The effect of individual differences, time-on-task and object versus space-based representations on production of systematic bias in spatial attention

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MATRIC NUMBER: 0904872B

SUPERVISORS NAME: Gregor Thut

DATE: 13/03/2013

LEVEL 4 PSYCHOLOGY

The contribution of individual differences, time-on-task and object versusspace-based representations to production of systematic bias in spatial

attention.

Declaration: I have read and understand the School of Psychologys guidelines onplagiarism (found in the class handbook) and declare that this essay (report) is entirely myown work. All sources have been acknowledged in the text and included in the referencesection. All quotations from other authors are marked as such in the text


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Abstract

Processing of visuo-spatial information is strongly modulated by hemispheric specialisation of

associated function. An example is a systematic leftward bias in spatial attention (pseudoneglect)

observed in general population which is believed to be the result of right hemisphere dominance for

visuo-spatial attention. However individual differences have been found both in direction and

magnitude of the bias. The leftward bias has been shown to shift to the left and even reverse in the

opposite direction as a result of increased fatigue. The role of both object-specific and space-specific

representations has been proposed to mediate spatial bias. Current study investigated the effects of

these factors on biases in line bisection and discusses the findings in relation to models of spatial

attention. Participants performed a computerised landmark task for 1 hour to induce a time-on-task

effect. Individual differences were accounted for by dividing participants into groups based on the

direction of their initial bias. The role of space-specific representations was investigated by

measuring the visual field advantages characterised by difference in reaction times to stimuli

presented to the left and right visual field. The results highlight the need for accounting for

individual stimuli in studies investigating biases in line bisection explaining the inability of some

studies to replicate leftward bias in line bisection. More importantly participants differing in initial

bias show different trends for the time-on-task effect in producing shifts in bias on line bisection task

questioning models assuming the involvement of distinct processes. No changes in visual field

advantages were recorded over the course of the experiment suggesting that the time-on-task effect

is better explained by object-specific rather than space-specific attentional processes. The research

invites further exploration into differences in neural connections underlying individual differences in

spatial biases and creation of models which would account for these differences.

1. Introduction

Processing of visuo-spatial information is strongly modulated by hemispheric specialisation of

associated function. As a result the information is not evenly distributed within the brain leading to

biases in spatial attention. A vivid example is unilateral, hemispatial neglect often manifesting after

right hemispheric brain damage characterised by difficulties reacting to and orienting attention

towards stimuli presented on the contralesional side of the visual field (Heilman, Watson, &

Valenstein, 2002). When asked to estimate a midpoint of a line, the majority of neglect patients

exhibit a strong rightward bias (Schenkenberg et al., 1980; Vallar, 1998; Fischer, 2001). In healthy

subjects there is a similar tendency to exhibit a spatial attention bias during a line bisection task

referred to as pseudoneglect(Bowers and Heilman, 1980). The direction of this bias is usually found

to be in the opposite direction (leftward bias) and of smaller magnitude than that of neglect patients


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(Jewell and McCourt, 2000). Pseudoneglect is a robust phenomenon being replicated in studies

differing in stimulus type and context factors. It was found for stimuli differing in length (McCourt

and Olafson, 1997), brightness, numerosity and size (Nicholls et al., 1999). However the magnitude

and direction of bias depend on a number of factors including selected context (Jewell and McCourt,

2000; Heber et al., 2010; Schmitz et al. 2011; Benwell CSY et al., 2012).

While being a relatively reliable phenomenon some studies were unable to find any

significant indication of bias during line bisection suggesting possible influence of individual

differences associated with visuo-spatial bias (Halligan et al., 1990a; Halligan, 1990b, Nichelli, 1989).

Halligan et al. (1990a) demonstrate a high inter-individual variation of subjects performance on a

line bisection task. Apart from a high inter-individual variation only about half of the subjects

demonstrated a typical leftward bias while the other half show bias in the opposite direction. As a

result no statistically significant effect of a bias was found by Halligan et al. (1990a-b) when all the

subjects were considered together, which might account for some of the difficulties replicating

leftward bias in pseudoneglect in some studies. The reported measure of bias will therefore often

depend on the proportion of right shifters and left shifters within the selected sample (Halligan et

al., 1990a). As such these differences pose constraints upon measuring the spatial attention bias in

healthy subjects as well as question some basic assumptions about the magnitude and direction of

such bias.

