11

Figure 1.1. Examples of hierarchical figures (Navon, 1977); the two on the left are

form-congruent (the global and local levels are the same; both letters or both numbers)

and the two on the right are form-incongruent (the global and local levels are

mismatched; letters and numbers are simultaneously present.

Similar to the EFT, a number of studies report conflicting findings when using

Navon figures, showing a lack of evidence for greater local processing biases in ASC

groups relative to controls (Iarocci, Burack, Shore, Mottron, & Enns, 2006; Mottron,

Burack, Iarocci, Belleville, & Enns, 2003; Mottron, Burack, Stauder, & Robaey, 1999;

Ozonoff, Strayer, McMahon, & Filloux, 1994; Plaisted et al., 1999; Rinehart et al., 2000;

Rondan & Deruelle, 2007; L. Wang et al., 2007). Only two studies have investigated the

effects of AQ on performance where hierarchical figure stimuli have been used. The

first revealed no difference between High- and Low AQ groups (Sutherland & Crewther,

2010), but in the second, which employed a non-conscious cueing paradigm, High AQ

individuals demonstrated a cueing effect of smaller local-level cues in hierarchical

arrow stimuli, whilst Low AQ individuals showed cueing effects from the global-level of

the stimuli (Laycock, Chan, & Crewther, 2017).

In response to these conflicting findings, a recent meta-analysis was conducted

that encompassed 56 studies assessing visual global and local processing in ASC (Van

der Hallen, Evers, Brewaeys, Van den Noortgate, & Wagemans, 2015). The authors of

the meta-analysis found that, in terms of overall accuracy, there is neither enhanced

local processing nor deficits in global processing. However, the analysis also showed

that global processing is slowed in individuals with ASC, especially on tasks where

participants have to attend to globally-presented targets whilst ignoring distractors

presented at the local level, suggesting that local-to-global interference is present

within individuals with ASC. The authors suggest that this finding partially supports

the WCC theory, and that future research should focus on the processes and

mechanisms that underlie perceptual functioning in ASC, rather than mere ability and

inability.


Perceptual Mechanisms Underlying ASC: Attention and

the Right Hemisphere

Global processing

For most neurotypical individuals, language and spatial processing are broadly

attributed to the left hemisphere (LH) and right hemisphere (RH) respectively (Pujol,

Deus, Losilla, & Capdevila, 1999; Whitehouse & Bishop, 2009). An absence or reduced

use of spoken language often pre-empts a subsequent diagnosis of ASC (Giacomo &

Fombonne, 1998) and often persists across the individual’s lifespan (Ballaban-Gil,

Rapin, Tuchman, & Shinnar, 2016). Substantial research has been devoted to

determining the neural underpinnings of impaired language in ASC, and one of the

most prevailing findings has been the suggestion that language is atypically lateralized

in these individuals (for a review, see Hollier, Maybery, & Whitehouse, 2014). However,

given that visuospatial ability is more often viewed somewhat as a strength in ASC

(Shah & Frith, 1983), it is surprising that relatively few studies have explored

behavioural implications that this atypical lateralization might have for visuospatial

processing in ASC individuals.

With respect to neurotypical individuals, it is possible that the drive for

coherence and preference for globally-arranged stimuli stems from the fact that global

processing is closely linked to the RH, which is generally more involved in visuospatial

processing, whereas local processing is associated with the LH (for a review see Ivry

and Robertson, 1998, see also: Evans, Shedden, Hevenor, & Hahn, 2000; Flevaris,

Bentin, & Robertson, 2010; Hübner & Studer, 2009; Lux et al., 2004; Malinowski,

Hübner, Keil, & Gruber, 2002; Volberg & Hübner, 2004; Weissman & Woldorff, 2005;

Yamaguchi, Yamagata, & Kobayashi, 2000). As a result, global processing regions may

be more readily activated than those associated with local processing, given that

performance on tasks requiring attention to be focused on particular regions in space

are relatively more dependent on intact RH functioning than on LH functioning

(Heilman, 1995; Hellige, 1993). Evidence for this asymmetrical lateralization of

visuospatial attention has primarily come from studies observing individuals with

unilateral brain injuries to either the left or right hemisphere. While injuries or lesions

to either hemisphere affect visuospatial ability, damage to the RH has by far the

greatest impact, severely impacting performance on a range of tasks, including picture

copying and line bisection (Gainotti, Messerli, & Tissot, 1972; Ivry & Robertson, 1998;

Kaplan, 1983; Poizner, Klima, & Bellugi, 1990). It is thought that the RH controls the

CHAPTER ONE 13

distribution of attention to both the left and right visual fields while the LH controls

attention only for the right visual field, and it is this absence of redundancies in the LH

that potentially explains why visuospatial deficits are more common following RH

damage than LH damage (Mesulam, 2000).

Following this notion, the reduced preference for global processing seen in ASC

could be attributed to atypically reduced RH functioning for visuospatial processing in

ASC. Supporting this view, many studies using behavioural and neurological measures

have reported cortical abnormalities in ASC that are specific to the RH, including

pragmatic language measures sensitive to right brain damage (Ozonoff & Miller, 1996;

Siegal, Carrington, & Radel, 1996), functional and structural magnetic resonance

imaging (Di Martino et al., 2011; Jou, Minshew, Keshavan, Vitale, & Hardan, 2010; A. T.

Wang, Lee, Sigman, & Dapretto, 2007; Yamasaki et al., 2010) and electrophysiological

measures (Keehn, Vogel-Farley, Tager-Flusberg, & Nelson, 2015; Lazarev, Pontes, &

DeAzevedo, 2009; McPartland, Dawson, Webb, Panagiotides, & Carver, 2004; Orekhova

et al., 2009). Arguably, deficits or abnormalities in the RH in ASC may be manifest as a

reduced preference for global processing.

Visual Neglect and Pseudoneglect

While indications of LH and RH activation levels may be obtained by observing

local and global processing respectively, it is not the only method of detecting atypical

hemispheric activation associated with visuospatial attention. If atypical hemispheric

activation of visuospatial attention drives abnormal global and local processing biases

in ASC, it should also yield atypical biases in other areas of visuospatial attention. For

instance, individuals with lesions in one hemisphere commonly fail to detect visual

stimuli presented in the contralateral visual field, a disorder known as hemispatial

neglect (Heilman, 1995), and this is substantially more common following RH than LH

damage (Costa, Vaughan, Horwitz, & Ritter, 1969; Gainotti et al., 1972). Patients with

RH lesions resulting in left hemispatial neglect generally complete visuospatial tasks

seemingly ignoring stimuli present in the left hemispace and consequently appearing

to over-attend to items presented in the right hemispace. For example, on a line

bisection task, which requires participants to mark the midpoint of a horizontal line,

RH lesioned patients erroneously place the midpoint to the right of the actual centre

(Bisiach, Bulgarelli, Sterzi, & Vallar, 1983; Harvey & Milner, 1999; Heilman, Watson, &

Valenstein, 1997; Parton, Malhotra, & Husain, 2004; Poizner et al., 1990). On a

cancellation task, which requires participants to place a cross over target items in an

array, the same patients fail to cross out most targets presented in the left hemispace


(Albert, 1973; Ferber & Karnath, 2001; Gauthier, Dehaut, & Joanette, 1989; Heilman et

al., 1997). Finally, on a landmark task, which requires participants to indicate if a

bisection on a horizontal line is closer to the left or right end of the line, patients with

RH damage tend to bisect the line to the right of the true midpoint, as if perceiving the

left half of the line to be shorter than it actually is or exaggerating how much space the

right-side of the line takes up (Harvey & Milner, 1999; Heilman et al., 1997; Milner,

Brechmann, & Pagliarini, 1992).

Critically, neurotypical individuals show an opposite attentional pattern. While

RH lesioned patients make errors that are in line with a rightward shift of attention,

neurotypical individuals generally err towards the left and, for example, bisect

horizontal lines slightly leftward of the actual centre (for review, see Jewell & McCourt,

2000). This leftward bias in neurotypical individuals was termed pseudoneglect

(Bowers & Heilman, 1980) due to the similarity it shares with hemispatial neglect, and

may also be attributed to the overall greater involvement of the RH in visuospatial

functioning (Fierro et al., 2000; Fink, Marshall, Weiss, Toni, & Zilles, 2002; Foxe,

McCourt, & Javitt, 2003; Harris & Miniussi, 2003; Siman-Tov et al., 2007; Vingiano,

1991).

Direct evidence for a link between pseudoneglect and the RH lateralization of

visuospatial attention stems from studies that manipulate hemispheric activation. One

study used excitatory stimulation delivered via transcranial direct current (tDCS), a

non-invasive technique that alters cortical excitability (for review, see Nitsche et al.,

2008 and Utz, Dimova, Oppenländer, & Kerkhoff, 2010). Stimulation over the left

posterior parietal cortex (PPC) eliminated pseudoneglect in neurotypical individuals,

presumably by rebalancing activation asymmetries between the left and right PPCs

(Loftus & Nicholls, 2012). Another research group had neurotypical participants

complete a landmark task whilst receiving transcranial magnetic stimulation (TMS) to

inhibit activation of the right PPC, finding that the TMS resulted in neglect-like

rightward deviations on the task (Fierro et al., 2000; Fierro, Brighina, Piazza, Oliveri, &

Bisiach, 2001). Finally, a study was recently conducted that used functional magnetic

resonance imaging (fMRI) to confirm changes in hemispheric lateralization following

TMS (Petitet, Noonan, Bridge, O’Reilly, & O’Shea, 2015). Neurotypical participants

initially showed pseudoneglect on a computerized target detection task that required

them to identify if visual targets were presented in the left, right, or both hemifields.

After receiving TMS to the right angular gyrus/intra-parietal sulcus, pseudoneglect was

CHAPTER ONE 15

abolished, and fMRI confirmed that the relative balance of parietal activity had shifted

from the RH to the LH.

With ASC, if the usual rightward hemispheric lateralization of attention is

altered by a reduction in RH activation, it follows that we might expect to see a

reduction in pseudoneglect, or even hemispatial neglect-like behaviours similar to

those found in RH lesioned patients. The few studies that have investigated this notion

have largely used face stimuli in their designs, as it is well documented that

neurotypical individuals tend to explore the LVF of centrally presented face stimuli

first, and for a longer duration, than the RVF (Butler et al., 2005; Everdell, Marsh,

Yurick, Munhall, & Paré, 2007; Guo, Meints, Hall, Hall, & Mills, 2009; Guo, Smith, Powell,

& Nicholls, 2012; Leonards & Scott-Samuel, 2005; Racca, Guo, Meints, & Mills, 2012;

Smith, Gibilisco, Meisinger, & Hankey, 2013). The chimeric face task, which requires

participants to decide which of two composite faces is a closer match for a given face

prime, shows reduced LVF bias for adults with Asperger syndrome compared to

controls (Ashwin, Wheelwright, & Baron-Cohen, 2005), and a later study using ASC

children and emotional faces found an intact LVF bias for only two (happiness and

anger) out of the six emotions presented (S. Taylor, Workman, & Yeomans, 2012). Eye-

tracking studies have also reported a reduced LVF bias for human faces in ASC adults,

ASC children, and infants with older ASC siblings, relative to controls (Dundas, Best,

Minshew, & Strauss, 2012; Dundas, Gastgeb, & Strauss, 2012; Guillon et al., 2014). The

pattern is also found in ASC children viewing stimuli of dog faces (Guillon et al., 2014),

indicating that reduced LVF bias in ASC is not limited to the domain of human faces.

However, it is currently unknown if reduced LVF bias in ASC is specific to face

stimuli or not, as studies using non-face stimuli are largely non-existent. While the only

study, to my knowledge, that has investigated this issue did not find any attentional

differences between a group of ASC and typically developing children (Rinehart,

Bradshaw, Brereton, & Tonge, 2002), several factors may have contributed to this

result that may have masked group differences. Critically, the typically developing

group may be somewhat atypical in that they did not show any pseudoneglect, which

conflicts with other reports (Dellatolas, Coutin, & De Agostini, 1996) and evidence

suggesting that pseudoneglect decreases with age (Jewell & McCourt, 2000).

Additionally, participants could not be screened for accuracy, and thus task

engagement, and participants completed fewer than the recommended number of

trials to ascertain a reliable bias (Nicholls, Bradshaw, & Mattingley, 1999).


It is possible that, when addressing these methodological issues, atypical

visuospatial biases linked to autism may be also present for non-face stimuli. This

could be considered potential evidence for the presence of atypical hemispheric

lateralization in ASC that generalizes to visuospatial processing regions beyond those

that are associated with face processing. Importantly, such findings may mark the

presence of a previously unknown neurocognitive phenotype for ASC. Furthermore, it

is also unknown if reduced LVF biases are present in individuals with high levels of

autistic-like traits for any stimulus type. Considering that autism is thought to lie on a

spectrum that extends into the neurotypical population, one may expect to find that the

degree of pseudoneglect expression may decrease with increasing level of autistic-like

traits. With this in mind, one aim of the present thesis is to test whether a relationship

exists between pseudoneglect and autistic-like traits in large samples of neurotypical

participants.

Representational Neglect and Pseudoneglect

Interestingly, RH damage appears to not just affect how visual space is

perceived (as in the tasks just described), but also biases spatially-organized mental

representations as well. One such representation is the mental number line. The

mental number line can be described as a series of numbers located on a horizontal

azimuth numerically ascending from left-to-right (Dehaene, Bossini, & Girau, 1993).

When asked to determine the midpoint between any two numbers, RH damaged

patients tend to select a number that is larger, or more ‘rightward’, than the actual

midpoint, with greater rightward errors of judgement corresponding with neglect

severity (Hoeckner et al., 2008; Zorzi, Priftis, & Umiltà, 2002). As with the line bisection

task, this outcome suggests that RH damaged patients fail to appropriately consider the

‘left’ half of the stimulus, and appear to exaggerate the distance between numbers on

the ‘right’ half of the mental number line. Similar patterns of behaviour by RH damaged

patients have also been observed on other tasks. For alphabetical sequences, which are

also thought to consist of a left-to-right mental organization (Gevers, Reynvoet, & Fias,

2003), RH damaged patients tend to perceive the alphabetical midpoint between two

letters to be rightward of the actual midpoint (Zorzi, Priftis, Meneghello, Marenzi, &

Umiltà, 2006). Furthermore, when tasked with recalling objects in a scene, RH

damaged patients tend to describe more items that would be positioned in their

imagined right visual field relative to the left visual field (Bisiach & Luzzatti, 1978;

Guariglia, Padovani, Pantano, & Pizzamiglio, 1993).

CHAPTER ONE 17

Complementing findings in neurotypical participants regarding pseudoneglect

for visual stimuli, healthy adults also show a small bias in mental number line

representation in favour of selecting a lower, more ‘leftward’ number, relative to the

true midpoint between any two numbers (Göbel, Calabria, Farnè, & Rossetti, 2006;

Loftus, Nicholls, Mattingley, Chapman, & Bradshaw, 2009; Longo & Lourenco, 2007;

Nicholls & McIlroy, 2010). Relatively greater activation of RH regions for visuospatial

attention also likely underlies this representational form of pseudoneglect, resulting in

the apparent exaggeration of the distance between numbers that are more “leftward”

on the number line (similar to the exaggeration of leftward stimulus features of visual

stimuli that results in pseudoneglect), shifting judgements as to the relative location of

the central point (Loftus et al., 2009).

Taken together, the outcomes of these studies strongly imply that there is a

common neural mechanism underlying physical and representational alterations of

spatial bias. However, while several studies have found evidence for both physical and

representational biases in the same patients (Aiello et al., 2012; Vuilleumier, Ortigue, &

Brugger, 2004; Zorzi et al., 2006, 2002), others report that neglect patients who show

rightward deviations on physical spatial tasks do not necessarily have similar biases in

the representational medium (Aiello et al., 2012; Doricchi, Guariglia, Gasparini, &

Tomaiuolo, 2005; Loetscher & Brugger, 2009; Loetscher, Nicholls, Towse, Bradshaw, &

Brugger, 2010; Pia et al., 2012; Rossetti et al., 2011; Storer & Demeyere, 2014; van

Dijck, Gevers, Lafosse, & Fias, 2012). These latter findings would suggest that, whilst

aspects of the RH are potentially responsible for both forms of neglect, it is likely that at

least partially disparate underlying networks are at play.

If disparate networks underlie physical and representational spatial biases for

patients with RH damage, we potentially may observe dissociations regarding

alterations of these spatial biases in individuals with ASC or neurotypical individuals

with high levels of autistic-like traits. Therefore, it is important to consider

representational as well as physical spatial biases in investigating atypical

lateralization of spatial attention in ASC. Autism-related reductions in pseudoneglect

on both physical and representational measures of spatial bias could be considered

evidence for a common neural mechanism. However, finding reductions in only the

physical measures of spatial bias would suggest that the mechanisms that underlie

atypical lateralization of attention in autism are tied to processes within the visual

perceptual system. Consequently, a second aim of the present thesis is to determine the


similarities or differences between pseudoneglect and representational pseudoneglect

for individuals differing in levels of autistic-like traits.

Modulating attentional biases by increasing right

hemisphere activation

Continuous performance task training

If reduced RH activation contributes to atypical attentional functioning in ASC,

it is possible that increasing RH activation might serve to align performance on

attentional tasks with that seen in neurotypical individuals. There is some evidence

that engaging in a continuous performance task (CPT) activates RH regions and alters

performance on visuospatial tasks for both neurotypical individuals and patients with

RH damage (Degutis & Van Vleet, 2010; Van Vleet, Hoang-duc, DeGutis, & Robertson,

2011). The CPT is a computerized attentional training task during which participants

must make speeded responses to the appearance of images of everyday objects on a

computer monitor, whilst inhibiting responses to images of a pre-designated target

category that comprises 10% of all trials. The task demands a high level of attention

from the participant, which is driven by increases in tonic and phasic attention. Tonic

attention, which refers to sustained attention over long periods of time, is linked to

activation of the right inferior frontal, inferior parietal and anterior cingulate regions

(Bartolomeo, 2014; Singh-Curry & Husain, 2009; Sturm & Willmes, 2001; Thiel, Zilles,

& Fink, 2004), whilst phasic attention, which deals with moment-to-moment attention

such as responding to the appearance of a stimulus, is linked to modulations of the

right ventral fronto-parietal network and anterior cingulate region (Corbetta &

Shulman, 2002).

In one study, RH lesioned patients completed landmark and visual search tasks

before and after engaging in three 12-minute rounds of CPT training (Degutis & Van

Vleet, 2010). It was reported that rightward biases for patients who completed the CPT

training were temporarily eradicated on the tasks in the follow-up session, whereas

patients who completed a control task that did not induce changes in tonic or phasic

attention saw no changes in task performance. However, subsequent follow-up

sessions showed that rightward biases in attention returned, indicating that the effects

of CPT training were temporary. The absence of rightward attentional biases

immediately following CPT suggests that the training task selectively activated regions

in the RH associated with visuospatial attention. In a second study, neurotypical

participants categorized global or local features of hierarchical figures (Navon, 1977)

CHAPTER ONE 19

in separate blocks in pre-CPT and post-CPT sessions (Van Vleet et al., 2011). Here, it

was found that, compared to completing a control training task, engaging in CPT

simultaneously reduced the interference produced by incongruent local features when

categorizing the global feature, and increased the interference produced by

incongruent global features when categorizing the local feature. Like the first study, it

was suggested that these modulations of attention were brought about due to

relatively greater activation of RH regions associated with global processing following

a brief engagement with CPT.

Critically, leftward shifts in attention (Degutis & Van Vleet, 2010) and increased

preference for globally-organized visual stimuli (Van Vleet et al., 2011) can both be

interpreted as behavioural evidence for relatively increased RH activation during the

short period following training. If attentional preferences in ASC are driven by

relatively reduced RH activation for visuospatial attention, then CPT may be an

effective tool for temporarily increasing RH activation and potentially ‘normalizing’

visuospatial attention for individuals with ASC and neurotypical individuals with high

levels of autistic-like traits. The third aim of the present thesis tests this notion, and

explores whether different levels of autistic-like traits impact the effectiveness of CPT.

It could be presumed that if, for any given reason, high trait neurotypical individuals

resist CPT training and attention remains unaltered, that the same results would likely

be seen in a clinical ASC sample. Therefore, this experiment will provide a preliminary

indication as to whether CPT in the broader context of ASC is a worthwhile pursuit.

Transcranial direct current stimulation

As mentioned earlier, the degree of lateralization of hemispheric activation can

be modulated via non-invasive techniques that temporarily alter cortical excitability,

with several studies showing that alterations of PPC excitability affect visuospatial

attentional biases. In each of these studies, increasing right PPC activation via anodal

tDCS induced a leftward shift in attention in both neurotypical individuals and RH-

damaged patients (Roy, Sparing, Fink, & Hesse, 2015; Sparing et al., 2009), whilst

disrupting right PPC activation (using cathodal tDCS or TMS) was generally associated

with a rightward shift in attention (Fierro et al., 2000, 2001; Petitet et al., 2015; Sparing

et al., 2009). On the other hand, stimulation of the left PPC produces the converse

pattern of effects (e.g. increased activation invokes rightward shifts of attention; Loftus

& Nicholls, 2012; Sparing et al., 2009). Furthermore, TMS over the right PPC also

causes a rightward shift on a number line bisection task (Göbel et al., 2006), suggesting


that the PPC is important to the allocation of spatial attention for both physical and

mentalized stimuli.

If reduced RH activation underlies variation in visuospatial attention in

individuals with ASC, it follows that increasing RH activation using techniques such as

tDCS may reduce or ameliorate these deficits. Previously in this introduction, it was

predicted that neurotypical individuals with high levels of autistic-like traits would

show reduced levels of pseudoneglect relative to individuals with low levels of autistic-

like traits – a consequence of relatively reduced RH activation in the high trait group.

Assuming that prediction is correct, could tDCS over the right PPC be used to increase

pseudoneglect in the high trait group? The fourth aim of the present thesis is to

examine this question by determining how anodal and cathodal tDCS alter visuospatial

biases on a visuospatial lateralization task for neurotypical individuals with high and

low levels of autistic-like traits. As prior work has demonstrated that anodal tDCS can

be effective at increasing pseudoneglect for a neurotypical sample (Roy et al., 2015;

Sparing et al., 2009), it stands to reason that the stimulation technique should be

particularly effective for a high autistic trait sample who potentially have RH baseline

activation levels that are lower relative to low trait individuals. Furthermore, the

potential presence of low trait individuals with higher pseudoneglect baselines may

account for the absence of right PPC stimulation effects in one study (Loftus & Nicholls,

2012).