It is generally believed that pseudoneglect is the result of a leftward bias resulting fromhemispheric differences in attention. The models of spatial attention propose involvement of both

hemispheres but to a different degree (Heilman and Van Den Abell, 1980; Mesulam, 1983, 1990).

Heilman and Valenstein (1979)propose the right visual field to be represented in both hemisphereswhereas the left visual field to be represented in the right hemisphere only. According to this model

the spatial attention biases are a result of the differential activation of the two hemispheres

resulting in an uneven distribution of attention , i.e. biased towards the hemispace contralateral to

the dominant hemisphere (Kinsbourne, 1970). The predominant right hemisphere activation during

a line bisection task (Foxe et al., 2003; Waberski et al., 2008) can therefore account for a leftward

shift of attention causing an increase in the perceived relative length of the associated side of line

thus shifting the mid-point towards the left from the veridical centre. However it is questionable

how this model would account for the variations observed with different context factors and for

individual differences in spatial attention bias demonstrated by Halligan et al. (1990a, 1990b).

An alternative explanation posits that the general (egocentric) location within the visual field

(left or right) and associated hemispheric activation is irrelevant for determining bias in line bisection

and the leftward bias is object-centred, i.e occurs because the left side of objects is in general


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perceived to be longer than the right side. Indeed, spatial attention is not governed solely by general

spatial (egocentric) information regardless of any objects in the scene, as would be suggested by the

widely accepted theory that attention behaves as a spotlight moving through space selecting

specific regions and objects within these regions as they come into focus (Posner, 1980). Research

with both healthy (Duncan, 1984; Kanwisher & Driver, 1992; Gibson & Egeth, 1994) and brain

damaged participants (Driver & Halligan, 1991; Behrmann & Moscovitch, 1994) has shown that

attention might be guided by object-based as well as location-based representations. In terms of

spatial neglect Tipper and Behrman (1996) and Egly et al. (1994) have demonstrated that biases in

attention are mediated based on the availability of object related information and are not limited to

spatial information.

Nicholls and Orr (2005) investigated the effect of object-based and general space-based

reference frames on pseudoneglect. Participants were asked to make luminance judgements

between two mirror reversed greyscales . The location of the greyscales differed throughout the

experiment being placed across various locations between the very-right and very-left side of visual

space. The greyscales were arranged so that when displaced the object and space- specific

information was congruent or incongruent. In the congruent condition the direction of expected bias

based on the luminosity judgement was aligned with that based on object location in space. In the

incongruent condition the bias associated with luminosity of the object was reversed to the bias

produced by the spatial information. Pseudoneglect was observed in the baseline condition and,with increased magnitude, in the congruent condition while no bias was recorded in the incongruent

condition. The results show that both object based and space-based attention processes affect

visuo-spatial bias and operate independently, creating additive effects in the congruent condition to

produce larger bias while cancelling each other out in the incongruent condition. Additionally the

object based bias increased with higher object centrality shedding doubt on models based purely on

spatial attention as proposed by Kinsbourne (1987, 1993) and Heilman (1979). The discussion of the

contribution of object specific and space specific attention creates a framework for the exploration

of some of the differences in direction and magnitude of attention biases as well as difficulties

replicating pseudoneglect across some studies.

Besides stimulus factors and individual differences state of arousal and general fatigue have

also been found to have effect on spatial attention bias. Manly et al. (2005) demonstrate a shift of

leftward bias to rightward bias after a sleep deprivation period in healthy subjects. In addition a

dynamic reversion of attention bias has been found when participants spent an hour practicing a

standardised line bisection task (Manly et al., 2005; Dufour et al., 2007). This time-on-task effect

suggests that pseudoneglect could indeed be a fairly fluid phenomenon modulated by an interaction


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between visuo-spatial and attentional processes. Benwell CSY et al. (2012) investigated the time-on-

task effect on attentional bias using a variation of a landmark task. Participants were asked to carry

out midpoint judgements on a series of short and long lines to account for the effect of stimulus

factors. It was shown that there was a significant time-on-task effect over an hour of trials causing a

rightward reversal of bias when the task was performed with long lines. However no time-on-task

effect was found for short lines despite similar recorded drop in overall vigilance. This lead the

authors to propose a resource based model of attention suggesting that the effect on spatial bias

depends on continuous performance but not on general fatigue. The right hemispheric dominance

causing the leftward attentional biases might decrease as a result of neuronal fatigue produced

throughout the task. Similarly it was assumed that processing of long lines to produce midpoint

judgements would require more attentional resources resulting in higher load on the neurons in the

right hemisphere than processing of short lines resulting in a time-on-task effect for long lines but no

or significantly lower time-on-task effect for short lines.