Summary

Social difficulties are a defining trait in ASC and it is likely that these difficulties

can be at least partly attributed to atypical attentional processes. Investigation into

atypical global/local biases has dominated this field thus far, but decades of research

has not led to firm conclusions. That said, the most recent meta-analysis of the

literature suggests that global processing is slowed in ASC relative to neurotypical

individuals (Van der Hallen et al., 2015). It is possible that atypical activation of the RH

is responsible for slowed global processing in ASC given that the RH typically has a

substantial role in global processing, and that several neurological studies have found

evidence for cortical abnormalities in this hemisphere linked to ASC. However, if broad

deficits in the RH are responsible for reduced global processing ability in ASC, it is

likely that other attentional abnormalities exist too. There is some evidence to suggest

that individuals with ASC show a reduced LVF bias when viewing faces that is not

dissimilar to RH lesioned patients. However, no research has yet explored whether this

CHAPTER ONE 21

atypical lateralization of attention in ASC extends to visual stimuli in general, or is

specific to faces.

Thesis overview

To broadly summarize the present thesis, I examine several aspects of attention

in individuals with high levels of autistic-like traits. To achieve this, I employ

established tasks, including the greyscales, landmark and mental number line tasks,

which have not previously been used in to explore how the distribution of attention

alters as a function of levels of autistic-like traits. Using these tasks, I provide novel

evidence of reduced RH activation during visual attentional tasks for individuals with

high levels of autistic-like traits compared to their low autistic trait counterparts.

Furthermore, I test whether pre-existing attentional differences in low and high

autistic-trait individuals can be reduced or eliminated using two different techniques

that are understood to increase RH activation – CPT training and anodal tDCS. As noted

earlier, observing the performance of neurotypical individuals differing in autistic-like

traits can be a useful method of conducting research in the field of autism, as generally

these individuals can be recruited in much larger numbers than individuals with

clinical ASC, and studies that use low-trait/high-trait comparisons often report findings

comparable to those gathered from neurotypical/ASC comparisons (Landry &

Chouinard, 2016). Thus, while the findings reported in this thesis for comparisons

between low and high autistic trait individuals are not conclusive of what might be

presented in a neurotypical/ASC comparison, the results can be considered highly

indicative of what might be found in such clinical studies, and certainly encourage

further investigation using clinical groups.

The present thesis has two distinct halves. In the first half (Chapters Two and

Three), I report two experiments that provide novel evidence for RH abnormalities in

individuals with high levels of autistic-like traits. In the second half (Chapters Four and

Five), I detail using two different techniques to selectively engage attentional

mechanisms in the RH and modify differences in biases in visuospatial attention

between individuals with low and high levels of autistic-like traits. In short, the thesis

provides novel evidence for reduced RH activation during attentional tasks for

individuals with relatively high levels of autistic-like traits, and examines methods that

may reduce ‘atypical’ hemispheric lateralization for these same individuals. The broad

aims and outcomes of each chapter are outlined below.


In the second chapter, a study is reported that shows a difference in attentional

bias on the greyscales task (Nicholls et al., 1999) for participants with low and high

levels of autistic-like traits. The greyscales task is similar to other previously described

lateralization tasks in that visuospatial biases are reflected in over-exaggeration of the

left or right side of a visual stimulus. In this task, participants are presented with two

horizontal bars that are evenly shaded, from the left, black-to-white for one, and white-

to-black for the other. While participants are tasked with selecting the bar they believe

is darker (i.e. has more black shading), a preference for selecting the bar that has the

black shading on the left side is indicative of pseudoneglect (i.e. exaggeration of

shading in the left visual field). The results of this study represent the first evidence

that pseudoneglect is reduced in individuals with high levels of autistic-like traits

relative to those with low trait levels, and as such, provide behavioural evidence of

reduced RH activation in high trait individuals for visuospatial attention.

This initial finding is followed up in the third chapter, which extends the results

to an alternative measure of physical spatial bias (landmark task) as well as examining

biases in representational space using the mental number line task. By using these

additional tasks, it could be determined whether the finding presented in Chapter Two

is specific to the greyscales task, and whether representational space, which does not

depend on the visual perceptual system, is also subject to reduced spatial bias in

individuals with high levels of autistic-like traits, consistent with a single underlying

mechanism for reduced physical and representational pseudoneglect. In line with the

findings in Chapter Two, high autistic trait participants showed a reduced bias on both

the greyscales and landmark tasks relative to low trait individuals, which provides

further evidence for reduced RH activation by high trait individuals for visuospatial

attention. In contrast, no group difference was found for the representational, mental

number line task, suggesting that the mechanism responsible for reduced leftward bias

in high trait individuals is tied to the visual perceptual system.

The fourth chapter moves from observation of attentional behaviours that are

consistent with RH impairments for individuals with high levels of autistic-like traits to

evaluating methods of modulating attention to reduce the differences between

individuals with low or high levels of autistic-like traits. Here, the continuous

performance task (CPT) described earlier is used as a training task. Recall that the CPT

is purported to activate regions in the RH associated with increases in global

preference in hierarchical Navon figures for neurotypical individuals (Van Vleet et al.,

2011) and with the reduction of left hemispatial neglect for individuals with RH

CHAPTER ONE 23

damage (Degutis & Van Vleet, 2010). In the study reported in Chapter Four, a short

period of CPT successfully increased the amount of attention individuals with high

levels of autistic-like traits directed to the global level of a hierarchical figure when

participants were instructed to observe the local aspects, but did not lead to a

complementary reduction in the degree of attention directed to local stimuli when

participants were instructed to attend to the global level. In summary, individuals with

high levels of autistic-like traits showed greater attendance to global arrangements of

stimuli following the CPT, which is a desirable result that encourages further exploring

the effectiveness of CPT in modifying patterns of attention to social stimuli that contain

global arrangements, such as faces.

In the fifth chapter, a second technique, tDCS, was used to influence RH

activation and the resulting distribution of visuospatial attention in individuals with

high and low levels of autistic-like traits on the greyscales task. It was predicted that

anodal tDCS would elicit the greatest increase in pseudoneglect for the High AQ group,

given their relatively lower baseline levels of LVF bias that were found in Chapters Two

and Three. Furthermore, it was expected that cathodal tDCS would show the greatest

reductions in pseudoneglect for the Low AQ group, given their relatively higher

baselines. While anodal tDCS did increase pseudoneglect for the High AQ group as

predicted, no increase was found for the Low AQ group, and cathodal tDCS failed to

reduce pseudoneglect for either group. However, considering that the primary focus of

the study was to increase pseudoneglect through stimulation of the right PPC for the

High AQ group, these results are particularly encouraging, and warrant further

investigation with a clinical autistic sample.

The final, sixth chapter consists of the General Discussion, which summarizes

the findings of this thesis and integrates them into the current literature. I examine

how the findings presented in Chapter Two and Three provide novel evidence of

reduced RH activation for visuospatial attention in individuals with high levels of

autistic-like traits, and speculate on what regions and pathways specific to the visual

perceptual system might be implicated, given that normal biases were found for a non-

perceptual, mental representation of space. Furthermore, I explore how the

modulations of attention examined in Chapters Four and Five may be used in the

context of intervention to potentially improve attentional outcomes for people with

ASC.


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CHAPTER TWO Individuals with autistic-like traits show reduced lateralization on a greyscales task Michael C.W. English, Murray T. Maybery and Troy A.W. Visser

Individuals with autism spectrum conditions attend less to the left side of

centrally presented face stimuli compared to neurotypical individuals,

suggesting a reduction in right hemisphere activation. To determine if this

difference extends to non-facial stimuli, we measured spatial attention bias

using the “greyscales” task in a large sample of neurotypical adults rated above-

or below-average on the Autism Spectrum Quotient. The typical leftward bias in

the below-average group was significantly reduced in the above-average group.

Furthermore, a negative correlation between leftward bias and the Social Skills

factor of the Autism Spectrum Quotient provides additional evidence for a link

between atypical hemispheric activation and social difficulties in autism

spectrum conditions that extends to non-facial stimuli and the broader autistic

phenotype.

Individuals with autism spectrum conditions (ASC) display many atypical behaviours

ranging from impaired social interactions to superior visuospatial skills (Shah & Frith,

1983; Soulières, Zeffiro, Girard, & Mottron, 2011). Evidence from pragmatic language

46 INDIVIDUALS WITH HIGH AQ SHOW REDUCED LATERALIZATION ON A GREYSCALES TASK

measures (Ozonoff & Miller, 1996; Siegal, Carrington, & Radel, 1996), functional and

structural magnetic resonance imaging (Di Martino et al., 2011; Jou, Minshew,

Keshavan, Vitale, & Hardan, 2010) and electrophysiological recordings (Lazarev,

Pontes, & DeAzevedo, 2009; Orekhova et al., 2009) suggests that right hemisphere

(RH) abnormalities may contribute to the development of these behaviours. Here, we

investigate whether high levels of autistic-like traits are also linked to RH-based

atypical biases in spatial attention.

There is substantial evidence that spatial attention is RH lateralized (Davidson

& Hugdahl, 1996; Hellige, 1993). Stroke patients with RH parietal lesions preferentially

focus on stimuli in the right visual field and ignore those in the left (Adair & Barrett,

2008; Bartolomeo, 2007), while neurotypical participants display the opposite pattern.

Neurotypical individuals bisect lines to the left of veridical centre (pseudoneglect;

Bowers & Heilman, 1980) and on the “greyscales” task (Mattingley, Bradshaw,

Nettleton, & Bradshaw, 1994), in which observers must choose the “darker” of two

symmetrical, equiluminant bars (see Figure 2.1), they preferentially choose the bar

that is darker on the left (Dellatolas, Coutin, & De Agostini, 1996; Jewell & McCourt,

2001). Both of these tasks elicit a reliable left visual field (LVF) bias and are suggestive

of relatively greater involvement of the RH in spatial attention (Nicholls, Bradshaw, &

Mattingley, 1999).

With respect to autism, findings on lateralization of attention are mixed. Adults

with ASC show reduced fixation-time to the LVF when viewing centrally-presented

faces (Dundas, Best, Minshew, & Strauss, 2012) and reduced LVF bias for identification

of chimeric faces (Ashwin, Wheelwright, & Baron-Cohen, 2005) compared to

neurotypical adults. However, on a numerosity-judgement task with non-facial stimuli,

adults with ASC show a LVF bias while neurotypical adults do not (Ashwin et al., 2005).

Finally, in children, Rinehart, Bradshaw, Brereton and Tonge (2002) reported that

neither a neurotypical group nor a group of children with ASC showed LVF biases1. The

significant disparities across these studies could reflect a unique role for face-specific

mechanisms or differences between adults and children. Alternatively, inconsistencies

might also stem from relatively small samples or insufficient trial numbers to obtain

reliable measures of spatial biases (Nicholls et al., 1999). Finally, task engagement

could not be monitored in Ashwin et al. (2005) and Rinehart et al. (2002) because

1 Rinehart et al. (2002) used non-face, greyscales stimuli, highly similar to those used in the present study.

CHAPTER TWO 47

stimuli in the greyscales task were actually equiluminant, making it impossible to

measure performance accuracy.

To address these issues, we modified the greyscales task (Nicholls et al., 1999)

by increasing the number of trials and introducing a luminance difference between the

bars on each trial. This allowed us to screen for task engagement, as well as estimate

spatial attention bias. We also tested a large sample of adults classified as having low

versus high levels of autistic-like traits using the Autism Spectrum Quotient

questionnaire (AQ; Baron-Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001). Past

research suggests there is a broader autism phenotype that extends into the ‘normal’

population (Baron-Cohen & Hammer, 1997; Baron-Cohen et al., 2001; Bishop et al.,

2004), with non-clinical individuals exhibiting high levels of autistic-like traits, as

measured by the AQ, showing similar behavioural patterns to ASC-diagnosed

individuals (Bayliss & Kritikos, 2011; Grinter, Van Beek, Maybery, & Badcock, 2009;

Grinter, Maybery, et al., 2009; Rhodes, Jeffery, Taylor, & Ewing, 2013; Russell-Smith,

Maybery, Bayliss, & Sng, 2012; Sutherland & Crewther, 2010). It should be noted that

some researchers have raised potential issues with the use of AQ-selected groups as

proxies for ASC (Gregory & Plaisted-Grant, 2013), arguing that differences between

low- and high-trait groups might be driven by undiagnosed individuals with an ASC

phenotype who are included in the high-trait group. We addressed this issue in two

ways; first, by testing a particularly large sample of participants and, second, by

comparing the entire distribution rather than just the extreme ends. Thus, while

undiagnosed individuals with ASC may form part of the high-trait group, they are

unlikely to be the sole source of group differences.

Methods

Participants

Participants were 332 right-handed psychology students (93 male; mean age

21.27) at the University of Western Australia.

Materials and Procedure

Questionnaires

The AQ (Baron-Cohen et al., 2001) is a 50-item self-report questionnaire

assessing autistic-like traits and behaviours in neurotypical individuals (items scored

1-4 using Austin's (2005) method; higher scores indicate more autistic-like traits). We


calculated scores for overall AQ and three subfactors: Social Skills, Details/Patterns,

and Communication/Mind Reading (Hurst, Mitchell, Kimbrel, Kwapil, & Nelson-Gray,

2007; Russell-Smith, Maybery, & Bayliss, 2011). Handedness was assessed using the

Edinburgh Handedness Inventory (Oldfield, 1971).

Greyscales Task

Stimuli were presented using custom software2, following the procedure of

Nicholls et al. (1999). Participants were seated approximately 50cm from a monitor

(HP L2245wg) and instructed to keep their eyes fixated on the display’s centre. Two

horizontal bars were presented above and below the display centre on each trial

(Figure 2.1) for 5000ms with one bar slightly darker than the other (200px difference).

Participants identified the darker bar by pressing the ‘T’ or ‘B’ key to indicate the top or

bottom bar respectively. A trial terminated when a response was made. This meant

that a response could still be recorded when the bars had been removed, although

participants were encouraged to respond while the bars were present. The next trials

began 1500ms after the previous response. Participants completed 96 trials, with the

darker bar appearing equally often on the top or bottom and shaded darker to the left

or right.

Figure 2.1. Example of a stimulus presented in the greyscales task.

2 Stimuli were generated, presented and participant responses recorded using custom software developed by Young Ho Kim, Mike Nicholls and Jason Mattingley downloaded from Mike Nicholls’ website accessible here: http://flinders.edu.au/sabs/psychology/research/labs/brain-and-cognition-laboratory/the-greyscales-task.cfm.

http://flinders.edu.au/sabs/psychology/research/labs/brain-and-cognition-laboratory/the-greyscales-task.cfm

http://flinders.edu.au/sabs/psychology/research/labs/brain-and-cognition-laboratory/the-greyscales-task.cfm

CHAPTER TWO 49

Results

Task accuracy was calculated as the percentage of correct identifications of the

darker bar on trials with response times from 200-5000ms. Participants with ≤50%

accuracy (n=43) or with more than 1/3 of trials with response times ≤200ms or

≥5000ms (n=16) were excluded from the analysis (four participants met both

exclusion criteria)3. The remaining 277 participants were divided into Low and High

AQ groups based on a median split of AQ scores (Median=105; see Table 2.1 for

descriptive statistics).

Table 2.1. Characteristics of AQ comparison groups.

Low AQ High AQ

N 135 (26 male) 142 (44 male)

AQ Age (years) AQ Age (years)

Mean 94.90 22.56 115.14 20.33

Median 97 19 113 19

Std. Deviation 7.84 8.50 8.56 3.64

Range 36 (68-104) 39 (18-57)* 44 (105-149) 18 (18-36)

* The 12 oldest participants in the study were all classified as Low AQ, explaining the

larger range and standard deviation for this group relative to the High AQ group. As

subsequent analyses showed the same pattern of results regardless of the inclusion or

exclusion of these participants, they were not removed from the dataset.

As summarized in Figure 2.2, mean accuracy for both groups was significantly

better than chance (both groups: p<.001, both d’s>2.54), and did not differ between

groups (p=.92, d=.01). Spatial bias was measured as the percentage of responses

indicating the bar shaded to the left was darker, irrespective of actual luminance. Both

groups made significantly more leftward responses than expected by chance (Low AQ:

p<.001, d=1.21; High AQ: p=.005, d=.48), indicating a leftward spatial attention bias. A

between-groups analysis of variance was conducted to determine if there was a

difference in spatial bias between AQ groups. As the High AQ showed a significantly

3 These exclusion criteria were used in order to exclude participants that were clearly not appropriately engaging with the task and the usable portion of their data could not be relied upon to be a true indicator of their attentional biases. For those interested, removing the accuracy criterion results in weaker, but still significant difference between the AQ groups (p = 0.03), and increasing the minimum accuracy to 60% also results in a slightly weaker, but still significant, difference between the AQ groups (p = 0.01).


greater proportion of male participants compared to the Low AQ group following a chi-

square test, χ2(1)=5.04, p=.03, participant sex was entered as a between-groups factor

in addition to AQ group in a 2x2 mixed-design analysis of variance. This analysis

yielded a main effect of AQ group, F(3,273)=5.28, p=.02, ηp2 = .02, with the Low AQ

group reporting a significantly greater proportion of leftward responses compared to

the High AQ group (see Figure 2.2). However, neither the main effect of sex nor the

interaction was significant (all p’s>.18, all ηp2<.01).

Figure 2.2. Proportion of trials in which participants correctly selected the darker bar,

and the bar with the black end oriented towards the left-side of the screen. Error bars

represent SEM.

Table 2.2 summarizes correlations between spatial bias and scores for the AQ

and its subfactors. As sex was not reported to significantly influence bias in the prior

analysis, data for the two sexes were combined for the correlational analysis. As can be

seen in Table 2.2, the negative correlation between spatial bias and overall AQ

approached significance, while the negative correlation of spatial bias with the Social

Skills subfactor was significant (illustrated in Figure 2.3)4.

4 It should be acknowledged that the correlation between Social Skills and Spatial Bias, though statistically significant, is relatively weak and may represent a Type I error. However, it is

50%

52%

54%

56%

58%

60%

62%

64%

Correct Responses 'Left' Responses

Low-AQ

High-AQ

CHAPTER TWO 51

Table 2.2. Correlation of spatial bias with AQ total and subfactor scores. Numbers in

parentheses indicate significance levels.

Overall AQ Social Skills

Details /

Patterns

Communication

/ Mindreading

Spatial Bias /

Left

Responses

-.09 -.12* -.01 -.04

(.12) (.046) (.90) (.56)

Figure 2.3. Scatterplot illustrating the distribution of spatial bias scores against the

AQ: social skills subfactor.

Finally, to address concerns raised by Gregory and Plaisted-Grant (2013), we

re-conducted all analyses whilst excluding participants whose AQ was above the 95th

percentile (AQ>127, 12 participants). This was designed to omit participants most

critical to note that it is in the expected direction, and is supported by previous work indicating that attentional differences between Low and High AQ groups is linked to this subfactor, and not the others (Russell-Smith et al. 2012), and thus we believe this is not a chance effect.


likely to be those with undiagnosed ASC. The resulting analyses yielded the identical

pattern of results5.

Discussion

The present results reveal a robust LVF bias of spatial attention that is

significantly reduced in High- compared to Low AQ observers. This echoes previous

results using facial stimuli (Ashwin et al., 2005; Dundas et al., 2012), and provides

additional evidence that ASC is linked to RH abnormalities. These findings also extend

this previous work by showing a reduced LVF bias with non-facial stimuli for

individuals with high levels of autistic traits. In contrast to Ashwin et al. (2005) and

Rinehart et al. (2002) who did not find a LVF bias in their control groups, we

demonstrate the typical LVF bias in our Low AQ group in conjunction with reduced LVF

bias for the High AQ group. In the case of Rinehart et al. (2002), this discrepancy is

unlikely to reflect effects of maturation, as previous studies have found robust LVF

biases in children (Dellatolas et al., 1996) and a decline in bias with age (Jewell &

McCourt, 2000). Instead, we suggest that differences stem from our larger sample,

additional trials, and screening to ensure task engagement.

Our correlation analysis revealed a somewhat surprising association between

LVF bias and the Social Skills subfactor of the AQ, where greater levels of social

difficulty were associated with reduced LVF bias. A similar pattern of results has been

previously reported by Russell-Smith et al. (2012), who found that high scores on the

Social Skills subfactor (greater social difficulty) were associated with superior

performance on the Embedded Figures Test (EFT; Witkin, 1971) in which participants

must locate a simple geometric shape within a relatively complex scene. Like the

greyscales task, High- and Low AQ groups perform differently on the EFT, with High AQ

participants typically outperforming Low AQ participants (Almeida, Dickinson,

Maybery, Badcock, & Badcock, 2010a, 2010b, 2013; Grinter, Van Beek, et al., 2009;

Grinter, Maybery, et al., 2009). The performance difference between groups is thought

to stem from High AQ individuals difficulties with global integration (Grinter, Maybery,

et al., 2009), a processing style that interferes with the task objective in the EFT by

drawing attention away from the details of a scene and instead towards the holistic

picture. Importantly, global processing is closely associated with activation of RH

regions (Fink et al., 1996; Heinze, Hinrichs, Scholz, Burchert, & Mangun, 1998; Hübner

5 Results of the independent samples t-test between AQ groups on spatial bias: t(263) = 3.00, p < 0.01, d = 0.37. Results of the correlation analysis between spatial bias and: AQ, r = -0.12, p = 0.05; AQ: Social Skills, r = -0.15, p = 0.02; other subfactors, n.s.

CHAPTER TWO 53

& Studer, 2009; Martinez et al., 1997) and thus High AQ individuals’ superior

performance on the EFT can be interpreted as further evidence of relatively reduced

activation in the RH.

Interestingly, both our study and Russell-Smith et al. (2012) found associations

between task performance and the Social Skills subfactor of the AQ, but not the

arguably more visuo-spatially oriented Details/Patterns subfactor. Furthermore,

similar patterns of behaviour have been reported between other social constructs and

visuo-spatial performance (Baron-Cohen & Hammer, 1997; Jarrold, Butler, Cottington,

& Jimenez, 2000; Pellicano, Maybery, Durkin, & Maley, 2006). However, the underlying

cause of this relationship remains unclear. One potential explanation is that face

processing, a skill that is particularly important in regard to processing non-verbal,

social information (Speer, Cook, McMahon, & Clark, 2007), is associated with increased

activation in RH regions, especially in the right fusiform gyrus (Clark et al., 1996; Haxby

et al., 1994, 1999; Kanwisher, McDermott, & Chun, 1997; McCarthy, Puce, Gore, &

Allison, 1997; Rossion, Schiltz, & Crommelinck, 2003; Sergent, Ohta, & Macdonald,

1992). If LVF bias, as measured by the greyscales task, is related to levels of relative RH

activity for spatial attention, then it is possible that a relative reduction in LVF bias may

also be associated with a reduction in activation of face processing regions and, by

extension, potentially-elevated levels of social difficulty. Of course, this explanation is

necessarily speculative and awaits further empirical testing.

On a final note, it is important to consider what brain regions might account for

the reduced LVF bias in High AQ observers. Loftus and Nicholls (2012) hypothesized

that pseudoneglect in neurotypical samples is due to asymmetrical activation of the

left-and-right posterior parietal cortices (PPC), with the right PPC more active than left

PPC. Consistent with this, they abolished pseudoneglect in neurotypical individuals by

administering anodal (excitatory) transcranial direct current to the left PPC. On this

account, we can conjecture that reduced leftward spatial bias in our High AQ group

reflects more balanced reliance on left and right PPCs. Further research will be

required to test this possibility and whether it reflects greater activation in left PPC or

reduced activation in right PPC. Finally, it is also critical to replicate the present

findings in samples with ASC in order to ensure the broad applicability of our results.


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CHAPTER THREE Reduced pseudoneglect for physical space, but not mental representations of space, for adults with autistic traits Michael C.W. English, Murray T. Maybery, Troy A.W. Visser

Neurotypical individuals display a leftward attentional bias, called

pseudoneglect, for physical space (e.g. landmark task) and mental

representations of space (e.g. mental number line bisection). However,

pseudoneglect is reduced in autistic individuals viewing faces, and neurotypical

individuals with autistic traits viewing ‘greyscale’ stimuli, suggesting attention

is atypically lateralized in autism. We investigated whether representational

pseudoneglect for individuals with autistic traits is also reduced by comparing

biases on greyscales, landmark, and mental number line tasks. We found that

pseudoneglect was intact only on the representational measure (i.e. the mental

number line task), suggesting that mechanisms for atypical lateralization of

attention in individuals with autistic traits are specific to processing of physical,

visual stimuli.

Asymmetries in brain hemispheric activation have been shown to have contralateral

effects on visual attention. Stroke patients with right hemisphere (RH) lesions, for

example, focus on stimuli presented primarily in the right visual field (RVF) and ignore

60 REDUCED PSEUDONEGLECT FOR PHYSICAL, NOT REPRESENTATIONAL, SPACE IN HIGH AQ ADULTS

(neglect) stimuli presented in the left (Adair & Barrett, 2008; Bartolomeo, 2007;

Heilman, Watson, & Valenstein, 2003). When completing line bisection tasks (which

require indicating the centre of a line) or landmark tasks (which require indicating

whether a pre-bisected line has been divided to the left or right of centre), these

patients reliably respond as if the centre of the line was towards the right of the true

centre.

Interesting, while neglect of the left visual field (LVF) is a common outcome

following RH damage, neglect of the RVF following left hemisphere (LH) damage, is

relatively less common (Stone, Halligan, & Greenwood, 1993). Increased connectivity

within the RH, as well as inter-hemispheric connectivity that favours information

transfer in a right-to-left direction (Siman-Tov et al., 2007), are factors that underlie

the widely accepted theory that the RH controls spatial attention for both LVF and RVF,

while the LH controls spatial attention only for the RVF (Mesulam, 1981) explaining

why neglect of the RVF following LH damage is relatively rare, whereas neglect of the

LVF following RH damage is common.

The relatively greater lateralization of spatial attention to the RH (Heilman,

1995; Hellige, 1993) can also be used to account for the fact that healthy individuals

show a leftward attentional bias (pseudoneglect; Bowers & Heilman, 1980). A side

effect of the RH lateralization for spatial attention is that stimulus properties in the

‘dominant’ LVF are exaggerated relative to stimulus properties presented in the other

visual field. As a result, neurotypical individuals bisect lines slightly left of the line’s

true centre (Jewell & McCourt, 2000) and show leftward attentional biases when

making stimulus judgements on the basis of brightness and numerosity (Nicholls,

Bradshaw, & Mattingley, 1999).

Atypical lateralization of spatial attention is not unique to neglect patients with

RH lesions. It has also been linked to autism spectrum conditions (ASC), neuro-

developmental conditions primarily characterized by deficits in social communication

and restricted and repetitive behavioural patterns (American Psychiatric Association,

2013). Relative to neurotypical participants, ASC adults usually show a reduced LVF

bias in target detection tasks (Wainwright & Bryson, 1996) and chimeric face

identification (Ashwin, Wheelwright, & Baron-Cohen, 2005), as well as reduced fixation

time to the left side of centrally presented faces (Dundas, Best, Minshew, & Strauss,

2012). Recently, we also reported that neurotypical individuals with relatively high

scores on the Autism Spectrum Quotient (AQ; a measure of autistic-like traits in

neurotypical participants; Baron-Cohen, Wheelwright, Skinner, Martin, & Clubley,

CHAPTER THREE 61

2001) show reduced pseudoneglect on the greyscales task (see a description and

example stimuli below) compared to participants with lower AQ scores (Chapter Two

of the present thesis, henceforth: English, Maybery, & Visser, 2015). This was the first

study to suggest that attenuations of attentional bias found in ASC are also present in a

high autistic-trait subset of the neurotypical population and adds to the growing

literature indicating that attentional characteristics associated with ASC also manifest

to a lesser degree in individuals with higher scores on measures of autistic-like traits

(Bayliss & Kritikos, 2011; Grinter, Van Beek, Maybery, & Badcock, 2009; Grinter,

Maybery, et al., 2009; Rhodes, Jeffery, Taylor, & Ewing, 2013; Russell-Smith, Maybery,

Bayliss, & Sng, 2012; Sutherland & Crewther, 2010; for a meta-analysis, see Cribb,

Olaithe, Di Lorenzo, Dunlop, & Maybery, 2016).

The present work examines whether spatially-organized mental

representations, like the mental number line, are also altered in individuals with high

levels of autistic-like traits. Figure 3.1 is a conceptual illustration of such a number line,

which is described as a series of numbers located on a horizontal azimuth numerically

ascending left-to-right (Brooks, Sala, & Darling, 2014). Past research has shown that

when patients with neglect are asked to make a judgement regarding numerical

distance, their responses are indicative of a rightward bias on the number line (e.g.

selecting ‘7’ as the midpoint between ‘1-9’), with the size of the bias corresponding

with neglect severity (Hoeckner et al., 2008; Loftus, Nicholls, Mattingley, Chapman, &

Bradshaw, 2009; Zorzi, Priftis, & Umiltà, 2002). Conversely, neurotypical participants

show a small bias in favour of a lower, more ‘leftward’ number (Göbel, Calabria, Farnè,

& Rossetti, 2006; Loftus et al., 2009; Longo & Lourenco, 2007; Nicholls & McIlroy,

2010). This representational pseudoneglect is hypothesised to occur due to

exaggeration of the distance between numbers that are more “leftward” on the number

line (similar to the exaggeration of leftward stimulus features of visual stimuli that

results in pseudoneglect), shifting perceptions for the relative location of the central

point (Loftus et al., 2009).


Figure 3.1. A simplified demonstration of how leftward representational bias occurs in

the mental number line. It is suggested that individuals exaggerate the distance

between numbers that are further “left” on the mental number line. Thus, when an

individual is asked to identify the midpoint of the range 0-10, ‘four’ is erroneously

reported more often than the correct answer of ‘five’.

While both visual and representational stimuli yield leftward biases in

neurotypical participants, it is unclear whether both types of bias would also disappear

for individuals high in autistic traits. This is partly because evidence from past studies

is equivocal with respect to the question of whether these two types of leftward bias

have a common neural substrate. One candidate region for such a substrate is the

posterior parietal cortex (PPC) due to its involvement in many aspects of visual

attention, including visual working memory (Berryhill & Olson, 2008), sustained

attention (Husain & Nachev, 2007; Malhotra, Coulthard, & Husain, 2009), visually

guided reaching (Clower et al., 1996), initiating visually guided saccades (Luna et al.,

1998), and activating mentalised spatial maps (Maguire et al., 1998). Non-invasive

stimulation of this region has been effective at modulating spatial biases. For example,

applying transcranial magnetic stimulation (TMS) to the right PPC of healthy

individuals temporarily disrupts its activation (Fierro, Brighina, Piazza, Oliveri, &

Bisiach, 2001), producing a rightward shift in the perceived midpoint of horizontal

lines similar to that shown by individuals with RH damage (Bjoertomt, Cowey, & Walsh,

2002). Likewise, applying TMS over the same region in healthy individuals produces a

rightward shift for representational space in the mental number line task, similar to

those observed in individuals with right parietal damage (Göbel et al., 2006).

Additionally, Loftus and Nicholls (2012) found that pseudoneglect, measured with the

luminance-judgement greyscales task (Nicholls et al., 1999), could be eliminated

following anodal transcranial direct current stimulation (tDCS) over the left parietal

cortex. Anodal tDCS is thought to increase neural excitation of the stimulated site and,

in line with Siman-Tov et al.'s (2007) activation-orientation model of attention, likely

caused a relative increase in the excitation of neurons in the left parietal cortex which

CHAPTER THREE 63

rebalanced the asymmetry in activation between the left and right hemispheres that

drives pseudoneglect.

While such studies provide evidence in favour of the notion that PPC activity

underlies both physical and representational pseudoneglect, and while several studies

have found evidence for both physical and representational biases in the same

individuals (Aiello et al., 2012; Longo, Trippier, Vagnoni, & Lourenco, 2015; Rotondaro,

Merola, Aiello, Pinto, & Doricchi, 2015; Vuilleumier, Ortigue, & Brugger, 2004; Zorzi,

Priftis, Meneghello, Marenzi, & Umiltà, 2006; Zorzi et al., 2002), other researchers have

found that neglect patients who show rightward deviations on physical spatial tasks do

not necessarily have similar biases in the representational medium, suggesting that at

least partially disparate underlying networks are at play (Aiello et al., 2012; Doricchi,

Guariglia, Gasparini, & Tomaiuolo, 2005; Loetscher & Brugger, 2009; Loetscher,

Nicholls, Towse, Bradshaw, & Brugger, 2010; Pia et al., 2012; Rossetti et al., 2011;

Storer & Demeyere, 2014; van Dijck, Gevers, Lafosse, & Fias, 2012). Moreover, there are

many fundamental differences between physical and representational stimuli that

might result in the recruitment of different neural mechanisms during processing. For

one, physical representations, like those in the line bisection and greyscale tasks, act as

visual cues that directly influence attentional processes, whereas few or no such cues

are present in the representational mental number line. Second, the differences could

reflect visual complexity of the task, with complexity generally greater in the visual

than representational tasks. Finally, it is possible that task relevant information, such

as letter or number identity in representational tasks, may recruit disparate brain

areas such as those responsible for memory and language that are unlikely to play a

role in processing abstract visual stimuli such as a line or greyscales stimulus.

The results of this study are therefore important for two reasons. First, they

address the issue of whether pseudoneglect for mental representations of space is

reduced in individuals with high levels of autistic-traits in line with the previously

reported reduced pseudoneglect for physical space (English et al., 2015). Second, they

provide new evidence concerning the question of whether common or disparate

mechanisms underlie physical and representational pseudoneglect. To whit, if

variations in levels of autistic-like traits produce similar variations in physical and

representational pseudoneglect, this would provide suggestive evidence for common

neural mechanisms. On the other hand, if pseudoneglect is not reduced for mental

representations of space in individuals with high autistic-trait levels, it would suggest

different mechanisms underlie spatial biases in physical and mental representations.


To address these questions, in the present work, we recruited two groups of

neurotypical individuals with scores in the upper or lower third of the AQ distribution

and measured their response biases on three tasks: greyscales and landmark tasks to

observe levels of physical pseudoneglect, and a number line bisection task to observe

levels of representational pseudoneglect. In line with our earlier findings, we expected

reduced levels of physical pseudoneglect in our High AQ group compared to our Low

AQ group (English et al., 2015) on both the greyscales and landmark tasks. This would

be a replication and a critical extension of this earlier result to a novel task (landmark).

If representational pseudoneglect is also attenuated in the group with higher AQ

scores, this would provide the first evidence that autistic traits modulate not only

physical but also representational pseudoneglect. This result would also provide

evidence for a shared mechanism responsible for attenuation of both forms of

pseudoneglect in High AQ (and possibly ASC) individuals.

Methods

Participants

Participants were 104 right-handed students recruited at the University of

Western Australia1. Students with AQ scores in the upper or lower third of a cohort

that had completed the AQ in a previous screening procedure were invited to

participate2. Fifty-two participants were in each of the Low and High AQ groups.

Materials

All tasks and questionnaires were presented using Presentation software

(Version 17.0, Neurobehavioral Systems) running on HP EliteOne 800 machines using

23” LCD monitors. Participants were seated approximately 50cm from the display.

Greyscales Task

Stimuli for this task were based on those previously used by English et al.

(2015). Each trial consisted of two horizontal bars presented above and below the

centre of the display. The shading on each bar changed from white to black

incrementally along the horizontal azimuth, with one bar effectively being a reversal of

the other (see Figure 3.2). The orientations of the bars were randomized such that the

bar that was shaded from the left, white-to-black, was presented equally often as the

1 No participant recruited for the study in the previous chapter participated in this study. 2 This recruitment method differs from that used in the previous chapter in order to maximise individual differences between the AQ groups, and allow for the study to be conducted with fewer participants.

CHAPTER THREE 65

top or bottom positioned bar. Finally, one bar from each pair was shaded ‘darker’ than

the other bar in the pair. This was achieved by randomly replacing 100 white pixels

with black pixels on one bar, and conversely randomly replacing 100 black pixels with

white pixels on the other bar, creating a 200-pixel difference in shading between the

two bars.

Figure 3.2. An example of a stimulus presented in the greyscales task.

Each trial began with a fixation cross that was displayed for 1500ms before the

greyscales stimuli were presented. Participants were instructed to observe the two

bars and determine which of the two was ‘darker’ overall. Participants made their

responses using the [T] and [B] keys on a standard keyboard, which indicated the “top”

or “bottom” bar respectively. Participants had 5s from the onset of the greyscales

stimuli to respond before the trial automatically ended. The task consisted of 120 trials,

with factorial combinations of stimulus characteristics such as length and orientation

counterbalanced across trials.

Landmark Task

Stimuli for this task consisted of a white horizontal bar presented in the centre

of the display on a black background. The bar was 481 pixels (px) long and 10px wide

and was bisected by a thin 1px wide, black vertical line. The position of the black line

was randomized across trials, appearing in the true centre or 10px, 5px, 3px, 2px, or

1px to the left or right of centre. A mask consisting of randomly placed black and white

pixels (white noise) covered the screen region occupied by the horizontal bar at the

beginning of each trial for 1s to prevent participants from using positions on the

previous trial when making judgements on the current trial.


Each trial began with a mask that was presented for 1000ms before the

landmark stimuli appeared. Participants were required to decide whether the black

vertical line on the white bar was closer to the left or right-hand side of the bar.

Participants responded using the [Z] and [/] keyboard keys to indicate that they

perceived the bisection to be closer towards the left or right-side of the bar

respectively. Participants had 5s from the onset of the landmark stimuli within which

to respond before the trial automatically ended. The task consisted of 132 trials, with

the bisection appearing at all 11 locations an equal number of times.

Number Line Task

Stimuli for this task were adapted from those used by Loftus et al. (2009). Each

trial consisted of a triplet of two-digit numbers, arranged with a central red number

and two yellow flankers. There were four sets of flankers (19_55, 53_99, 42_98 and

17_83). For each set, the red central number was always shifted 1, 2 or 5 greater or less

than the actual midpoint between the two flankers (e.g. for the flanker pair ‘19_55’, the

midpoint is 37 and the possible central numbers included 32, 35, 36, 38, 39 and 42).

Whereas the Loftus et al. study presented all numbers on the same horizontal plane, we

used a diagonal configuration so that participants made top/bottom judgements

instead of left/right judgements (see Figure 3.3). This change was made to dissociate

participants’ potential bias towards making left/right key presses from left/right

spatial judgements. Number triplets varied factorially with respect to the spatial

configuration (spatially ascending/descending (SA/SD), left-to-right) and direction of

numerical sequence (numerically ascending/descending (NA/ND), left-to-right). This

resulted in four different possible spatial configurations for each triplet configuration

(SA-NA, SA-ND, SD-NA, and SD-ND), four sets of flankers, and six possible values for the

central number, yielding a total of 96 unique trials presented without repetition.

CHAPTER THREE 67

Figure 3.3. Examples of the four spatial configurations, with each configuration

containing one, red central number and two, yellow flanker numbers. Left-to-right; A)

spatially ascending, numerically ascending, B) spatially ascending, numerically

descending, C) spatially descending, numerically ascending, D) spatially descending,

numerically descending.

Each trial began with a centrally presented fixation cross which was displayed

for 1000ms, after which a number triplet was presented. Participants were required to

determine if the top flanker or bottom flanker in a triplet was closer in numerical

position to the red central number. Participants were instructed to not use arithmetic

when making their judgements. Triplets were presented for 3s, after which the triplet

disappeared and participants had 2s to make a response using the [T] and [B] keys to

indicate “top” and “bottom” respectively. If a participant did not make a response

within 2s, the trial automatically ended.

Questionnaires

Levels of autistic-like traits were measured using the Autism Spectrum

Quotient (Baron-Cohen et al., 2001), a 50-item self-report questionnaire that assesses

autistic-like traits and behaviours in neurotypical individuals. Items were scored 1-4

using Austin's (2005) method; higher scores represent more autistic-like traits. This

scoring method was used rather than the 0-1 method reported by Baron-Cohen et al.

(2001) to take advantage of the range of potentially useful information in each item

and increase the variability of total AQ scores. Handedness was assessed using the

Edinburgh Handedness Inventory (Oldfield, 1971).

Procedure

Testing took place in a single session that typically lasted 45-60 minutes. At the

beginning of the testing session, participants were given a verbal overview of the


testing procedure, including the general nature of the tasks and questionnaire involved.

Participants then completed the three computer tasks in counterbalanced order across

participants. Task-specific instructions were presented to participants on the computer

immediately prior to commencing the relevant task. Participants also had the

opportunity to complete a number of practice trials at the beginning of each task and

take short breaks between tasks.

Results

Descriptive statistics for the High and Low AQ groups are presented in Table

3.1. Participant performance was screened for outliers on each of the tasks; if a

participant was deemed an outlier for a particular task, their data were excluded from

analysis only on that task. No participant was identified as an outlier on more than one

task. We conducted one-tail t-tests on the differences in bias between AQ groups

because we predicted a reduced left bias in our High AQ group given prior results

(English et al., 2015).

Table 3.1. Characteristics of the Low AQ and High AQ comparison groups (standard

deviations in parentheses).

Low AQ (n = 52, 5 male) High AQ (n = 52, 18 male)

Mean Age (years) 20.63 (4.25) 20.06 (3.39)

Mean AQ 89.60 (7.15) 120.04 (6.31)

The two groups did not differ in mean age (p = 0.45, r = 0.07), but the Low AQ

group contained fewer male participants than the High AQ group [χ2(1) = 9.43, p <

0.01], and so we conducted preliminary versions of the analyses reported below with

sex as an additional between-subjects variable to AQ group. No main effects of sex or

interactions between sex and AQ group were reported on any of the task bias measures

(all p’s > 0.13, all ηp2 < 0.02). Given this result, we chose to conduct the following

analyses without sex entered as a factor in order to focus on effects related to AQ

group.

Data Screening

Several criteria were implemented to ensure that the participants were

appropriately engaged with the tasks. Participants were excluded from the analysis of a

single task if: A) more than a third of responses were faster than 200ms (indicating the

CHAPTER THREE 69

participant was anticipating target onset), B) the proportion of top/bottom key presses

exceeded the group mean by more than 2.5 standard deviations for the greyscales or

mental number line task or C) the proportion of left/right key presses for the two most

extreme (and easiest) bisection locations exceeded the group mean by more than 2.5

standard deviations for the landmark task. One participant met criteria A (landmark

task, High AQ), five participants met criteria B (greyscales task, two Low AQ; mental

number line task, one Low AQ and two High AQ) and seven participants met criteria C

(three Low AQ, four High AQ). No participant met exclusion criteria for multiple tasks.

Finally, one participant was excluded from all analyses because her accuracy on

multiple tasks was less than 50% (Low AQ), and one participant was excluded from the

analysis for the mental number line task due to a data recording error (Low AQ).

Greyscales Task

Task accuracy was calculated as the percentage of trials for which a participant

correctly identified the darker of the two bars on a given trial. Leftward spatial bias

was calculated as the percentage of trials for which a participant selected the bar which

had the darker end oriented towards the left-side of the screen (e.g. the top bar in

Figure 3.2), regardless of whether the selected bar was actually darker overall.

Accuracy for both the Low AQ (M=56.23%, SD=6.17%) and High AQ group

(M=57.38%. SD=6.66%) was above chance level (largest p < 0.001, smallest r = 0.71),

with no difference between the groups (p = 0.37, r = 0.09). Pseudoneglect (leftward

spatial bias) differed significantly from chance levels for the Low AQ group

(M=60.38%, SD=18.23%; p < 0.001, r = 0.50), but only marginally so for the High AQ

group (M=54.54%, SD=16.32%; p = 0.05, r = 0.27). Additionally, pseudoneglect was

significantly higher in the Low AQ group relative to the High AQ group, t(99) = 1.70, p =

0.046, r = 0.17. Taken together, these results show an even stronger pattern than in our

previous work (English et al., 2015), with only marginal levels of pseudoneglect seen in

the High AQ group and a larger difference in pseudoneglect between AQ groups (r=0.17

vs. r=0.12 in our previous study). Greater levels of autistic traits in the High AQ group

of the present study relative to the previous study might account for this stronger

pattern (High AQ: M = 120.04 vs M = 115.14; see Cribb et al. (2016) for simulations of

the influence of AQ group separation).

Landmark Task

Task accuracy was calculated as the percentage of trials on which participants

correctly identified the location of the bisection towards the left or right side of the


horizontal bar, excluding trials where the bisection was presented at centre. Leftward

spatial bias was calculated as the overall percentage of trials in which participants

indicated that the bisection was closer to the right-side of the bar (as increased

rightward responses suggest a relative perceptual exaggeration of the left side of

space).

Accuracy for both the Low AQ (M=69.43%, SD=8.27%) and High AQ group

(M=68.48%, SD=8.64%) was above chance level (largest p < 0.001, smallest r = 0.91),

with no difference between the groups (p = 0.59, r = 0.06). Pseudoneglect differed

significantly from chance levels in the Low AQ group (M=55.89%, SD=15.72%; p = 0.01,

r = 0.35), but not in the High AQ group (M=50.57%, SD=15.15%; p = 0.51, r = 0.04).

Additionally, pseudoneglect was significantly higher in the Low AQ group than in the

High AQ group, t(93) = 1.68, p = 0.048, r = 0.17. Taken together, these findings suggest

the Low AQ group showed pseudoneglect while the High AQ group did not,

conceptually replicating findings using the greyscales task here and in our previous

study (English et al., 2015).

Number Line Task

Response accuracy was measured as the percentage of trials on which

participants correctly identified the flanker number that was closest in numerical

position to the central number. A summary of the mean accuracy for each of the four

configurations (illustrated in Figure 3.3) is provided in Table 3.2. A repeated measures

analysis of variance (ANOVA) with the factors configuration type and AQ group

revealed a main effect of configuration type on accuracy, F(7,96) = 7.81, p < 0.001, ηp2 =

0.08. However, there was no main effect of AQ group (p = 0.60, ηp2 < 0.01), or

configuration x AQ group interaction (p = 0.96, ηp2 < 0.01). Accuracy for the Low AQ

group (M=65.48%, SD=7.17%) and High AQ group (M=64.63%, SD=8.64%) were both

above chance levels (largest p < 0.001, smallest r = 0.86), with no difference between

the groups (p = 0.89, r = 0.01).

Table 3.2. Number task accuracy across trial configurations (standard deviations in

parentheses).

Configuration A Configuration B Configuration C Configuration D

Low AQ 67.35%

(11.29%)

67.09% (9.33%) 65.22%

(10.98%)

62.22%

(11.13%)

High AQ 66.42%

(11.09%)

67.00%

(12.37%)

64.33% (9.89%) 60.75%

(13.37%)

CHAPTER THREE 71

Representational pseudoneglect (leftward representational bias) was

calculated as the percentage of trials in which participants indicated that the flanker

that was of a higher value was closer in numerical position to the central number. In

the context of this study, in keeping with convention (Brooks et al., 2014), we refer to

the numerically higher number as being positioned “rightward” of the central number,

as the mental number line is considered to be a series of numbers that ascend in value

from left-to-right. Therefore, an increased percentage of rightward responses would be

indicative of exaggeration of the left side of the mental representation of the number

line. Note that the words “left” or “right” in this task refer to a number’s position on the

mental number line, not to its location on the display. To ensure that participants were

not preferentially responding to the flanker number that was physically on the left side

or right side of the screen, single-sample t-tests were conducted on both AQ groups to

determine if responses to items presented on the left side of the screen differed from

chance levels. Neither AQ group reported a significant bias towards flankers that were

presented on one side of the screen (smallest p = 0.44, largest r = 0.11).

Representational pseudoneglect for both the Low AQ group (M=58.17%,

SD=11.80%) and High AQ group (M=54.53%, SD=12.62%) exceeded chance levels

(smallest p = 0.01, largest r = 0.34). There was no significant difference in the levels of

representational pseudoneglect between AQ groups (p = 0.21, r = 0.08), which

substantially differs from the results reported for the greyscales and landmark tasks, as

well as our previous study (English et al., 2015), where physical pseudoneglect was

attenuated in High AQ participants.

Discussion

The aim of this study was to determine if representational pseudoneglect is

attenuated in a High AQ sample as has been reported for physical pseudoneglect

(English et al., 2015). This was achieved by measuring response biases on tasks that

require visual assessment of physical stimuli (the landmark and greyscales tasks), and

a task that requires assessment of an internalized representation of space (the mental

number line bisection task). If pseudoneglect were reduced for all tasks in the High AQ

group relative to the Low AQ group, this would be evidence for a common neural

mechanism underlying reduced pseudoneglect for physical and representational

measures of space in individuals with high levels of autistic traits. However, if

pseudoneglect were only reduced on the physical spatial tasks, and unaltered on the

mental number line task, this would instead suggest that an attentional difference is


present in High AQ individuals that specifically affects attentional bias in processing of

visually-presented stimuli.

Regarding physical pseudoneglect, the study has two key findings. First, it

replicates our previous evidence for attenuated pseudoneglect amongst High AQ

participants on a greyscales task (English et al., 2015). Second, it extends and

generalize our previous findings by showing that physical pseudoneglect on the

landmark task is also reduced in a High AQ group. This is particularly important, as it

may have been possible that the attenuated pseudoneglect reported in the previous

study was specific to the greyscales stimuli. To the contrary, the present results

confirm that High AQ participants show a broad reduction in physical pseudoneglect

relative to their Low AQ peers.

The results from the mental number line task answer two key questions about

the relationship between AQ and representational pseudoneglect. First, whereas bias

scores for the High AQ group did not significantly differ from chance levels on either

measure of physical pseudoneglect, this was not the case for the number line task.

Here, the High AQ group showed a significant level of representational pseudoneglect.

Second, whereas physical pseudoneglect was reduced in the High AQ group relative to

the Low AQ group for the landmark and greyscales task, levels of representational

pseudoneglect were comparable between AQ groups in the number line task. Taken

together, these results suggest high levels of autistic-like traits do not influence

spatially-organized mental representations, indicating that unique mechanisms

underlie physical and representational spatial biases in this group.

Why might pseudoneglect differ between physical and mental spatial

representations for High AQ participants? The limited research into asymmetries in

spatial attention with respect to ASC restricts our ability to form definite conclusions.

However, one likely possibility is that attenuation of physical pseudoneglect arises

from processes directly related to the visual processing system that are not involved in

the number line task (which does not require judgements based on visual stimulus

characteristics). It is well documented that attention-related visual processing in

autism is markedly different than in neurotypical individuals. In particular, one of the

most widely reported differences is with respect to the global and local processing of

visually presented stimuli. Whereas most neurotypical individuals show a preference

for processing visual stimuli in a holistic or global manner, ASC and High AQ

individuals show a preference for a more piecemeal local processing style (Bayliss &

Kritikos, 2011; Cribb et al., 2016; Grinter, Maybery, et al., 2009; Grinter, Van Beek, et al.,

CHAPTER THREE 73

2009; Rhodes et al., 2013; Russell-Smith et al., 2012; Sutherland & Crewther, 2010).

This difference in preferential processing styles is particularly relevant here because,

like pseudoneglect, global processing is linked to RH activation while local processing

is associated with LH activation (Evans, Shedden, Hevenor, & Hahn, 2000; Flevaris,

Bentin, & Robertson, 2010; Hübner & Studer, 2009; Lux et al., 2004; Malinowski,


Yamaguchi, Yamagata, & Kobayashi, 2000).

Given this parallel, we tentatively hypothesize that differences in hemispheric

activation in High AQ and ASC specifically affect processes related to visual attention

and stimulus evaluation. On this notion, in the absence of the need to evaluate a visual

stimulus, hemispheric activation biases do not affect performance. While this

dissociation might reflect differences unique to High AQ and ASC, it may alternatively

reflect similar mechanisms underlying dissociations in RH-damaged patients, where

neglect on visual and representational tasks has not always been found to co-occur

(Doricchi et al., 2005; Loetscher & Brugger, 2009; Loetscher et al., 2010; Pia et al.,

2012; Rossetti et al., 2011; Storer & Demeyere, 2014; van Dijck et al., 2012). These

dissociations may be driven by the activation of different regions depending on the

physical or representational nature of the task. For example, line bisection judgements

are associated with activation of the striate, extrastriate visual cortex and parietal

lobes, with noted RH lateralization (Doricchi & Angelelli, 1999; Fink et al., 2000).

However, judgements of number positioning are relatively less lateralized to the RH,

and are associated with bilateral intraparietal sulcus activation, as well as activation in

the left precentral gyrus and prefrontal areas (Dehaene, 2004; Dehaene, Piazza, Pinel, &

Cohen, 2003; Doricchi et al., 2005; Walsh, 2003). If spatial attention shows reduced RH

lateralization for individuals with high levels of autistic traits, these regional

differences in activation may account for why we found differences in between the two

AQ groups on physical but not representational pseudoneglect.

Overall, the absence of reduced representational pseudoneglect for High AQ

individuals is evidence against the notion of a common mechanism underlying physical

and representational pseudoneglect. What then, may account for the results of

previous studies, that found that using TMS or tDCS to alter the activation of the

posterior parietal cortex shifted attentional biases for physical and representational

measures of space in the contralateral direction of the stimulated hemisphere (Göbel et

al., 2006; Göbel, Walsh, & Rushworth, 2001; Loftus & Nicholls, 2012)? As mentioned

earlier, while both task types are associated with parietal activation, several cortical


regions are only activated when completing either physical or representational spatial

tasks specifically (Dehaene, 2004; Dehaene et al., 2003; Doricchi & Angelelli, 1999;

Doricchi et al., 2005; Fink et al., 2000; Walsh, 2003). A potential explanation that is

compatible with the account above is that stimulation of the PPC may lead to

attentional changes for both task types, while other key structures are unaffected. This

may be of key importance as unaffected structures might compensate for changes in

parietal activity. For example, in the absence of external stimulation, relatively intact

right prefrontal working memory structures, which are linked to building up the

mental number line (Doricchi et al., 2005) may compensate for reduced parietal

activation that is related to high levels of autistic traits.

To summarize, the purpose of this study was to determine if previously

reported alterations in physical spatial bias found between AQ groups also occurred in

spatially-organized mental representations, and to generalize our previous findings

(English et al., 2015) using an alternative measure of attentional spatial bias (the

landmark task). While we replicated the attenuation of physical pseudoneglect in High

AQ participants on both tasks, Low and High AQ groups showed comparable

representational pseudoneglect on the mental number line task. We also hypothesize

that the PPCs play a specific role in processing physical spatial representations but not

in tasks requiring judgements about mental spatial representations, thus potentially

explaining task dissociations in our High AQ group. Finally, it should also be noted that

although the findings of this study are likely to be indicative of what might be found in

a clinical ASC sample, it is important to replicate our findings using such a sample in

future work. While research into lateralization of spatial attention in regard to ASC is a

relatively novel line of inquiry, atypical asymmetries are relatively simple to observe

and, with further understanding of the underlying mechanisms, such measures could

be valuable in the assessment and monitoring of neurological functioning in ASC.

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CHAPTER FOUR Modulation of global and local processing biases in adults with autistic-like traits

Michael C.W. English, Murray T. Maybery and Troy A.W. Visser

Previous work shows that doing a continuous performance task (CPT) shifts

attentional biases in neurotypical individuals towards global aspects of

hierarchical Navon figures by selectively activating right hemisphere regions

associated with global processing. The present study examines whether CPT

can induce similar modulations of attention in individuals with high levels of

autistic traits who typically show global processing impairments. Participants

categorized global or local aspects of Navon figures in pre- and post-CPT blocks.

Post-CPT, high trait individuals showed increased global interference during

local categorization. This result suggests that CPT may be useful for

temporarily enhancing global processing in individuals with high levels of

autistic traits and possibly those diagnosed with autism.

Autism is a disorder well-known to be characterized by repetitive patterns of behavior,

restricted interests and impaired social functioning (American Psychiatric Association,

2000). Less well known is that autism is also commonly associated with a bias to

preferentially process individual (“local”) components over holistic (“global”)

constructs (Behrmann, Thomas, & Humphreys, 2006; Behrmann, Avidan, et al., 2006;

82 MODULATION OF GLOBAL AND LOCAL PROCESSING BIASES IN ADULTS WITH HIGH AQ

Isomura, Ogawa, Yamada, Shibasaki, & Masataka, 2014a, see Happé & Frith, 2006, for a

review). For example, in the Embedded Figures Test (Witkin, 1971), which requires

locating a smaller target shape (e.g., a triangle) that forms part of a larger, more

complex image (e.g., a grandfather clock), individuals with autism spectrum conditions

(ASC) are reliably faster at locating the smaller target than neurotypical individuals

(for a meta-analysis see Muth, Hönekopp, & Falter, 2014). Similarly, for hierarchical

figures – stimuli that consist of a larger character and multiple embedded characters

(Navon, 1977) – individuals with ASC show faster identification of the smaller

characters and/or slowed identification of the larger character compared to

neurotypical individuals (also see Muth et al., 2014).

According to the “weak central coherence” theory (Shah & Frith, 1983), the

enhanced ability to attend to local-level stimuli in ASC arises from an impairment in the

usual automatic propensity to process information in a holistic fashion seen in

neurotypical individuals. Consequently, when attending to local-level stimuli,

individuals with ASC are less distracted by holistic, global-level inputs, leading to

improved performance. However, when attending to global-level stimuli, the relatively

weaker drive for coherence results in greater interference from local-level stimuli,

leading to poorer performance. While Shah and Frith’s argument posits a potential

deficit in global processing in ASC, many studies have found that individuals with ASC

can actually process global information with comparable performance to neurotypical

individuals, especially if their attention is explicitly directed to the global construct

with a prime or instruction (López, Donnelly, Hadwin, & Leekam, 2004; Plaisted,

Swettenham, & Rees, 1999). This suggests that rather than having a structural deficit in

global processing, individuals with ASC simply display a preference for prioritizing

local information.

The present work explores the possibility that processing in ASC could be

shifted towards global constructs using a behavioral task that increases right

hemisphere (RH) activation. There is substantial evidence that activation of cortical

regions in the RH is associated with global processing (Evans, Shedden, Hevenor, &

Hahn, 2000; Flevaris, Bentin, & Robertson, 2010; Hübner & Studer, 2009; Malinowski,


Yamaguchi, Yamagata, & Kobayashi, 2000). Moreover, the RH has also been linked to

variations in tonic (sustained) and phasic (short-term) attention. For example,

increases in tonic attention lead to greater activation in the right inferior frontal,

inferior parietal and anterior cingulate regions (Bartolomeo, 2014; Singh-Curry &

CHAPTER FOUR 83

Husain, 2009; Sturm & Willmes, 2001; Thiel, Zilles, & Fink, 2004), while phasic

attentional changes modulate activity in the right ventral fronto-parietal network and

anterior cingulate (Corbetta & Shulman, 2002). Together, this suggests that a shift

towards processing global constructs might be accomplished using a task that

increases tonic and phasic awareness.

Recent work by Degutis and Van Vleet (2010) directly supports this suggestion.

They had RH-damaged stroke patients complete a continuous performance task (CPT)

over nine days that required participants to withhold a response to the presentation of

pre-designated target images (teapots; 10% of trials), but to quickly respond to non-

targets (all other objects; 90% of trials). This task is similar to the sustained attention to

response task (Robertson, Manly, Andrade, Baddeley, & Yiend, 1997) in terms of

response ratios, but differs in that the CPT used by Degutis and Van Vleet (2010) has

variable and unpredictable inter-trial intervals, as opposed to fixed and predictable

intervals. The combination of relatively rare inhibited responses and random inter-trial

intervals demanded a high level of task engagement, thereby enhancing tonic and

phasic attention. Consistent with the link between tonic/phasic attention and RH

activity, left visuospatial neglect in the RH damaged stroke patients decreased

following CPT but was unchanged by a similar training task that did not modulate tonic

or phasic attention. Subsequently, Van Vleet, Hoang-duc, DeGutis, and Robertson

(2011) showed that CPT training could also modulate global and local attentional

biases to hierarchical figures (Navon, 1977) in a neurotypical sample. Following a

brief1 period of CPT, participants were better able to ignore local distractors when

directed to identify the global aspect of hierarchical stimuli, measured as a significant

decrease in the difference between RTs to trials with and without the local distractor.

Simultaneously, participants were worse at ignoring global distractors when tasked

with identifying the local aspect. Again, these changes were not present following a

training task that did not modulate tonic or phasic attention.

The work of Van Vleet and colleagues strongly suggests that behavioral

training, in the form of a CPT task, can increase RH activation and global preference in

RH-damaged and neurotypical individuals. However, no study, to our knowledge, has

yet attempted to produce similar results using an ASC sample. Thus, we do not know if

behavioral training can modulate attentional biases for individuals with ASC. It is

entirely possible that processing preferences are relatively rigid for individuals with

1 Approximately 10 minutes in duration.


ASC, and may exhibit resistance to behavioral training. However, in light of evidence

linking atypical behaviors and functioning in ASC to cortical abnormalities specific to

the RH (Di Martino et al., 2011; Jou, Minshew, Keshavan, Vitale, & Hardan, 2010;

Lazarev, Pontes, & DeAzevedo, 2009; Orekhova et al., 2009; Ozonoff & Miller, 1996;

Siegal, Carrington, & Radel, 1996), in the present work, we conducted two experiments

to assess whether processing in individuals with high levels of autistic traits could be

shifted towards a global aspect using CPT training to increase tonic and phasic

attention.

In the first experiment, we began by replicating the experimental findings

reported by Van Vleet et al. (2011) using a modified2 version of their paradigm adapted

for our laboratory. This is critical not only because it shows the benefits of CPT are

generalizable across laboratories and minor variations in methodology, but also

because the experimental outcome provides a baseline against which to compare

subsequent findings. In the second experiment, we sought to determine how CPT

training influences global and local processing in neurotypical individuals with low or

high levels of autistic traits. An increasing body of research has found that low and high

autistic-trait comparisons reveal similar patterns of results as found in neurotypical

and clinical ASC comparisons, especially with regard to global/local processing (for a

meta-analysis see Cribb et al., 2016; see also: Bayliss & Kritikos, 2011; Grinter, Van

Beek, et al., 2009; Grinter, Maybery, et al., 2009; Rhodes et al., 2013; Russell-Smith et

al., 2012; Sutherland & Crewther, 2010). Thus, we expected the findings here to

provide broad indications about the effectiveness of CPT training in altering local

processing biases in neurotypical individuals high in autistic traits, and potentially by

extension, individuals with ASC.

Experiment 1

In this experiment, we aimed to replicate the main findings presented by Van

Vleet et al. (2011). To achieve this, we modified the experimental procedure laid out by

these authors using our own stimuli and equipment. Participants began by categorizing

the global and local aspects of a series of hierarchical figures (e.g. Figure 4.1; Navon,

1977) to obtain measures of task interference in global-categorization and local-

categorization task conditions. Task interference was calculated by taking the

difference in RTs between trials that were “congruent” (i.e. the global and local aspect

2 Modified in the sense that while we recreated the experiment based on the parameters outlined by Van Vleet et al. (2011) to the best of our ability, the experiments may not be exactly identical. No intentional modifications were made.

CHAPTER FOUR 85

of the hierarchical figure were the same category – both numbers or both letters) and

trials that were “incongruent” (i.e. the global and local aspect of the hierarchical figure

were different categories – one letters and the other numbers). This difference

provides a measure of how well individuals can maintain their attention on the aspect

of the hierarchical figure that they were instructed to attend to. Larger interference

scores, corresponding to longer RTs on incongruent trials relative to the congruent

trials, therefore represent a relatively greater tendency to attend to the to-be-ignored

aspect of the task.

Figure 4.1. Examples of hierarchical figures (Navon, 1977) adapted from Van Vleet et

al. (2010); the two on the left are form-congruent (the global and local features are the

same; both letters or both numbers) and the two on the right are form-incongruent

(the global and local features are mismatched; letters and numbers are simultaneously

present.

Participants then completed training consisting of the CPT, or a categorization

control task (CCT) that used similar stimuli but does not invoke changes to

tonic/phasic attention (Van Vleet et al., 2011). Finally, participants repeated the

hierarchical figures task to assess changes in global and local processing. We predict

that, following a period of CPT, participants’ attention to global details will be relatively

greater than prior to training; specifically, local interference for global-categorization

should be reduced, and global interference for local-categorization should be increased.

In contrast, participants who complete CCT should experience no significant changes in

interference for either categorization type.

Method

Participants

Seventy-two first year psychology students (CPT group: n=36 (10 male), mean

age 18.39 (SD=1.59); CCT group: n=36 (18 male), mean age 19.31 (SD=2.75)) at the

University of Western Australia participated in the study in exchange for partial credit


towards a course requirement. Participants were selected to complete either the CPT

or CCT by order of attendance to the laboratory.

Materials

Participants were seated approximately 500mm in front of BenQ XL2420T

displays (refreshing at 100 Hz) that were connected to computers running Windows 7.

Presentation 17.0 software (Neurobehavioral Systems) was used to generate and

display task stimuli and record participant responses.

The Hierarchical Figure Task (HFT)

Stimuli comprised the letters A, E, F, H, L and P, and the numbers 2, 3, 4, 6, 7

and 9. For each stimulus, one letter or number was randomly chosen as the larger

global form and was created using a different randomly-chosen smaller letter or

number as the local form arranged on a 4x5 grid (see Figure 4.1). Every combination of

letters and numbers as the global and local forms was used to create 132 figures. Half

of the figures were congruent, with global and local forms chosen from the same

category (i.e. global letter constructed from local letters, or global number constructed

from local numbers), while the other half were incongruent, with the global and local

forms chosen from different categories (i.e., global letter constructed from local

numbers or vice-versa).

The task was divided into two blocks, with participants required to categorize

(letter or number) the global form in one, and categorize the local form in the other.

The blocks were presented in counterbalanced order. Each unique hierarchical figure

was presented twice, yielding 264 trials per block. In the global-categorization block,

the hierarchical figure’s global forms were 1.90° wide x 2.53° high and were created

using font size 10 characters. In the local-categorization block, the hierarchical figure’s

global forms were 1.43° wide x 1.90° high and were created using font size 9

characters. In a pilot study, Van Vleet et al. (2011) found that these stimulus sizes

produced optimal interference from the task-irrelevant global or local form.

Each trial began with a fixation cross presented in the center of the display for

500ms, followed by a hierarchical figure presented for 750ms, and then a blank screen.

Participants were directed to categorize either the global or local form of the

hierarchical figure using two keys on a keyboard - the ‘Z’ key if the target form was a

letter or the ‘/’ key if the target form was a number. Participants were prompted to

make their responses as quickly as possible whilst also retaining high task accuracy.

Responses could be made during the presentation of the hierarchical figure or the

CHAPTER FOUR 87

blank screen, with a response immediately ending the trial. The fixation cross then

reappeared marking the start of the next trial.

Continuous Performance Task (CPT)

The CPT task was based on the paradigm previously used by Degutis and Van

Vleet (2010), and Van Vleet et al. (2011). Task stimuli were 90 images (6.97° wide x

3.49° high) selected from the Caltech-256 Object Category dataset (Griffin, Holub, &

Perona, 2007), which was used due to the diverse nature of the images in the dataset,

and was also the source of CPT stimuli in Degutis and Van Vleet (2010), and Van Vleet

et al. (2011). Eight of the images were of teapots and were designated as targets, while

the remaining 82 images were distractors comprised of random, everyday objects.

Participants were instructed to withhold responses to target images, whilst dismissing

all non-targets with a response.

Each trial began with a fixation cross presented in the center of the display for

600, 1800 or 3000ms (randomly chosen on each trial) to prevent participants from

anticipating the onset of the image. An image then replaced the fixation cross and

remained onscreen for 500ms or until a response was recorded. Participants were

directed to press a response key (either ‘Z’ or ‘/’) as quickly as possible if a distractor

image appeared, but to withhold a response if a target image (a teapot) appeared and

wait for the image to disappear. Images were presented in random order, with each

image appearing four times, yielding 360 trials. The task took participants

approximately 16 minutes to complete.

Continuous Categorization Task (CCT)

The CCT required participants to view a series of images and respond to each

by indicating its orientation. Images were identical to those used in the CPT, except that

50% of the images (randomly-chosen) were inverted prior to presentation. Stimulus

presentation conditions were also identical to the CPT. However, participants were

directed to make a non-speeded orientation response when an image was displayed,

pressing the ‘Z’ key if the image was upright, or the ‘/’ key if the image was inverted.

The equal frequencies of presentation of upright and inverted images and non-speeded

responses result in a task that is less focused on tonic and phasic modulations of

attention than the CPT.

Procedure

The experimental structure was broken into three main parts. Participants

began with two HFT blocks (pre-training), which were followed by the CPT or CCT, and


then two further HFT blocks (post-training). To control for task order effects, the four

possible orders in which HFT blocks (local vs. global classification) could be completed

were counterbalanced across participants. Participants received instructions for all

tasks at the beginning of the experimental session and were also presented with task-

specific instructions on the computer immediately before beginning each task. In

addition, each task began with 40 practice trials so that participants could adjust to the

task demands.

Results

Training Task Performance

Participants who completed the CPT responded to non-targets with a mean RT

of 376ms (SD=26ms), missed responding to 15.95% (SD = 10.06%) of non-targets, and

incorrectly responded (false alarm) to 40.54% (SD=15.13%) of target images.

Participants who completed the CCT had comparable accuracy for upright and inverted

images (respectively, M=93.69%, SD=4.68% and M=94.11%, SD=4.29%).

General Hierarchical Figure Task Performance

Trials for the Hierarchical Figure Task were excluded from the calculation of

interference scores if they were outside the range of the mean RT ± 3SD for a given

participant (if the lower bound of the acceptable range fell below 200ms for a

participant, the lower bound was set to 200ms). Additionally, RT analyses excluded

incorrect trials.

Hierarchical Figure Task accuracy was examined using a Training Group (CPT

vs CCT) x Session (pre- vs post-training) x Categorization Type (global-categorization

vs local-categorization) repeated measures analysis of variance (ANOVA). The results

are summarized in Table 4.1. A main effect of Session was found, F(1, 35) = 16.37, p <

0.001, ηp2 = 0.19, which indicated that accuracy was lower across tasks in the post-

training session. A main effect of Categorization Type was also present, F(1, 35) = 5.63,

p = 0.02, ηp2 = 0.07, which indicated that participants completed the local-

categorization task with greater accuracy than the global-categorization task3. No other

3 It should be acknowledged that, as outlined earlier, neurotypical participants show superior performance for global processing relative to local processing. However, this is not always the case and certain experimental manipulations can change this pattern of performance. For example, as mentioned later in the General Discussion, alterations of the width of the attentional spotlight can affect performance. In the present study, it may be that the use of a small fixation cross presented before the hierarchical figure stimuli primed attention towards the local level, resulting in overall superior local performance. Importantly, this does not affect the results of the interference produced, the main aspect of this study.

CHAPTER FOUR 89

significant main effects or interactions were obtained. This suggests that HFT accuracy

was comparable for the CPT and CCT groups, meaning that potential differences found

in RTs between the groups is likely the result of different training conditions.


Ta

ble

4.1

. Hie

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re t

ask

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(m

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um

mar

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n (

pre

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d

po

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, Tra

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p (

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T a

nd

CC

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tim

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(co

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(st

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P

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54

1 (

61

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52

2 (

62

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Po

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PT

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52

8 (

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51

8 (

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)

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)

51

0 (

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)

Rea

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n T

ime

(ms)

Pre

-CP

T (

i)

63

7 (

11

8)

58

6 (

82

)

Pre

-CC

T (

i)

60

5 (

74

)

55

4 (

77

)

Pre

-CP

T (

c)

61

8 (

11

2)

57

6 (

84

)

Pre

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T (

c)

58

2 (

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54

6 (

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)

Acc

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3.9

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Pre

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T

92

.03

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2.9

9%

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Pre

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92

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92

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4.3

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Glo

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CHAPTER FOUR 91

Identical analyses were conducted on HFT RTs. A main effect of Session was

found, F(1, 35) = 88.76, p < 0.001, ηp2 = 0.56, with RTs faster during the post-training

session. A main effect of Categorization Type was also present, F(1, 35) = 17.58, p <

0.001, ηp2 = 0.20, with RTs faster for local-categorization compared to global-

categorization. A Session x Categorization Type interaction was present, F(1, 35) =

19.23, p < 0.001, ηp2 = 0.22, which indicated that RTs for global-categorization were

slower relative to local-categorization prior to training, but were comparable following

training. Finally, a Training Group x Categorization Type interaction was also found,

F(1, 35) = 4.02, p = 0.05, ηp2 = 0.05, though interpretation of this interaction is not

particularly germane to the present study given that RTs were collapsed across pre-

and post-training sessions.

Global/Local interference changes following training

Finally, to determine if CPT or CCT influenced participant’s ability to direct

their attention to the global or local aspect of the hierarchical figure, task RTs were

then subjected to further analysis to examine differences in HFT stimuli with congruent

or incongruent global and local levels between CPT and CCT training groups, and pre-

and post-training sessions (see Table 4.1). To determine the specific effect of CPT or

CCT training on the ability to focus on relevant global or local forms, global and local

interference scores were then calculated from the raw RTs. A measure of local

interference was created by subtracting RTs on global-categorization congruent trials

from RTs on global-categorization incongruent trials. A measure of global interference

was calculated by subtracting RTs on local-categorization congruent trials from RTs on

local-categorization incongruent trials. Interference scores were calculated separately

for pre- and post-training sessions, and then were contrasted with each other to

determine the impact that CPT or CCT training had on participants’ ability to ignore the

distracting information presented at either the local or global level.

Levels of task interference are summarized in Figure 4.2. Change in task

interference following attentional training was assessed using a Training Type (CPT vs

CCT training) x Categorization Type (local vs global-categorization) x Session (pre- vs

post-training) x Categorization Order (global-categorization first vs local-

categorization first in post-training test blocks) mixed-design analysis of variance

(ANOVA). The Categorization Order variable was included to evaluate Van Vleet et al.'s

(2011) suggestion that training effects may be short-lived and thus only present in the

first post-training HFT block. A main effect of Categorization Type was revealed, F(1,

68) = 4.42, p = 0.04, ηp2 = 0.06, which indicated that greater levels of interference were


present during the global task relative to the local task across all conditions. The

ANOVA also revealed a significant interaction between Session and Categorization

Type, F(1, 68) = 8.36, p < 0.01, ηp2 = 0.11, indicating that interference levels for global

and local-categorization showed substantially dissimilar changes as a result of

attentional training. The ANOVA revealed no other main effects or interactions (all ps >

0.06, all ηp2 < .06).

Figure 4.2. Interference pre- and post-training for each training condition (error bars

represent mean standard error. Paired sample t-tests with ps < 0.05 indicates with a *.

The absence of a Training Type x Categorization Type x Session interaction

suggests that effects did not differ across different types of training. However,

examination of Figure 4.2 appears to indicate that while training effects had a similar

pattern across conditions, effects were more pronounced in the CPT compared to the

CCT condition. For this reason, we also conducted two follow-up ANOVAs on the CPT

and CCT data separately, following the analytical procedure of Van Vleet et al. (2011).

The notable result of these additional analyses was a significant Categorization Type x

Session interaction that was present in the CPT training condition only, F(1, 35) = 8.20,

p < 0.01, ηp2 = 0.19, and not in the CCT training condition, F(1, 35) = 1.82, p = 0.19, ηp

2 =

0.05.

Finally, in keeping with Van Vleet et al.'s (2011) analytical procedure, a priori t-

tests were conducted to examine potential changes in task interference as a result of

attentional training. A paired samples t-test on the global-categorization task

comparing pre- and post-CPT performance revealed a significant decrease in local

CHAPTER FOUR 93

interference following CPT training, t(35) = 2.27, p = 0.03. An identical t-test on the

local-categorization task revealed a significant increase in global interference following

CPT training, t(35) = 2.17, p = 0.04. In contrast, identical analyses comparing pre- and

post-CCT performance showed no changes (both ps > 0.34, both rs < 0.09).

Discussion

The present experiment successfully replicated the chief findings of Van Vleet

et al. (2011) using our own equipment and modified experimental design. Similar to

their study, following CPT training, there was a significant increase in global

interference and a significant decrease in local interference for the local and global-

categorization tasks respectively. Importantly, similar changes were not reported in

the CCT group, indicating that the increased tonic and phasic attention were

responsible for shifts in global preference rather than general characteristics of the

task or stimuli. In short, the results validate our instantiation of the CPT training,

replicate the benefits to global processing found in previous work, and provide a useful

baseline against which to judge performance in Experiment 2.

That said, our results did depart from Van Vleet et al.'s (2011) in one respect.

While they found the effects of CPT training dissipated after a single block of trials,

training effects in our experiment persisted throughout the HFT. Participants in both

studies completed a similar number of training trials, so this difference cannot be

explained in terms of training length. However, it does appear that there were

differences in engagement with the CPT training between the studies. Specifically, our

participants made faster responses to non-target images (376 vs 462ms) at the cost of

reduced correct response omissions to target images (59 vs 77%). This suggests that a

focus on making speeded responses in the CPT task, presumably reflecting a relative

reduction in inhibition to targets, leads to a relatively larger global shift observed post-

training.

Experiment 2

As outlined earlier, neurotypical individuals high in autistic-like traits and

those with ASC both show an atypical bias towards local processing relative to their

neurotypical, low autistic trait peers (for a meta-analysis see Cribb et al., 2016; see

also: Bayliss & Kritikos, 2011; Grinter, Van Beek, et al., 2009; Grinter, Maybery, et al.,

2009; Rhodes et al., 2013; Russell-Smith et al., 2012; Sutherland & Crewther, 2010). In

this experiment, using the same paradigm as in Experiment 1, we aimed to determine

whether CPT training increases global processing in neurotypical individuals with high


levels of autistic traits, and compare this to effects on individuals with low levels of

autistic traits. This study may be the first to show that behavioral training designed to

increase RH functioning can shift processing towards global constructs in individuals

high in autistic-like traits. It would also provide preliminary insight into potential

effects of training on a clinical ASC sample.

Method

Participants

Participants were 128 right-handed, first-year psychology students at the

University of Western Australia who participated in the study in exchange for partial

credit towards a course requirement. Because non-right-handed individuals show

reduced lateralization of brain functions (Banich, 1997), we recruited only right-

handed participants to ensure that any atypical lateralization reflected the contribution

of autistic traits only. A measure of autistic traits, the Autism-spectrum Quotient (AQ;

Baron-Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001) was obtained in a

separate screening procedure completed by the entire first-year cohort, and

participants were invited to the study if their AQ scores were in the upper or lower

quartile of the cohort. Participants were selected to complete either the CPT or CCT by

order of attendance to the laboratory. No participants had completed Experiment 1.

Descriptive statistics for the participants are summarized in Table 4.2.

Table 4.2.

Descriptive statistics for participants in Experiment 2 (standard deviations in

parentheses)

CPT CCT Low AQ High AQ Low AQ High AQ N 32; 7 male 32; 12 male 32; 6 male 32; 5 male Age 19.63 (4.38) 21.69 (7.46) 22.56 (8.95) 19.16 (1.44) AQ 89.91 (6.55) 125.47 (6.71) 90.28 (7.35) 127.17 (8.30)

Materials

Experiment 2 used the same materials as Experiment 1 with the addition of the

AQ (Baron-Cohen et al., 2001), and the Edinburgh Handedness Inventory (EHI;

Oldfield, 1971). The AQ is a 50-item self-report questionnaire that assesses traits and

characteristics associated with autism in neurotypical individuals. Items were scored

CHAPTER FOUR 95

using the 1-4 method described by Austin (2005) with higher scores indicating greater

levels of autistic-like traits. This scoring method was used to take advantage of the

range of potentially useful information in each item, increasing the variability of total

AQ scores. The EHI is a 10-item self-report questionnaire used to assess handedness of

participants.

Procedure

Apart from the preliminary screening with the AQ and EHI, the procedure was

identical to that of Experiment 1.

Results

Training task performance

Low and High AQ performance on the CPT and CCT was comparable

(summarized in Table 4.3), with independent samples t-tests comparing response

times, misses and false alarms showing no significant differences between the groups

(all ps > 0.33, all rs < 0.04).

Table 4.3. Overall CPT and CCT performance (standard deviations in parentheses).

Low AQ High AQ

CPT

Reaction Time (non-targets) 379ms (23ms) 376ms (29ms)

Misses (non-targets) 14.95% (8.47%) 14.04% (11.94%)

False Alarms (targets) 42.76% (15.67%) 46.75% (17.07%)

CCT

Accuracy (upright) 81.10% (16.76%) 79.48% (19.47%)

Accuracy (inverted) 82.75% (17.67%) 84.88% (11.53%)

General Hierarchical Figure Task Performance

Similar to the first experiment, trials for the Hierarchical Figure Task were

excluded from the calculation of interference scores if they were outside the range of

the mean RT ± 3SD for a given participant (if the lower bound of the acceptable range

fell below 200ms for a participant, the lower bound was set to 200ms). Additionally, RT

analyses excluded incorrect trials.


Hierarchical Figure Task accuracy was examined using a Training Group (CPT

vs CCT) x AQ Group (Low vs High AQ) x Session (pre- vs post-training) x Categorization

Type (global-categorization vs local-categorization) repeated measures ANOVA, with

results summarized in Table 4.4. A main effect of Session was found, F(1, 124) = 34.22,

p < 0.001, ηp2 = 0.22, which indicated that accuracy was lower in the post-training

session. No other effects reached significance (all ps > 0.10, all ηp2s < 0.02). Critically,

HFT accuracy for the CPT and CCT groups is comparable between training groups and

AQ groups across the session, meaning that any differences in RTs are the result of

different training tasks.

CHAPTER FOUR 97

Ta

ble

4.4

. Hie

rarc

hic

al f

igu

re t

ask

per

form

ance

(m

ean

s an

d S

Ds)

fo

r gl

ob

al a

nd

loca

l-ca

tego

riza

tio

n s

um

mar

ized

acr

oss

Ses

sio

n (

pre

- an

d

po

st-t

rain

ing)

, Tra

inin

g G

rou

p (

CP

T a

nd

CC

T),

AQ

Gro

up

(L

ow

an

d H

igh

AQ

) an

d, f

or

reac

tio

n t

imes

, co

ngr

uen

cy (

con

gru

ent

(c)

and

inco

ngr

uen

t (i

)) (

stan

dar

d d

evia

tio

ns

in p

aren

thes

es).

Po

st-C

PT

(i)

53

8 (

73

)

56

0 (

12

0)

55

3 (

58

)

53

6 (

82

)

Po

st-C

CT

(i)

55

5 (

83

)

54

2 (

91

)

54

2 (

80

)

54

9 (

92

)

Po

st-C

PT

(c)

52

6 (

75

)

54

8 (

10

5)

53

4 (

58

)

52

1 (

83

)

Po

st-C

CT

(c)

54

6 (

77

)

52

4 (

91

)

52

5 (

77

)

53

9 (

94

)

Rea

ctio

n T

ime

(ms)

Pre

-CP

T (

i)

60

0 (

61

)

62

0 (

12

5)

61

7 (

76

)

56

7 (

92

)

Pre

-CC

T (

i)

63

3 (

96

)

61

7 (

11

2)

57

3 (

73

)

56

4 (

81

)

Pre

-CP

T (

c)

57

7 (

55

)

60

7 (

12

9)

60

2 (

78

)

56

0 (

99

)

Pre

-CC

T (

c)

61

4 (

98

)

59

9 (

10

4)

55

7 (

70

)

55

1 (

76

)

Po

st-C

PT

91

.34

% (

4.2

4%

)

90

.46

% (

7.4

8%

)

91

.43

% (

4.4

8%

)

90

.51

% (

7.8

1%

)

Po

st-C

CT

91

.34

% (

4.2

4%

)

88

.29

% (

7.5

1%

)

91

.25

% (

5.9

2%

)

91

.43

% (

4.7

3%

)

Acc

ura

cy

Pre

-CP

T

92

.73

% (

4.0

8%

)

91

.20

% (

6.9

9%

)

93

.07

% (

3.4

2%

)

92

.31

% (

5.8

0%

)

Pre

-CC

T

92

.41

% (

4.3

8%

)

91

.51

% (

4.9

0%

)

92

.64

% (

4.5

9%

)

91

.08

% (

4.9

6%

)

Lo

w A

Q

Hig

h A

Q

Lo

w A

Q

Hig

h A

Q

Lo

w A

Q

Hig

h A

Q

Lo

w A

Q

Hig

h A

Q

Glo

bal

Cat

ego

riza

tio

n

Lo

cal

Cat

ego

riza

tio

n

Glo

bal

Cat

ego

riza

tio

n

Lo

cal

Cat

ego

riza

tio

n


Identical analyses were conducted on HFT RTs. A main effect of Session was

found, F(1, 124) = 110.01, p < 0.001, ηp2 = 0.47, with RTs significantly faster during the

post-training session. A main effect of Categorization Type was also present, F(1, 124)

= 14.36, p < 0.001, ηp2 = 0.10, with faster responses recorded for the local-

categorization task. A Session x Categorization Type interaction effect was found, F(1,

124) = 19.49, p < 0.001, ηp2 = 0.14, which indicated that participants were initially

slower on global-categorization prior to training, but performed both categorization

tasks comparably in the post-training session. Further statistically significant effects

included a Categorization Type x Session x AQ Group interaction, F(1, 124) = 4.18, p =

0.04, ηp2 = 0.03, as well as a Categorization Type x Session x Training Group interaction

F(1, 124) = 10.37, p < 0.01, ηp2 = 0.08. The first interaction indicated that both AQ

groups performed similarly on both categorization tasks, except prior to training when

the Low AQ group was slower at the local-categorization task. The second interaction

indicated groups receiving CPT and CCT training performed similarly on both

categorization tasks, except prior to training when the CPT group was slower at the

local-categorization task. Finally, a Categorization Type x AQ Group x Training

interaction was found, F(1,124) = 11.73, p < 0.01, ηp2 = 0.09, but was not examined

further as the effects were meaningless due to the collapsing of pre- and post-training

RTs for this comparison.

Global/Local interference changes following training

Finally, to determine if CPT or CCT influenced participant’s ability to direct

their attention to the global or local aspect of the hierarchical figure in the presence of

competing information from the to-be-ignored aspect, task RTs were subject to further

analysis to examine differences in HFT stimuli with congruent or incongruent global

and local levels between CPT and CCT training groups, between AQ groups, and pre-

and post-training sessions (see Table 4.4). Global and local task interference was

calculated in the same way as detailed in Experiment 1 to compare the effects of CPT

and CCT training on the ability to focus on relevant global or local forms across AQ

groups. Before conducting the critical analysis, a preliminary analysis was conducted to

determine if pre-CPT training differences existed between the AQ groups using a

repeated measures ANOVA with the factors Categorization Type (local vs global) and

AQ Group (Low vs High AQ). If pre-existing differences between the AQ groups were

present, it could affect the interpretation of subsequent analyses. The only statistically

significant result from the analysis was a main effect of AQ Group, F(1, 62) = 4.81, p =

0.03, ηp2 = 0.07, indicating that interference was greater for the Low AQ group relative

CHAPTER FOUR 99

to the High AQ group across both global and local-categorization. The other effects did

not reach statistical significance (both ps > 0.09, both ηp2s < 0.05).

Levels of task interference are illustrated in Figure 4.3. Changes in task

interference following attentional training was assessed using a Training Type (CPT vs

CCT) x Categorization Type (local vs global) x Session (pre- vs post-training) x AQ

Group (Low AQ vs High AQ) x Categorization Order (global-categorization first vs local-

categorization first in post-training test blocks) mixed-design ANOVA. Replicating the

results of Experiment 1, a significant interaction was found between Session and

Categorization Type, F(124, 1) = 4.26, p = 0.04, ηp2 = 0.03, indicating that interference

changed following attentional training. All other comparisons were non-significant (all

ps > 0.19, all ηp2 = 0.01).


Figure 4.3. Pre- and post-CPT and CCT training for the Low and High AQ groups (error

bars represent mean standard error). Paired sample t-tests with ps < 0.05 indicates

with a *.

AQ group was not found to interact with any of the variables and the critical

Training Type x Categorization Type x Session x AQ Group interaction only reached p =

0.51, ηp2 < 0.01. However, as noted earlier, baseline levels of CPT differed between AQ

Groups, which may have masked subsequent interaction effects. Furthermore, the

results of Experiment 1 demonstrated that similarities in training effects across the

two training tasks obscured otherwise significant interactions in the CPT condition. As

such, and in keeping with Van Vleet et al.'s (2011) analytical procedure, a priori t-tests

were conducted to determine if CPT training yielded interference changes similar to

Experiment 1 for each of the AQ groups; specifically, if CPT reduced local interference

for global-categorization, and increased global interference for local-categorization. In

CHAPTER FOUR 101

the Low AQ group, local interference on the global-categorization HFT decreased

following CPT training, t(31) = 2.05, p < 0.05, r = 0.25, while global interference on the

local-categorization task did not change (p = 0.64, r = 0.07). In the High AQ group,

global interference on the local-categorization HFT increased following CPT training,

t(31) = 2.28, p = 0.03, r = 0.22, while local interference on the global-categorization

HFT showed no change (p = 0.84, r = 0.02). Follow-up tests to examine the specific

changes in interference following CCT training failed to reveal any significant

differences between AQ groups (all ps > 0.14, all rs < 0.18).

Discussion

The chief aim of this experiment was to determine whether CPT training could

modulate global processing bias in individuals high in autistic-like traits and to

compare these changes to those seen in individuals low in autistic-like traits. As in

earlier studies using RH-damaged and neurotypical participants, there was substantial

evidence that CPT training shifted processing towards a global aspect in both Low and

High AQ participants. This is consistent with the suggestion that CPT increases RH

activation via changes in tonic and phasic attention. However, Low and High AQ

individuals did not show the same pattern of benefits from CPT-training. Specifically,

the Low AQ group showed only a decrease in local interference during global-

categorization, while the High AQ group showed only an increase in global interference

during local-categorization.

It should also be noted that, as in Experiment 1, the Categorization Order

variable did not influence the pattern of results. This is a positive outcome, as it

indicates that CPT training benefits may persist for significantly longer than found by

Van Vleet and colleagues. Also, as in Experiment 1, we found that our participants

completed the CPT with faster RTs and fewer correct response omissions to target

images compared to Van Vleet et al.'s (2011) participants. This provides further

evidence that emphasizing response speed over accurately withholding responses to

items to be ignored may prolong the effects of CPT training on global processing bias.

General Discussion

ASC is associated with a preference for processing stimuli in a manner that

emphasizes the local features relative to the global, coherent aspect (Behrmann,

Thomas, & Humphreys, 2006; Behrmann, Avidan, et al., 2006; Isomura, Ogawa,

Yamada, Shibasaki, & Masataka, 2014a, see Happé & Frith (2006) for a review).

Importantly, this processing bias seems to be associated with the social processing


deficits that characterize the disorder. For example, greater difficulty on face and

emotion recognition tasks has been linked to a reduced preference for globally

organized stimuli (Behrmann, Avidan, et al., 2006; Gross, 2005; Rutherford & McIntosh,

2007; Walsh, Vida, & Rutherford, 2014).

The present investigation stems from studies that associate global processing

with regions primarily located in the RH (Evans et al., 2000; Flevaris et al., 2010;

Hübner & Studer, 2009; Lux et al., 2004; Malinowski et al., 2002; Volberg & Hübner,

2004; Weissman & Woldorff, 2005; Yamaguchi et al., 2000) and evidence linking

atypical behaviors and functioning in ASC to cortical abnormalities specific to the RH

(Di Martino et al., 2011; Jou et al., 2010; Lazarev et al., 2009; Orekhova et al., 2009;

Ozonoff & Miller, 1996; Siegal et al., 1996). The goal of the present work was to

determine whether global processing could be increased in neurotypical individuals

with high levels of autistic traits using CPT training (Van Vleet et al., 2011). This task

increases tonic and phasic attention and thus is thought to boost RH activation

(Bartolomeo, 2014; Corbetta & Shulman, 2002; Singh-Curry & Husain, 2009; Sturm &

Willmes, 2001; Thiel et al., 2004).

In Experiment 1, we replicated the results of Van Vleet et al. (2011) using a

modified experimental design with an unselected neurotypical sample. This allowed us

to verify our methodology and ensured that the results from Experiment 2, which

included separate groups of Low and High AQ neurotypical individuals, could be

directly attributed to variations in autistic traits rather than any variations in

methodology from prior work. Indeed, in Experiment 2, consistent with the possibility

that CPT increases global processing, we found that the High AQ group showed

increased global interference on the local-categorization HFT.

While the key finding from the present experiments is that CPT increased

global preference in neurotypical participants who were high in autistic-like traits, it is

interesting that this change manifested itself exclusively in increased global

interference in the local-categorization condition. This contrasts with the pattern

shown in the Low AQ group who manifested increased global preference as decreased

local interference in the global-categorization condition. The differential effects of CPT

training for the two forms of categorization across the Low and High AQ groups could

be due to differences in pre-training performance across AQ groups. Because of this

group difference in pre-CPT interference, it was comparatively more difficult to detect

a significant increase in already-high global interference for the Low AQ group and a

significant decrease in already-low local interference in the High AQ group following

CHAPTER FOUR 103

CPT training. However, if pre-training differences were not present, as was the case

between AQ groups in the CCT condition, it is possible that CPT effects on local and

global categorization would have been similar for the two AQ groups if baseline

interference levels had been comparable. Given the absence of ASC-related research in

regard to behavioral training of global and local processing, at this point in time there

is no strong prior basis for a particular profile of attentional training effects for the

different AQ groups. Regardless, the fact that CPT was effective at increasing global

interference for local categorization, and the potential remains for CPT to be effective

at reducing levels of local interference for global categorization, highlights the need for

further investigations in this area.

That said, other options aside from pre-training group differences could also

account for this pattern of results. For example, the impact of CPT could differ across

AQ groups. However, for this dissociation to be true, it would require that CPT-related

changes of decreased local interference during global-categorization and increased

global interference during local-categorization to be attributed to two different

mechanisms, with one operating in Low AQ participants and the other in High AQ

participants. Perhaps this notion could be plausible if there were indication that CPT

affected attention to both globally and locally presented stimuli, but this is unlikely

given prior evidence that it selectively activates RH regions, which are more likely to be

substantially involved in global rather than local processing.

Another possibility is that significant differences in both global and local

interference across AQ groups would have been found with additional CPT training.

Here, we used a single 16-minute session of CPT as outlined in Van Vleet et al. (2011)

as it was sufficient to produce a measurable change in a neurotypical group. However,

in Degutis and Van Vleet's study (2010) with neglect patients, a decrease in left neglect

severity was observed after a much longer training regimen consisting of nine days of

36 minute sessions. Given that several papers have suggested that individuals with ASC

may also have RH abnormalities (Di Martino et al., 2011; Jou et al., 2010; Lazarev et al.,

2009; Orekhova et al., 2009; Ozonoff & Miller, 1996; Siegal et al., 1996), this might

imply that additional training would have led to greater benefits, particularly in the

High AQ group. While this is clearly a topic for future research, it is still apparent from

the present findings that even a single session of CPT is sufficient to increase global

processing in a High AQ sample.

A final possible account involves the relative size of the attentional spotlight or

window for our Low and High AQ groups. The attentional spotlight has been described


as moveable “beam” that can be expanded and narrowed to facilitate processing of

visual stimuli (Eriksen & Yeh, 1985; Posner, 1990). Previous work has demonstrated

that children with ASC show an impaired ability to widen the attentional spotlight

relative to typically developing children (Mann & Walker, 2003) and, assuming our

Low AQ group in Experiment 2 had generally broader attentional windows than our

High AQ group, this could account for their relatively longer RTs for local-

categorization, as their wider spotlight is more likely to be captured by the global

information rather than local-level detail.

Importantly, the size of the attentional window can be altered. For example, a

LH-damaged patient with right neglect were better at detecting right-sided targets

presented on a small circle stimulus when their attentional window was broadened by

including trials with larger circles (Hillis, Mordkoff, & Caramazza, 1999). CPT training

may similarly broaden the size of the attentional window, suggesting an alternative

account for CPT training benefits reported by Degutis and Van Vleet (2010). Rather

than CPT increasing RH activation and consequently shifting attention leftward, CPT

may have broadened the attentional spotlight resulting in more ‘leftward’ parts of

landmark stimuli being attended. Applying a similar logic to the present study, CPT

training may have broadened the attentional spotlight, making global-level information

more accessible. This could explain why training increased global interference for

local-categorization in our High AQ group, but not in our Low AQ group. This

explanation is, of course, speculative at this point, but clearly warrants more detailed

investigation in future work.

A separate analysis of CPT and CCT training effects, identical to the analytic

procedure used by Van Vleet et al. (2010), indicated that only CPT training significantly

shifted processing towards global aspects of the HFT. However, in combined analyses,

neither the relevant three-way (Experiment 1) or four-way (Experiment 2) interaction

involving training type were significant, suggesting both types of training yielded

similar behavioral change. One likely explanation for this discrepancy is simply that

our analyses lacked adequate statistical power to detect these higher-order

interactions. Nevertheless, the similar patterns of change across tasks clearly visible in

Figures 4.2 and 4.3 suggests that other factors may also be at work. In particular, the

tonic and phasic attentional effects arising from CPT may also be generated by the CCT

task, albeit in a weaker form, given the absence of CCT-related training effects.

Importantly, this does not diminish the relevance of the present findings, which clearly

show that behavioral training can alter global processing in individuals with high AQ.

CHAPTER FOUR 105

However, it does suggest that more appropriate control tasks be used instead of CCT as

low levels of attentional training effects induced by CCT may otherwise mask group

differences between CPT and CCT training groups. Such similarities reduce the ability

to detect CPT training effects and may explain why our higher-order interactions that

included training type did not reach significance.

Due to the substantively similar pattern of results across studies that have

compared ASC versus neurotypical individuals, and high versus low autistic trait

individuals (for a meta-analysis "see Cribb et al., 2016; see also: Bayliss & Kritikos,

2011; Grinter, Van Beek, et al., 2009; Grinter, Maybery, et al., 2009; Rhodes et al., 2013;

Russell-Smith et al., 2012; Sutherland & Crewther, 2010), the results of this study can

be interpreted as a preliminary indication that CPT may also increase global processing

in clinically diagnosed individuals with ASC. These training effects could potentially

benefit people with ASC by encouraging the use of global processing styles when they

might otherwise employ a default local processing style. As an illustration of how such

training might manifest itself in changes in task performance, consider the study of

Rutherford and McIntosh (2007) who demonstrated that individuals with ASC show a

greater tolerance for faces with exaggerated facial features which could be the result of

depending upon strategies that use rule-based, piecemeal processing to identify shapes

and infer emotions, rather than a holistic strategy that compares the face stimulus to a

known gestalt or template. Applying the findings of our study to this context, we would

hypothesize that if the ASC participants were administered a session of CPT before the

face processing task, they would be more inclined to use a global strategy rather than a

rule-based strategy, potentially reducing their tolerance for unrealistic emotional

expressions. Naturally, the precise ability for CPT to influence the processing of face

stimuli has yet to be examined, with demonstrations of the effectiveness of CPT

currently limited to hierarchical Navon figures. The degree to which this behavioral

task could influence perceptions of face stimuli should be explored.

This study provides the first evidence, to our knowledge, that local attentional

biases can be modulated in individuals with high levels of autistic traits. In so doing, it

suggests that CPT, or other techniques to enhance RH activation, such as transcranial

direct current stimulation, might be useful in enhancing global processing and

potentially related skills such as face recognition and emotion identification. Of course,

the implications of these results for ASC are necessarily speculative at this point.

Further research into the usefulness of CPT in the context of autism is needed.


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CHAPTER FIVE

Modulating attentional biases of adults with autistic traits using transcranial direct current stimulation: a pilot study

Michael C W English, Emma S Kitching, Murray T Maybery and Troy

A W Visser

While neurotypical individuals over-attend to the left-side of centrally-

presented visual stimuli, this bias is reduced in individuals with autism/high

levels of autistic traits. Because this difference is hypothesized to reflect

relative reductions in right-hemisphere activation, it follows that increasing

right-hemisphere activation should increase leftward bias. We administered

transcranial direct current stimulation (tDCS) over the right posterior parietal

cortex to individuals with low levels (n = 19) and high levels (n = 15) of autistic

traits whilst completing a greyscales task. Anodal tDCS increased leftward bias

for high-trait, but not low-trait, individuals, while cathodal tDCS had no effect.

This outcome suggests that functional imbalances in hemispheric activation for

attentional mechanisms in high-trait individuals can be restored following

right-hemisphere stimulation.

112 MODULATING ATTENTIONAL BIASES OF ADULTS WITH HIGH AQ USING TDCS

Whilst neurotypical individuals tend to show pseudoneglect, an attentional bias

towards stimulus features presented in the left hemifield (Jewell & McCourt, 2000)

driven by the relatively greater lateralization of spatial attention to the right

hemisphere (RH) (Siman-Tov et al., 2007), this attentional bias is reduced in

individuals with autism spectrum conditions (ASC). Compared to controls, adults with

ASC and infants with older ASC siblings show reduced eye-gaze to the left side of

centrally-presented faces (Dundas, Best, Minshew, & Strauss, 2012; Dundas, Gastgeb, &

Strauss, 2012), whilst adults show reduced leftward bias on face-identity matching

tasks (Ashwin, Wheelwright, & Baron-Cohen, 2005). Similar patterns are also found for

neurotypical individuals with high levels of autistic-like traits (High ALT) viewing non-

face stimuli (Chapter 2 & 3 of the present thesis; henceforth English, Maybery, & Visser,

2015, 2017).

Another aspect of attention linked to RH regions is global processing; the ability

to integrate multiple independent stimuli into a coherent and meaningful whole

(Hübner & Studer, 2009; Malinowski, Hübner, Keil, & Gruber, 2002; Weissman &

Woldorff, 2005; Yamaguchi, Yamagata, & Kobayashi, 2000). Here too, individuals with

ASC/High ALT show a reduction in global processing relative to neurotypical peers (for

meta-analyses, see Cribb, Olaithe, Di Lorenzo, Dunlop, & Maybery, 2016; Van der

Hallen, Evers, Brewaeys, Van den Noortgate, & Wagemans, 2015). In turn, this reduced

global processing has been linked with poorer face identification and emotional

recognition (Behrmann, Thomas, & Humphreys, 2006; Gross, 2005) – key elements of

social processing.

It is possible that reductions in both pseudoneglect and global processing are

the result of a functional imbalance in the activation of the two hemispheres for spatial

attention in individuals with ASC/High ALT. If that were the case, non-invasive

transcranial direct current stimulation (tDCS) might be effective in shifting this

imbalance and modulating associated attention-related task performance. TDCS has

been previously used to elicit shifts in spatial attention by stimulating or disrupting

posterior parietal cortex (PPC) activation in neurotypical individuals (Loftus &

Nicholls, 2012; Roy, Sparing, Fink, & Hesse, 2015; Sparing et al., 2009) and to improve

social functioning outcomes for individuals with ASC via disruption of the left

dorsolateral prefrontal cortex (D’Urso et al., 2014, 2015). However, to our knowledge,

no study has attempted to invoke attentional shifts using tDCS in individuals with

either ASC or High ALT.

CHAPTER FIVE 113

The present study aims to provide preliminary examination of the attentional

changes induced via anodal and cathodal tDCS of the right PPC of neurotypical adults

with Low and High ALT. Attentional changes as a result of tDCS were assessed by

measuring performance on the greyscales task (Nicholls, Bradshaw, & Mattingley,

1999), which has been shown to be sensitive to differences in attentional bias between

Low and High ALT groups (English et al., 2015, 2017).

Method

Participants

Thirty-eight right-handed undergraduate students from the University of

Western Australia participated in the study in exchange for partial course credit.

Materials

Questionnaires

ALT levels were assessed using the Autism Spectrum Quotient (AQ; Baron-

Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001), a 50-item self-report

questionnaire, scored using Austin's (2005) 1-4 method. Handedness was assessed

using the ten-item Edinburgh Handedness Inventory (Oldfield, 1971).

Greyscales Task

The task was adapted from English et al. (2015). Stimuli were generated using

Presentation software (Version 17.0, Neurobehavioral Systems) and presented on a

24” BenQ XL2420T monitor. Participants were seated approximately 50cm from the

display. Trials consisted of a central fixation cross presented for 1500ms, followed by

two horizontal bars presented above and below the display’s centre. From the left, one

bar was shaded white-to-black, with the number of black pixels increasing evenly

across the stimulus. The other bar was shaded similarly but in the reverse direction

(Figure 5.1). Finally, one bar was ‘darker, achieved by randomly replacing 100 white

pixels with black pixels evenly across the bar, and similarly replacing 100 black pixels

with white pixels across the other bar. The top/bottom positions of the left-to-right

shaded bar and the overall darker bar were varied randomly but counterbalanced

across trials. Participants were instructed to select the bar they perceived was ‘darker’

overall by pressing the T (“top”) or B (“bottom”) keys. Participants had 5s to make their

response – if no response was recorded, the trial was repeated after the remaining

trials.


Figure 5.1. Example of a stimulus presented in the greyscales task

Transcranial Direct Current Stimulation (tDCS)

Stimulation was applied to the right PPC using a battery-driven stimulator

(Dupel Iontophoresis System, MN) via a pair of 6x4cm electrodes placed on the scalp in

saline-moistened sponge pouches. Electrode configuration followed that used in prior

work (Loftus & Nicholls, 2012; Roy et al., 2015; Sparing et al., 2009). During anodal

stimulation, the reference was placed at the Cz position according to the International

10-20 System (Klem, Lüders, Jasper, & Elger, 1999), and the active electrode at P4. This

configuration was reversed for cathodal stimulation. For anodal and cathodal

stimulation, the current was gradually ramped up to 2mA over 30s and maintained at

this level for the duration of the session. Mean stimulation duration was 9.76 min (SD =

2.06) and a repeated measures analysis of variance (ANOVA) showed that durations

did not differ across stimulation conditions or ALT groups (all ps > 0.55, all ηp2s < 0.02).

Procedure

Participants completed the experiment over two days separated by a minimum

24-hour period (see Figure 5.2 for a diagram of the experimental flow). On each day,

participants completed two 168-trial sessions of the greyscales task. There were four

session types. In the initial practice session, participants completed the task without

wearing the tDCS apparatus. In the anodal and cathodal sessions, the respective

stimulation was applied during the task. In the sham (placebo) condition, the current

was ramped up to 2mA, but immediately ramped down again over 30s. This was

intended to create a physical experience like anodal and cathodal stimulation with

minimal or no effect on neural excitability (Loftus & Nicholls, 2012; Sparing et al.,

2009). The administration order of the anodal and cathodal conditions was

CHAPTER FIVE 115

counterbalanced across participants and always followed the practice/sham sessions

to avoid carry-over stimulation effects between sessions.

Figure 5.2. Order of administration of the tDCS conditions, with half of the sample

receiving the first order and half the second.

Results

Low- and High-ALT groups were created using a median split of AQ scores

(median = 107) following previous methodology (English et al., 2015). Accuracy was

calculated as the percentage of trials on which participants selected the darker

stimulus. Pseudoneglect was calculated as the percentage of trials on which

participants selected the bar with most black pixels oriented towards the left side of

the screen.

Four participants (1 Low ALT, 3 High ALT) data were omitted because their

pseudoneglect scores across the anodal, cathodal and sham sessions were identified as

multivariate outliers (Mahalanobis Distance: χ2 > 7.82, p < 0.05). The remaining

participants descriptive statistics are presented in Table 5.1. An independent-samples

t-test confirmed that AQ scores differed between Low- and High-ALT groups, t(32) =

8.20, p < 0.001, d = 2.90.


Table 5.1. Descriptive statistics for the Low and High ALT groups (standard deviations

presented in parentheses)

Low ALT (n=18) High ALT (n=16)

Sex 8 male, 10 female 10 male, 6 female

Mean age (years) 21.28 (1.23) 21.19 (2.00)

Mean AQ 95.67 (9.75) 120.63 (7.72)

A one-sample t-test verified that mean task accuracy (M = 55.25%, SD = 5.87%)

was significantly greater than chance levels (50%), t(31) = 5.21, p < 0.001, d = 1.68. We

also verified that any changes in pseudoneglect were not attributable to variations in

accuracy by submitting mean accuracy scores to a 2 (ALT Group; Low or High) x 3

(Stimulation Type; anodal, sham and cathodal) repeated-measures ANOVA. No main

effects or interactions were found (all ps > 0.23, all ηp2s < 0.04).

To test our main research question, mean pseudoneglect scores were

submitted to a 2 (ALT Group) x 3 (Stimulation Type) ANOVA (mean scores illustrated

in Figure 5.3). This analysis revealed no main effect of AQ Group1, p = 0.36, ηp2 = 0.03,

but a main effect of Stimulation Type, F(2,34) = 14.06, p < 0.001, ηp2 = 0.31, and,

importantly, an ALT Group x Stimulation Type interaction, F(2,34) = 5.83, p < 0.01, ηp2

= 0.15. To determine the source of the interaction, we used paired-samples t-tests

(Bonferroni corrected) to compare pseudoneglect scores in the sham condition relative

to the anodal and cathodal conditions separately for each ALT group. The High ALT

group showed a significant difference between sham and anodal stimulation

conditions, t(15) = 4.09, p < 0.001, d = 0.65, and a near-significant difference between

sham and cathodal stimulation conditions, p = 0.06, d = 0.28. For the Low ALT group,

sham stimulation did not differ from either anodal or cathodal stimulation (both

ps>0.35, both ds<0.07).

1 It could be argued that an absence of a main effect of AQ group here can be considered a failure to replicate the findings presented in Chapter 2 and 3. However, it is more likely that the participant sample in this study was merely not large enough to obtain a significant comparison. This is not problematic, as the purpose of the current study is to address whether tDCS differentially influences spatial biases between AQ groups – a question that can be answered without the larger sample sizes recruited in the previous studies.

CHAPTER FIVE 117

Figure 5.3. Pseudoneglect levels associated with each of the three types of stimulation

(error bars represent one standard error of the mean). The dashed line highlights the

level at which no pseudoneglect is present. *** p < 0.001.

Discussion

Following behavioral evidence that reduced levels of pseudoneglect and global

processing in individuals with ASC/High ALT potentially reflect relatively lower

activation of the RH (English et al., 2015, 2017), the present study examined whether

tDCS applied over the right PPC could alter attentional biases in these individuals.

Consistent with this hypothesis, anodal tDCS significantly increased pseudoneglect in

our High ALT group relative to sham stimulation, while not yielding a significant

increase in pseudoneglect in our Low ALT group. Such a pattern of results would be

expected as the result of baseline pre-stimulation differences in RH activation between

the two groups (English et al., 2015; Loftus & Nicholls, 2012). That is, the increased

cortical excitability arising from anodal tDCS would more effectively increase RH

activation in our High ALT group with lower pre-stimulation baseline, than in the Low

ALT group, with relatively higher pre-stimulation RH activity. In turn, this would yield


greater increases in levels of pseudoneglect in the High ALT group than in the Low ALT

group. However, this account is somewhat speculative given that a baseline group

difference in hemispheric activation was not observed in the present study and further

investigation would be needed to confirm this interpretation.

Our findings also offer a fresh perspective on the failure of Loftus and Nicholls

(2012) to observe an effect of right-PPC anodal stimulation on pseudoneglect in an

ALT-unselected neurotypical sample. While the authors proposed that this outcome

reflected generally high levels of right PPC pre-stimulation activation, our results

suggest that this explanation may not adequately account for individual differences

arising from variations in levels of autistic traits. Put differently, it is likely that testing

an unselected neurotypical sample which, collectively, likely had relatively high levels

of RH activation, masked effects of anodal tDCS – effects which may be more readily

apparent in a subset of individuals with relatively higher levels of autistic traits (and

thus lower RH baseline activation).

Cathodal tDCS failed to produce any significant reductions in pseudoneglect for

either ALT group. One plausible explanation is that factors other than baseline

activation more strongly modulate the impact of cathodal stimulation. Indeed, effects of

cathodal stimulation have not been observed in numerous studies (for a review, see

Jacobson, Koslowsky, & Lavidor, 2012), and a multitude of factors, including

stimulation duration, location and intensity, as well as task complexity, seem to

contribute to situations where anodal and cathodal stimulation do not lead to

systematic changes (Vallar & Bolognini, 2011). Given that the effects of cathodal

stimulation are generally less robust than those arising from anodal stimulation, we

suggest that further research is required to better understand the possible implications

of the lack of cathodal stimulation effects on pseudoneglect.

In summary, this study is the first to our knowledge to show that atypical

attentional biases in individuals with High ALT may be modulated using non-invasive

cortical stimulation. If pseudoneglect can be shifted (at least, temporarily), what other

aspects of attention could be similarly altered regarding autism? Spatial processes that

have a relatively greater reliance on regions in the RH, such as global processing,

potentially stand to benefit from techniques such as tDCS. For example, tDCS could

assist with increasing otherwise low levels of RH activation that may be contributing to

slowed global processing in autism (Van der Hallen et al., 2015), which is itself linked

with less accurate/efficient processing of socially relevant information such as faces

(Behrmann, Avidan, et al., 2006; Gross, 2005). Furthermore, determining the extent to

CHAPTER FIVE 119

which attentional modulations in individuals with ASC/High ALT can be made to

persist over time will help establish the scope of changes that could arise from

techniques to stimulate RH activity. A limitation of the present study that could be

addressed in future work is the relatively small sample size observed, and a replication

with a larger sample would assist in confirming the present findings. Finally, it is also

critical to confirm the present findings using a sample with ASC as while individuals

with High ALT are susceptible to attentional modulations following tDCS, this may not

be true of individuals with ASC.

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CHAPTER SIX General Discussion

Central Aims of this Thesis

While early conceptualizations of autism focused on the role of the child’s social

environment in the disorder, more contemporary theories have incorporated research

findings from studies of cognitive and perceptual mechanisms including spatial

attention. The weak central coherence model, for example, suggests that many autistic

behaviours stem from a failure to integrate independent pieces of information into a

coherent whole, something that is demonstrated by autistic individual’s superior

attention to detail on spatial tasks such as the Embedded Figures Test (EFT). With the

development of psychometric tools, such as the Autism Spectrum Quotient (AQ; Baron-

Cohen, Wheelwright, Skinner, Martin, & Clubley, 2001), it is now known that atypical

spatial attention is not strictly limited to those individuals diagnosed with autism

spectrum conditions(ASC). Similar atypical attention effects are often seen in

neurotypical individuals with high levels of autistic traits (High AQ) compared to

individuals with low levels of autistic traits (Low AQ). That said, many important

questions remain about the mechanisms that underlie atypical distribution of spatial

attention seen in both ASC and High AQ individuals.

The objective of the present thesis was to investigate whether relatively

reduced right hemisphere activation contributes to atypical spatial attention seen in

individuals with High AQ. This objective was divided into two separate, but related,

research aims. The first aim was to produce novel behavioural evidence that reduced

RH activation mediating spatial attention is linked to increasing levels of autistic-like

traits. This was achieved by comparing the spatial biases of Low and High AQ

124 GENERAL DISCUSSION

participants on several attentional measures, such as the greyscales and landmark

tasks. The second aim extended the first by examining whether aspects of spatial

attention considered atypical in ASC and High AQ, such as poor global processing, may

be overcome in High AQ individuals by using techniques that increase RH activation. In

what follows, findings and implications drawn from two studies investigating the first

aim are summarized and discussed, before moving on to a similar treatment of the two

studies that investigated the second aim.

Part 1: New Evidence for Reduced Right Hemisphere

Activation for adults with autistic-like traits

Individuals with autistic-like traits show reduced lateralization on a

greyscales task

In the first experimental chapter of this thesis, Chapter Two, attentional biases

on the greyscales task (Nicholls, Bradshaw, & Mattingley, 1999) were observed for

neurotypical adults with low or high levels of autistic traits, as measured using the AQ

(Baron-Cohen et al., 2001). Tasks that index attentional lateralization, such as

greyscales, landmark and line bisection tasks, are useful indicators of underlying

asymmetries in hemispheric activation, and are often used to observe left hemispatial

neglect following right-sided cortical lesions (Heilman, Watson, & Valenstein, 1997).

Previous work has found a reduced leftward bias by ASC individuals when

viewing face stimuli (Ashwin, Wheelwright, & Baron-Cohen, 2005; Dundas, Best,

Minshew, & Strauss, 2012; Guillon et al., 2014), but it was not clear if this attention-

related behaviour would generalize to other types of visual stimuli or to neurotypical

individuals with High AQ. However, given that both ASC and High AQ participants show

superior performance, relative to healthy controls and Low AQ participants

respectively, for attentional tasks that do not use face stimuli, such as the EFT (Cribb,

Olaithe, Di Lorenzo, Dunlop, & Maybery, 2016; Muth, Hönekopp, & Falter, 2014), it was

predicted that High AQ participants would also show a reduced leftward attentional

bias, or pseudoneglect (Bowers & Heilman, 1980), on the greyscales task.

To maximize the likelihood of detecting an effect of AQ scores on

pseudoneglect, a relatively large number of participants (n = 277) were recruited

across the entire distribution of possible scores. When analysing the sample as two

groups based on whether participant’s AQ scores were above or below the sample

median (Low vs High AQ), pseudoneglect was found to be significantly lower in the

CHAPTER SIX 125

High AQ group compared to the Low AQ group, though still above chance levels

(indicating that pseudoneglect was still present in the High AQ group). Furthermore,

while overall AQ scores did not directly correlate with levels of pseudoneglect, the

social skills factor score of the AQ (Russell-Smith, Maybery, & Bayliss, 2011) was

associated with pseudoneglect levels, with increased social difficulty linked to reduced

pseudoneglect.

Beyond greyscales: reduced pseudoneglect for other tasks

The experiment outlined in Chapter Three extended the findings presented in

Chapter Two. The goal here was to replicate the initial results obtained using the

greyscales task and to determine whether differences in attentional bias between Low

and High AQ individuals could also be observed on a landmark task and a mental

number line task. As the findings presented in Chapter Two were the first to show

differences in attentional lateralization as a function of autistic traits, a replication was

prudent. The inclusion of the landmark task was designed to test if some unique aspect

of the greyscales stimuli was driving the reduced pseudoneglect seen for High AQ

participants in Chapter Two, or if reduced pseudoneglect could be observed across

variations in visual stimuli. The inclusion of a mental number line task, which does not

require judgements about visual stimuli, allowed us to examine whether Low and High

AQ individuals also showed different attentional biases for mental representations of

space.

The findings reported in Chapter Two were replicated, with significantly

reduced levels of pseudoneglect present in the High AQ group for both the greyscales

and landmark task in this study. Furthermore, unlike the findings in Chapter Two,

pseudoneglect was absent in the High AQ group in both of these tasks. However,

analyses of pseudoneglect on the mental number line task differed to all the findings

presented thus far, with significant and statistically indistinguishable levels of

pseudoneglect observed for the Low and High AQ groups.

Part 1: Discussion

Given the evidence that global processing is impaired in ASC (for review, see

Van der Hallen, Evers, Brewaeys, Van den Noortgate, & Wagemans, 2015), and that

less-active RH regions responsible for global processing may, at least partially,

contribute to this impairment (for review, see Happé & Frith, 2006), it is somewhat

surprising that few studies have sought converging behavioural evidence for RH-based

spatial attention impairments in ASC. While several studies do demonstrate that ASC


individuals (Dundas, Best, et al., 2012; Guillon et al., 2014), and infants with ASC

siblings (Dundas, Gastgeb, & Strauss, 2012), show a reduced leftward bias for viewing

faces indicative of reduced RH lateralization for spatial attention, these findings have

not been replicated for non-face stimuli. The finding reported in Chapter Two is

therefore the first, to my knowledge, to show that attentional biases are atypical in a

group of High AQ individuals viewing non-face stimuli. Confirming this finding, the

study presented in Chapter Three replicated this effect and demonstrated that a similar

pattern exists for the landmark task. Should similar patterns also be present in a

clinical ASC sample, reduced (or absence of) pseudoneglect could be interpreted to be a

potentially new phenotypic expression of ASC that is indicative of reduced RH

activation for spatial attention. The importance of extending the present findings with

neurotypical individuals to ASC groups is discussed in greater detail below.

It should be noted that one study offers evidence that potentially conflicts with,

or at least qualifies, the present findings. Rinehart, Bradshaw, Brereton and Tonge

(2002) reported that spatial biases measured using the greyscales task in children with

ASC were no different to those seen in typically developing (TD) children. On the face

of it, this result would seem to indicate a disparity between Low/High AQ and TD/ASC

comparisons for pseudoneglect. However, the interpretability of the results is

obfuscated by the fact that the TD control group in Rinehart et al. (2002) failed to show

any pseudoneglect, which conflicts with other demonstrations of pseudoneglect in

children (Dellatolas, Coutin, & De Agostini, 1996), and evidence that pseudoneglect is

more prominent in younger children and decreases with age (Jewell & McCourt, 2000).

It is also possible that the present significant findings stem from the use of a larger

sample, additional trials, and the ability to screen participant engagement based on

task accuracy (Rinehart et al.'s stimuli were equiluminant and therefore their task did

not have ‘correct’ or ‘incorrect’ trials). In the end, a study that combines the present

experimental methodology with Rinehart et al.'s ASC and TD comparison groups would

help provide definitive answers as to the reasons for the disparate findings.

The results reported in Chapter Two regarding the relationship between levels

of pseudoneglect and scores on individual factors within the AQ were unexpected.

Specifically, the social skills component score and greyscales pseudoneglect index were

negatively correlated (i.e., greater social difficulties were linked to reduced

pseudoneglect), whereas no such relationship was apparent between the

pseudoneglect index and the details/patterns component score of the AQ. A similar

result was reported by Russell-Smith and colleagues (2012) who found that superior

CHAPTER SIX 127

EFT performance correlated positively with scores for the social skills factor of the AQ

(i.e. faster EFT reaction times (RTs) were associated with greater social difficulty), but

not with scores for the details/patterns factor. It is interesting that, in both cases,

attentional performance was more closely associated with the social ability factor of

the AQ, which suggests that the mechanisms responsible for reduced pseudoneglect

and faster EFT RTs in High AQ individuals potentially have links to social functioning.

However, it should also be noted that this relationship was somewhat weak in the

results presented in Chapter Two, and it is possible that the association with the social

skills factor is in part a result of the factor also being the largest (and therefore best

reflection of overall AQ) out of the three.

That being said, previous research has noted that the individual subfactors

comprising the AQ are relatively independent, as scores for factors relating to social

ability and factors encompassing rigid interests, details and patterns are not observed

to correlate (Dworzynski, Happé, Bolton, & Ronald, 2009; Russell-Smith et al., 2011).

The absence of a close relationship between these factors is consistent with how

pseudoneglect and EFT performance (Russell-Smith et al., 2012) correlate with the one

factor, but not the other. However, further inspection of the AQ factor scores for the

study reported in Chapter Two reveals that the social skills and details/patterns factor

scores were, in fact, modestly correlated (Pearson correlation: r = 0.18, p < 0.01). Thus,

even in populations for which the two factors do correlate, it is the social skills scores,

and not the details/patterns scores, that are associated with perceptual performance.

Assuming the relationship between pseudoneglect and social ability is not

coincidental, why are the two associated? One possibility is that both reduced

pseudoneglect and social difficulty are partly driven by relatively reduced RH

activation for spatial attention. Pseudoneglect itself likely manifests due to the RH

lateralization of spatial attention in neurotypical individuals (Fierro et al., 2000; Fink,

Marshall, Weiss, Toni, & Zilles, 2002; Foxe, McCourt, & Javitt, 2003; Harris & Miniussi,

2003; Siman-Tov et al., 2007; Vingiano, 1991). The RH is also linked to central

coherence (Evans, Shedden, Hevenor, & Hahn, 2000; Flevaris, Bentin, & Robertson,

2010; Hübner & Studer, 2009; Lux et al., 2004; Malinowski, Hübner, Keil, & Gruber,

2002; Volberg & Hübner, 2004; Weissman & Woldorff, 2005; Yamaguchi, Yamagata, &

Kobayashi, 2000) and face processing ability (Clark et al., 1996; Haxby et al., 1994,

1999; Kanwisher, McDermott, & Chun, 1997; McCarthy, Puce, Gore, & Allison, 1997;

Rossion, Schiltz, & Crommelinck, 2003; Sergent, Ohta, & Macdonald, 1992), which, in

turn, contributes towards communication ability and overall social functioning


(Dawson, Webb, & McPartland, 2005). Further investigation is required to confirm that

reduced RH activation underlies levels of pseudoneglect expression, central coherence,

and social ability as measured using the AQ.

The absence of group differences for the mental number line task suggests that

spatially-organized mental representations are not influenced by higher levels of

autistic traits like their perceptually derived counterparts. This dissociation resembles

the results of previous studies of patients with RH damage that showed dissociations

between effects on perceptual and representational biases (Doricchi, Guariglia,

Gasparini, & Tomaiuolo, 2005; Loetscher & Brugger, 2009; Loetscher, Nicholls, Towse,

Bradshaw, & Brugger, 2010; Pia et al., 2012; Rossetti et al., 2011; Storer & Demeyere,

2014; van Dijck, Gevers, Lafosse, & Fias, 2012). This would also suggest that the

mechanism that leads to reduced pseudoneglect with increasing autistic traits is likely

linked to the sensory system, given that no differences are present in the absence of

needing to visually inspect stimuli.

A key question arising from the findings reported in Chapters 2 and 3 concerns

the nature of cortical changes underlying performance differences between High and

Low AQ individuals. Previous work has shown that changes in relative activation of the

left and right posterior parietal cortex (PPC) appear to induce systematic attentional

shifts on tasks that index the lateralization of spatial attention. In neurotypical

individuals, using anodal tDCS to increase right PPC activation is associated with a

leftward shift in attention (Roy, Sparing, Fink, & Hesse, 2015; Sparing et al., 2009).

Similarly, using TMS to disrupt right PPC activation or cathodal tDCS to increase left

PPC activation leads to a rightward shift in the perceived midpoint of horizontal lines

(Fierro, Brighina, Piazza, Oliveri, & Bisiach, 2001; Loftus & Nicholls, 2012). TMS-related

changes in hemispheric activation have been confirmed with fMRI, as TMS over the

right angular gyrus/intra-parietal sulcus in neurotypical individuals has been found to

abolish pseudoneglect and shift the relative balance of parietal activity from right to

left (Petitet, Noonan, Bridge, O’Reilly, & O’Shea, 2015). It can be inferred from the

results of these studies, that a strong candidate for the neural substrate for reduced

pseudoneglect in the present High AQ groups is the right PPC.

Notably, changes in PPC activation have also been strongly linked with

attentional shifts for mental representations of space – for example, disrupting right

PPC activation of neurotypical participants using TMS leads to rightward shifts on a

number line bisection task (Göbel, Calabria, Farnè, & Rossetti, 2006; Göbel, Walsh, &

Rushworth, 2001). Nevertheless, significant differences between Low and High AQ

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groups on the mental number line task in Chapter Three were not observed. This may

suggest that areas other than the PPC can be linked to representational pseudoneglect

in High AQ participants. One potential candidate could be right prefrontal working

memory structures, which are linked to maintaining the mental number line (Doricchi

et al., 2005). These areas do not show activity changes during perceptual-based

lateralization tasks, which are instead strongly associated with activation of the striate

and extrastriate visual cortex and parietal lobes (Doricchi & Angelelli, 1999; Fink et al.,

2000). It is possible that functioning in these memory-related regions remains intact

for individuals with higher levels of autistic-like traits and that this compensates for

reduced right PPC activation when accessing mental representations of space. Further,

given these prefrontal areas are not readily involved in lateralization judgements of

visual stimuli, the absence of compensatory mechanisms in the presence of visual

cortex and parietal lobe deficits may account for the disparate results across

perceptual and representational measures of space.

Rather than a deficit in a single brain region, it is also possible that a neuronal

pathway abnormality may underlie reduced pseudoneglect for High AQ individuals.

Visual processing beyond the primary visual cortex is largely divided into two separate

pathways. The ventral stream/parvocellular pathway is associated with the processing

of higher spatial frequencies and stimulus recognition. The dorsal

stream/magnocellular pathway, is associated with processing lower spatial

frequencies, motion and spatial relations between objects (Culham, He, Dukelow, &

Verstraten, 2001; Felleman & Van Essen, 1991; Livingstone & Hubel, 1988; Merigan,

Byrne, & Maunsell, 1991; Merigan & Maunsell, 1993; Young, 1992). Spencer et al.

(2000) and Milne et al. (2002) suggested that deficits may exist in the dorsal stream for

ASC individuals after finding that individuals with ASC have atypically large thresholds

for the detection of coherent global motion. Consistent with this possibility, global

coherence thresholds are inversely related to EFT performance for children with ASC,

indicating a link between ASC and reduced involvement of global processing for both

static and moving global arrangements (Pellicano, Gibson, Maybery, Durkin, & Badcock,

2005), with identical findings reported when comparing neurotypical individuals with

High and Low AQ (Grinter et al., 2009). Critically, the dorsal stream has greater input

into parietal regions than the ventral stream (Milner & Goodale, 2006) and there may

be weakness within the dorsal stream that leads to the right PPC. In short, this account

suggests that reduced pseudoneglect in High AQ individuals is potentially due to


deficits existing in the dorsal stream/magnocellular pathway that connects to the right

PPC, rather than deficits existing in the right PPC itself.

While reduced RH activation may result in reduced pseudoneglect on tasks that

index lateralization, and greater attentional preferences to the local-level of visual

stimuli, it remains unknown the extent to which these two aspects of attention are

directly associated. Potentially, global processing styles dominate when observing the

visual stimuli presented in Chapter Two and Three (Ciçek, Deouell, & Knight, 2009;

Fink et al., 2002; B. H. Lee et al., 2004; Nicholls, Mattingley, & Bradshaw, 2005), and the

relatively greater RH activation that results from engaging in global processing in turn

drives greater levels of pseudoneglect. Consequently, High AQ participants may be less

inclined to engage in global processing when viewing these stimuli, resulting in

reduced hemispheric asymmetry in activation levels and reduced pseudoneglect.

Evidence in support of this notion comes from a study conducted by Nicholls,

Mattingley and Bradshaw (2005), who reported that neurotypical participant’s

pseudoneglect for greyscales stimuli is reduced when the use of local processing

strategies is facilitated by segmented the stimuli into halves or quarters.

With this finding in mind, one avenue for future research would be to

determine how pseudoneglect in Low and High AQ participants is altered due to

segmentation – if only the Low AQ group sees reductions in pseudoneglect, an absence

of change in the High AQ group could be interpreted as evidence suggesting that they

are already using a more locally-oriented processing style, which in turn alters

hemispheric asymmetries in activation that potentially accounts for the reduced

pseudoneglect in this group. This line of reasoning also offers another possible account

for why differences in pseudoneglect between AQ groups were not present in the

mental number line task: an absence of a visual stimulus to inspect meant that

global/local processing preferences were not engaged.

Future studies should also examine whether a common neural mechanism

underlies pseudoneglect and the expression of global/local biases on measures like the

EFT. Evidence from neuroimaging studies suggests that the ability to perceive target

shapes in EFT stimuli (disembedding) is linked to LH functioning. In one study, EFT

completion was associated with greater left inferior and left superior parietal cortex

activation relative to completing a visual search control task for neurotypical

participants (Manjaly et al., 2003). Another study revealed that ASC children

completing the EFT also showed left superior parietal activation, relative to TD

children who showed bilateral activation of the same region (P. S. Lee et al., 2007).

CHAPTER SIX 131

Finally, in neurotypical individuals, superior EFT disembedding is associated with

greater volumes of grey matter in the left inferior parietal lobule (Hao et al., 2013).

Given this evidence, together with the findings that suggest that relative left and right

PPC activation influences pseudoneglect, I expect that reduced pseudoneglect arising

from relatively reduced RH activation, would be associated with superior EFT

disembedding. To my knowledge, only one study has examined the two constructs

together, confirming that reduced pseudoneglect is associated with superior EFT

disembedding (Glicksohn & Kinberg, 2009). However, further work is required to

establish the strength of this relationship and the contribution of autistic traits to it.

Research is presently being conducted in the laboratory to investigate this extension of

the work reported in this thesis.

Part 2: Increased Right Hemisphere Activation

‘Corrects’ Attention in Adults with Autistic-Like Traits

Chapters Four and Five were a logical progression of the research reported in

the preceding two chapters, shifting from the observation of spatial attention

differences linked to reduced RH activation in High AQ individuals, to directly testing

whether increasing RH activation can alter the distribution of spatial attention in these

individuals. The experimental designs used in Chapters Four and Five were selected on

the basis that, in unselected neurotypical participants, they had been shown to alter

aspects of spatial attention considered atypical in ASC and High AQ. These aspects

included global processing ability (Cribb et al., 2016; Van der Hallen et al., 2015) and

the distribution of spatial attention, as demonstrated in Chapters Two and Three, and

other studies (Ashwin et al., 2005; Dundas, Best, et al., 2012).

Improving global processing using a RH-activating continuous

performance task

In Chapter Four, I tested whether an attentional training task could alter biases

towards the global and local levels of hierarchical Navon figures, in different ways for

groups selected to differ in levels of autistic-like traits. Previous work has found that

engaging in a continuous performance task can result in leftward spatial shifts on a

landmark task (Degutis & Van Vleet, 2010) and increases in attention directed towards

the global level of Navon figures (Degutis & Van Vleet, 2010) for RH-damaged patients

with left neglect. Critically, both of these attentional changes can be explained by

relative increases in RH activation arising from training on the CPT. Given that the

present thesis presents additional evidence for reduced RH activation for spatial


attention in High AQ individuals (Chapters Two and Three), and global processing is

closely linked with RH activation (for a review see Ivry and Robertson, 1998), it

seemed appropriate to investigate whether CPT could increase attention directed

towards the global-level of visual stimuli for neurotypical individuals with High AQ.

To this end, two experiments were carried out. The first experiment was

designed to confirm the findings reported Van Vleet et al. (2011) with an unselected

sample, while the second was designed to test whether CPT training influenced global

processing in neurotypical participants with high levels of autistic like traits. The

results of the replication study showed the same key findings as Van Vleet et al. (2011)

– after training, participants were better at ignoring the local level of Navon figures

when directed to categorize the global level, and worse at ignoring the global-level

when directed to categorize the local-level. In the second experiment, which included

neurotypical individuals with Low or High AQ scores, an interesting dissociation was

found between the two AQ groups. Following CPT training, Low AQ individuals were

better able to ignore local-level distractors during global categorization, while

interference from global distractors during local categorization was unchanged. In

stark contrast, High AQ participants were less able to ignore global-level distractors

during local categorization, but saw no changes in their attention to local-level

distractors during global-categorization, following the CPT training. Also of

significance, the present findings did depart from Van Vleet et al. (2011) in one key

respect as training effects in both experiments persisted for a longer duration.

Increasing pseudoneglect using cortical stimulation

In Chapter Five, the final study of this thesis, the activation level of the right

PPC in High and Low AQ individuals was modulated using tDCS and the resulting

changes in the distribution of spatial attention observed using the greyscales task. The

greyscales task was chosen as the measure for observing tDCS-related effects on

attention because it is simple in design (all trials contributing to a single pseudoneglect

index) and showed reliable differences between Low and High AQ participants in

earlier studies. In contrast, the Navon figure categorization task described in Chapter

Three is more complex (four conditions required to index the two forms of

interference) and it did not provide reliable baseline differences between samples of

Low and High AQ individuals.

The study in Chapter Five was motivated by previous work demonstrating that

tDCS and TMS are effective at shifting attention towards the left or right-side of visual

CHAPTER SIX 133

space depending on whether stimulation excites or disrupts activation of the left or

right PPC (Fierro et al., 2000, 2001; Loftus & Nicholls, 2012; Petitet et al., 2015; Roy et

al., 2015; Sparing et al., 2009). In particular, Sparing et al. (2009) showed that

excitatory, anodal tDCS over the RH was effective at reducing left hemispatial neglect

symptoms of RH-damaged patients, shifting attention leftward. With these results in

mind, it seemed reasonable to hypothesize that evidence for atypical lateralization for

spatial attention seen in High AQ participants in Chapters Two and Three might be

altered using tDCS applied over the right PPC.

Consistent with this possibility, anodal tDCS applied over the right PPC led to

significantly greater levels of pseudoneglect for a High AQ group relative to a sham

(control) stimulation condition. In contrast, no changes in pseudoneglect were

observed in a Low AQ group. However, cathodal tDCS was not effective at producing

systematic changes for either the High or Low AQ group, suggesting only excitation of

the RH is able to alter spatial bias in High AQ individuals.

Part 2: Discussion

While substantial research has investigated the extent to which attention in

ASC deviates from neurotypical profiles, to my knowledge, there are no studies that

have attempted to overcome any of these deviations using methods that increase RH

activation. This is somewhat surprising, given that prominent theories of autism, like

the weak central coherence account (Frith, 1989), have posited a central role for

attentional deficits that are linked to reduced RH activation. It is timely then that the

results presented in Chapters Four and Five essentially provide the first indications

that attentional patterns associated with ASC are not necessarily fixed and, at least for

neurotypical individuals with High AQ, may be modulated with techniques that

increase RH activation.

In both experiments in Chapter Four, CPT-related training effects were more

enduring than those recorded by the original authors (Van Vleet et al., 2011). In

comparing results of these studies, it was noted that reaction times during the CPT

tended to be faster in the present study, but accuracy worse, indicative of a speed-

accuracy trade-off. This pattern suggests that CPT-related attentional changes may be

chiefly driven by participants making speeded responses, rather than correctly

inhibiting responses per se. This notion is further supported by the fact that

participants who completed the categorization control task (CCT) were not required to

make speeded responses and did not show changes in global processing. Of course, CCT


also differed from CPT in that participants were required to respond to all stimuli,

rather than inhibit certain stimuli classes, as was the case for CPT. Consequently, it is

unknown to what extent the attentional mechanisms responsible for making speeded

responses and those mechanisms responsible for correct response inhibition underlie

attentional shifts towards the global level of Navon figures. Subsequent studies using

this training paradigm may want to consider using variations of the current CCT to

determine the extent to which speeded responses or correct inhibition make

contributions towards CPT-related training effects. This could be achieved by altering

CCT, which requires participants to make non-speeded categorizations of two stimulus

classes (upright vs inverted images) each with 50% presentation rates, to a design

where participants make speeded categorizations. With this change, it could be

determined whether speeded-CCT induces attentional changes similar to CPT. Such a

finding would indicate that attentional mechanisms responsible for speeded responses,

rather than response inhibition, are associated with attentional shifts in global/local

biases.

The present findings that individual differences in AQ scores lead to varying

CPT-related attentional changes, has implications for the interpretation of Van Vleet et

al.'s (2011) results. While their findings seemingly suggest that all neurotypical

individuals show alterations on globally-directed and locally-directed variants of the

Navon figure task as a result of CPT, the present study does not support this claim.

Instead, it is likely that training effects varied with AQ levels, but that these variations

were unobservable since AQ levels were not assessed. This notion is supported by the

fact that training effects seen in Experiment 2, when collapsed across AQ groups,

appear identical to those reported in Experiment 1 and in Van Vleet et al. (2011). This

has important implications for future studies wishing to use this training paradigm, as

such individual differences appear to have the capacity to significantly influence

attentional training outcomes.

Alternatively, the differences between AQ groups may have in part arisen due

to differences in baseline levels of task performance. Specifically, relatively higher

baseline levels of global interference for local categorization seen in Low AQ

individuals would have made it difficult for further increases in global interference to

be detected post-CPT training (i.e. ceiling effects). Similarly, for the High AQ group,

lower baseline levels of local interference for global categorization would have made it

difficult for further reductions in local interference to be detected post-CPT training

(i.e. floor effects). These differences in baseline levels that were present in the CPT

CHAPTER SIX 135

condition may not be reliable as the AQ groups who received the control training task

were comparable in terms of pre-CPT performance. It is unfortunate that baseline

levels were not well-matched for the High and Low AQ groups across the CPT and

control training conditions. This issue could be addressed in subsequent studies by

having participants complete both types of training, thus controlling for individual

differences.

A further possibility is that additional training would have increased effect

sizes, perhaps, enhancing changes in task interference that did not reach conventional

levels of significance in Experiment 2. The present design followed the methodology

outlined in Van Vleet et al. (2011) and administered a single 16-minute session of CPT,

which was sufficient to produce measurable changes in attention in a neurotypical

sample. By comparison, in Degutis and Van Vleet's (2010) study with RH-damaged

neglect patients, three 12-minute sessions were administered each day over nine

consecutive days. Given the possibility that individuals with ASC may also have

reduced RH activation for spatial attention, a longer training period may have been

required to produce the full-breadth of attentional changes, especially in the High AQ

group.

One final alternative explanation that could account for CPT-related

performance alterations is changes in the size of the spatial attentional spotlight or

window. Like its physical namesake, the attentional spotlight is described as a

moveable “beam” that can narrow or widen its focus in order to facilitate the

processing of visual stimuli (Eriksen & Yeh, 1985; Posner, 1990). Previous work

suggests that children with ASC exhibit impairments with broadening the attentional

spotlight. This was demonstrated on a task where children had to determine whether

the vertical or horizontal line in a crosshair was longer. ASC children were slower and

less accurate than TD children when responding to crosshairs that were larger than the

crosshair presented immediately prior (Mann & Walker, 2003), with the authors

suggesting that ASC is associated with a deficit in the ability to broaden the spatial

spread of attention. This in consistent with the conclusions drawn from a meta-analysis

of global/local processing studies on ASC which indicated that global processing is

slowed in ASC (Van der Hallen et al., 2015).

By this logic, it is possible that the High AQ participants in Chapter Four did not

as readily broaden their attentional spotlights given the opportunity. For example,

prior to CPT training, High AQ participants’ faster reaction times during local

categorization may be partly accounted for by a superior ability to maintain a small


attentional spotlight when shifting between the small fixation cross and the Navon

figure. In contrast, Low AQ participants may have automatically widened the spotlight

in response to the larger Navon figure, slowing their ability to efficiently categorize the

local level of the Navon figure. This might also indicate that the capture of attention in

Low AQ individuals by bottom-up or stimulus-driven mechanisms is stronger than

what is seen for High AQ individuals. However, this account remains uncertain since

similar group differences in baseline RT were not present for participants who

completed CCT. Furthermore, would increasing the size of the fixation cross facilitate

performance for global categorization in High AQ individuals? Presumably, a wider

fixation cross would “prime” the attentional spotlight to a more appropriate size for

processing the global level of the Navon figure, compensating for deficits in the ability

to broaden the attentional spotlight. These possibilities are clearly worth investigating

in future studies.

It is possible that CPT training may further broaden the size of the attentional

spotlight, explaining why global interference increased for the High AQ group following

CPT training, but did not change for the Low AQ group. The findings reported by

Degutis and Van Vleet (2010) could also be explained if CPT increased the size of the

spatial attentional spotlight: rather than increasing relative RH activation of neglect

patients and inducing a ‘leftward’ attentional shift on a landmark task, training could

allow more of the landmark stimulus to be attended to, including the previously

neglected left-side. However, it is unlikely that the anodal stimulation effects arising for

High AQ participants in Chapter Five can be accounted for by a similar widening of the

attentional window, given that a wider window would allow for a greater amount of

the stimulus to be attended to and should therefore reduce pseudoneglect. Further,

there is no documented overlap between the brain region stimulated in Chapter Five,

the right PPC, and regions that are noted to become more active due to CPT-related

attentional demands (Bartolomeo, 2014; Corbetta & Shulman, 2002; Singh-Curry &

Husain, 2009; Sturm & Willmes, 2001; Thiel, Zilles, & Fink, 2004).

In sum, it appears that CPT is a useful tool that can modulate attention in such a

manner that the global level of Navon figures is more salient. However, there remains

several gaps in our understanding with respect to the training paradigm. For example,

it is uncertain the extent to which speeded responses or correctly inhibited responses

during CPT enhances global attentional biases. Furthermore, it is unclear whether CPT

directly facilitates global processing, or if changes in global attentional biases are a

secondary effect from other modulations, such as a widening of the attentional

CHAPTER SIX 137

spotlight. It is also necessary to determine whether CPT can enhance attention to other

globally-organized visual stimuli for High AQ and ASC individuals. For example, could

the training paradigm improve the speed at which High AQ individuals process facial

expressions (English, Maybery, & Visser, 2017), or impair performance on the EFT

(Cribb et al., 2016)? It is clear that substantially more investigation is required to

understand the mechanisms and potential of this training task.

The findings reported in Chapter Five provide interesting new insights into the

results of a similar study conducted by Loftus and Nicholls (2012). These authors

administered anodal tDCS over the right PPC of an unselected neurotypical sample but

observed no changes in pseudoneglect on a greyscales task. They accounted for this

outcome by suggesting that baseline RH activation in their sample was already high,

limiting the ability to further activate this region. However, an alternative explanation

arises from the fact that their unselected sample presumably contained a mix of High

and Low AQ participants. It is possible that anodal tDCS altered pseudoneglect in the

High AQ subset without affecting the majority of Low AQ participants who likely had

already-high levels of right PPC activation. As AQ scores are known to be normally

distributed in the general population (Baron-Cohen et al., 2001; Hoekstra, Bartels,

Verweij, & Boomsma, 2007), is likely that a low proportion of High AQ participants in

the sample tested by Loftus and Nicholls (2012) resulted in no significant effects being

detectable.

Regarding the absence of cathodal tDCS effects, one possibility is that cathodal

stimulation is simply insufficient to overcome relatively high baseline levels of RH

activation. Similar failures to obtain effects following cathodal stimulation have been

previously noted (for a review, see Jacobson, Koslowsky, & Lavidor, 2012), and it

would appear that systematic changes following anodal and cathodal stimulation

should not necessarily be expected given the interaction between a multitude of

competing factors, including stimulation duration, location and intensity, as well the

complexity of the task (Vallar & Bolognini, 2011). Regardless, it would be worth

conducting a follow-up study in which cathodal tDCS is administered to the left PPC in

an attempt to increase pseudoneglect in High AQ individuals. Theoretically, cathodal

tDCS is unlikely to affect the distribution of spatial attention for Low AQ individuals

given that the left PPC is relatively less active for this group and the present finding

suggesting that anodal tDCS further stimulating the dominant right PPC also did not

result in attentional shifts. However, given that the balance of left and right PPC

activation appears not to favour the right PPC as strongly for High AQ individuals as it


does for Low AQ individuals, it is more likely that a leftward shift in attention for a High

AQ group may result from left PPC cathodal tDCS. Should this prediction be correct, it

would provide converging evidence for atypical hemispheric lateralization of spatial

attention in High AQ individuals.

It is encouraging that two different techniques used on two different

attentional tasks were at least partially successful at modulating spatial attention bias

in High AQ participants, essentially ‘reducing the gap’ between their attentional profile

and that of Low AQ participants. These outcomes suggest that the RH is responsive to

different methods of activation, and relatively malleable in its activation levels. While

there is clearly a need for further exploration regarding the possible effects of CPT

training, especially with respect to its effect on behaviours linked to autism, it is

nonetheless apparent that even a single session of CPT is sufficient to increase biases

towards global levels in a High AQ sample. Furthermore, given that global processing is

associated with the RH (Evans, Shedden, Hevenor, & Hahn, 2000; Flevaris, Bentin, &

Robertson, 2010; Hübner & Studer, 2009; Lux et al., 2004; Malinowski, Hübner, Keil, &

Gruber, 2002; Volberg & Hübner, 2004; Weissman & Woldorff, 2005; Yamaguchi,

Yamagata, & Kobayashi, 2000) and understood to be slowed in ASC (Van der Hallen et

al., 2015), it is possible that tDCS may be used to enhance attendance to globally-

arranged stimuli a well.

Implications for ASC and Future Directions

While the findings presented within this thesis further our understanding of

spatial attention and hemispheric activation in individuals with High AQ, and by

extension ASC, they are also not without limitation and future research will certainly be

necessary to explore the limits of results obtained here. The most obvious constraint is

that these studies used Low/High AQ comparisons, rather than neurotypical/ASC

comparisons, and therefore the results do not directly inform the mechanisms

underlying clinical cases of autism. While it will be necessary to confirm the present

findings in an autistic sample, previous attention-related work has shown similar

patterns of results in studies that recruited Low/High AQ and neurotypical/ASC

comparison groups (e.g. performance on the EFT (Cribb et al., 2016; Horlin, Black,

Falkmer, & Falkmer, 2014)) and thus there is no immediate reason to expect that the

present results from High AQ participants would not also be found for a clinical ASC

sample. Though the techniques used in Chapters Four and Five were successful with a

High AQ group, one could question whether a clinical ASC group might be resistant to

such interventions, potentially due to the greater severity of their symptomology.

CHAPTER SIX 139

However, this suggestion is countermanded by the fact that CPT and tDCS techniques

have both been successful at invoking leftward shifts in attention for RH-lesioned

patients (Degutis & Van Vleet, 2010; Sparing et al., 2009). In fact, if anything, stronger

modulation effects may be obtained with an ASC sample because individuals with ASC

should have lower baseline levels of RH activation compared to High AQ individuals.

Assuming pseudoneglect is also reduced (or absent) for individuals with ASC, it

is possible that methods of observing the lateralization of spatial attention, such as the

greyscales or landmark tasks, may hold some diagnostic value, especially as they are

relatively affordable and simple to administer. While by no means diagnostic on its

own, some consideration could be made as to whether such tasks should be included in

the clinicians ‘toolbox’, alongside other attentional tests like the Block Design test and

the EFT, that can potentially detect reduced attendance to the coherent, global

structure. Similarly, reduced (or absent) pseudoneglect on the greyscales task could

indicate the likely presence of social difficulties, given that a correlation was found

between levels of pseudoneglect and the social skills factor of the AQ in Chapter Two

and in the combined analysis reported in this chapter. This notion is not too dissimilar

to how tasks that index attentional lateralization are used to detect diagnose left

hemispatial neglect, which also commonly results following RH damage (Costa,

Vaughan, Horwitz, & Ritter, 1969; Gainotti, Messerli, & Tissot, 1972; Heilman, 1995).

However, further examination of the greyscales task, and others tasks that index

lateralization of spatial attention, is needed to check their reliability and validity and

their association with AQ scores and ASC status. It is especially important that biases in

attention for ASC children receive further investigation. While tasks like line bisection

have been reported to show decreasing levels of pseudoneglect across the lifespan

(Jewell & McCourt, 2000), there has been limited research on how attentional biases

for non-face stimuli may change across the developmental period with respect to ASC.

This gap in the literature could be filled with the incorporation of attentional

lateralization tasks into longitudinal studies examining infants at risk of ASC (e.g.,

infants with an autistic sibling) as they progress through childhood.

One obvious limitation of the results reported in Chapters Four and Five is that

the attentional modulations following increased RH activation are observed for

relatively abstract stimuli that are limited in their real-world ecological validity.

Whether these activation techniques could alter attentional performance for face

perception, for example, remains to be seen, and is a logical ‘next step’ in this line of

investigation. For example, can the otherwise reduced leftward attentional bias for


faces in ASC individuals (Dundas, Best, et al., 2012; Guillon et al., 2014) be increased

using either of these methods, similar to how anodal tDCS increased pseudoneglect in

the High AQ group in Chapter Five? Additionally, as a reduced preference for globally-

organized visual stimuli is associated with impairments in face identity and emotion

recognition in ASC (Behrmann et al., 2006; Gross, 2005), it follows that increased

activation of global processing regions in the RH could benefit face processing in ASC

individuals. Given the present findings, it is feasible that individuals with ASC would

stand to benefit from techniques like CPT and tDCS that result in greater RH activation,

improving their ability to interpret facial cues. Furthermore, as the ability to process

faces accurately and efficiently is essential for successful social interactions (Dawson et

al., 2005), it is possible that such interventions would ultimately lead to increases in

general social functioning – a highly desirable outcome with respect to ASC.

It is also important to investigate how long the performance modulations

arising from CPT and tDCS last, as any potentially therapeutic value that these

techniques might have is likely to depend on the durability of their effects. Regarding

CPT, it is difficult to determine how long training effects should be expected to last. The

study by Van Vleet et al. (2011) found that the benefits from a single session of CPT

were observable for approximately 5-10 minutes, whereas the effects observed in the

replication study in Chapter Four lasted the entire duration of the post-training period

(approximately 12 minutes). Further complicating matters is the study by Degutis and

Van Vleet (2010), in which CPT was administered 36 minutes/day for nine days before

attentional changes on a landmark task were observed. While the RH-damaged

patient’s left neglect remained ameliorated during the day immediately following the

end of the CPT training regimen, neglect had returned by the next follow-up period

which was 14-days post-training. This means that training effects lasted anywhere

between 1 and 13 days. Furthermore, as a longer training period may have been

necessary to induce training effects in the latter study of RH-lesioned patients, and as

RH deficits may also be present in ASC for spatial processing, inducing attentional

changes in ASC individuals may also require relatively longer periods of CPT than what

was administered to the Low/High AQ participants in Chapter Four. The limited extent

of the training may account for why only partial training effects were observed in the

High AQ group post-CPT. A more comprehensive set of training effects may have been

elicited with prolonged, repeated training sessions, as per Degutis and Van Vleet's

(2010) methodology.

CHAPTER SIX 141

Substantially more research exists regarding the effects of tDCS, and studies

demonstrate that the after-effect of stimulation is generally longer lasting than for the

equivalent period of CPT, especially when tDCS is delivered repeatedly and with

greater intensity (for a review, see Paulus, Antal, & Nitsche, 2013). As with CPT, the

amount of tDCS required to produce observable changes in attention and the duration

of these changes may be different for ASC individuals relative to neurotypical

individuals. Unfortunately, very few studies have administered tDCS to ASC

participants (Amatachaya et al., 2014, 2015; D’Urso et al., 2014; Hupfeld & Ketcham,

2016), and none of these, to my knowledge, have stimulated the PPC or explored tDCS-

related effects on spatial attention. Several studies have found that cathodal tDCS

administered to the dorsolateral prefrontal cortex is associated with positive outcomes

on measures of autistic behaviours (Amatachaya et al., 2014, 2015; D’Urso et al., 2014).

For example, multiple sessions of tDCS administered over a period of several weeks

resulted in positive changes in social and health/behaviour domains as measured by

the autism treatment evaluation checklist (Amatachaya et al., 2015; Geier, Kern, &

Geier, 2012). Furthermore, another study concluded that, although there are some

difficulties, administering tDCS to children with ASC is feasible and capable of inducing

long-lasting behavioural changes (Hupfeld & Ketcham, 2016). These reports, together

with the findings in the present thesis, suggest that the field of ASC could stand to

benefit from further exploration of the potential therapeutic use of tDCS.

It should also be noted that a large portion of the present thesis discusses

spatial attention of High AQ individuals with respect to behavioural evidence for

reduced RH lateralization of spatial attention and associated attentional changes. While

behavioural evidence can be a good indicator of the relative activation of specific brain

regions, it is still limited by the fact that it is only an indirect measure of hemispheric

activation. As such, attempts should be made to confirm that actual differences in

hemispheric activation map on to the behavioural measures used in the present thesis

in the expected directions. The use of neuroimaging techniques, like functional

magnetic resonance imaging and near-infrared spectroscopy, could assist with

answering several pertinent questions. For example, neuroimaging could be used to

confirm that High AQ individuals show relatively reduced RH activation during the

greyscales task and probe which brain regions are more active when engaging in CPT

relative to CCT. The answers to such questions are necessary to further our

understanding of the precise mechanisms that underlie the attentional differences and

training effects reported in the present thesis.


Final thoughts

In summary, the primary aims of this thesis were to provide novel evidence

that High AQ individuals have reduced RH activation that results in abnormal

lateralization of spatial attention, and to explore how techniques that activate the RH

could modulate these attentional shifts. The present research highlights a relatively

novel aspect of attention (pseudoneglect) that reduces as a function of increasing levels

of autistic-like traits, which may be useful for detecting atypical lateralization of spatial

attention in ASC. Furthermore, this research also demonstrates that attentional

differences between Low and High AQ individuals can be reduced using both indirect

(CPT) and direct (tDCS) methods of RH activation, and that these techniques might also

be applied to other aspects of visual attention, such as face perception.

It is my sincere hope that aspects of the research presented in this thesis may

one day be used to help design new methods of early intervention for children with

ASC. As stated several times throughout this thesis, it is possible that the RH-activating

properties of CPT and tDCS have the potential to increase activation of regions

responsible for global processing, which in turn is closely associated with face

processing ability. If reduced RH activation for spatial attention in ASC ultimately

contributes to impaired facial processing, and consequently social functioning, then

encouraging greater use of the RH at early ages may result in improved outcomes in

these areas. For example, taking the key components that underlie RH activation

following CPT and embedding them into a game that a child can play several times a

week may encourage and strengthen activation of RH regions responsible for accurate

and efficient face processing, and short tests assessing lateralization of attention could

provide a quick assessment of the impact of CPT training. Although such a scenario is

currently a distant goal, the studies presented within this thesis take several strides

towards achieving such a reality.

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