This was taken as support for study of Schmitz et al. (2011) who found conflicting findings

when replicating the study by Manly et al. (2005) demonstrating no significant shift of participants

bias suggesting general fatigue as produced by sleep deprivation does not affect pseudoneglect . An

alternative explanation however might be the role of individual differences in the direction of bias

and their modulation of the time-on-task effect. Taking into account different distribution of right

shifters and left shifters across studies would potentially account for why Manly et al. (2005) wereable to find an effect of general fatigue on spatial bias and Schmitz et al. were not despite following

a highly similar research paradigm.

Further information on models of attention orienting in line bisection has been provided by

neuroimaging. These studies reveal a predominant involvement of the right hemisphere in line

bisection demonstrating a right-lateralized occipito-parieto-frontal network during task processing

(Cicek, Deouell &Knight, 2009; Fink et al., 2000; Foxe, McCourt, & Javitt, 2003). This was further

supported by studies employing non-invasive methods such as transcranial magnetic stimulation

(TMS) to induce a temporary localised lesion to produce hemispatial neglect. Fierro et al. (2006)

demonstrated the role of posterior right parietal cortex in neglect and pseudoneglect using

repetitive transcranial magnetic stimulation (rTMS). Another rTMS study by Kim et al. (2005) showed

that trains of rTMS delivered over the right posterior parietal cortex (PPC) during a line bisection task

enhance the bias whereas rTMS delivered over the left PPC induces a rightward bias thus providing

support for the activation/orientation model proposed by Kinsbourne (1970). Similarly Giglia et al.

(2011) demonstrated a rightward shift in bias for pseudoneglect after application of dual transcranial

direct current stimulation (tDCS), similar in magnitude to that observed by Benwell CSY et al. (2012)


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as a result of time spent on task. Schmitz et al. (2011) however point out the importance of alertness

and associated alerting and attention orienting networks. This is mainly because noradrenergic (NA)

transmission in the locus coeruleus (LC) have been found to form particularly dense formations in

the right frontal and parietal regions, which are essential for orienting attention and its ability to

select relevant information (Posner & Petersen, 1990).

In summary the research on neglect and pseudoneglect has demonstrated a dominance of

the right hemisphere in mediating spatial bias. Both space and object specific information has been

demonstrated to affect spatial attention bias and are believed to be associated with specific parieto-

frontal regions in the right hemisphere. It is questionable, however, to what degree spatial biases in

line bisection are influenced by visual space based processes and to what degree they are

modulated by object specific orientation of attention. Studies have also recorded a time-on-task

effect on spatial bias however with somewhat mixed findings regarding the effect of general

vigilance. Furthermore there seems to be considerable individual differences which put constraints

upon making generalisations about the mechanisms underlying neglect and pseudoneglect. These

individual variations in spatial biases have received very little attention and to this day their

influence on some of the mechanisms of pseudoneglect outlined by previous studies has not been

studied.

The aim of the current study is to investigate the effect of these individual differences both

on the time-on-task effect and on the influence of space and object-based processes in productionof the attentional biases in line bisection. The study utilises a modification of the landmark task used

by Benwell CSY et al. (2012) to investigate shifts in spatial attention bias due to time-on-task during

line bisection while introducing a simple reaction time task using lateralised, unilateral visual stimuli

assess space-based attention bias by measuring the effect of visual field advantages due to

hemispheric differences in spatial attention. The reaction time task was completed before and after

the landmark task to investigate whether the time-on-task effect found to affect spatial bias in line

bisection will transfer to general spatial attention when no object is present in the scene. The

individual differences were accounted for by dividing participants into two separate groups based on

their initial bias identifying right shifters and left shifters as proposed by Halligan et al. (1990a).


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2. Method

2.1. Participants

Thirty participants, mostly undergraduate students, took part in the experiment. Participants were

recruited using the Glasgow University subject pool and were offered 6 pounds or course credits for

participation. Data from one participant could not be used due to a software malfunction. Nine

participants were removed from the analysis due to unsatisfactory level of performance using a

median absolute difference (MAD) method for outlier removal as a removal criterion (a participants

performance was considered to be an outlier if the test statistic z for MAD was higher than 3.5).

The remaining twenty participants (4 male, 16 female, mean age = 23, SD = 4.6) were all right

handed as confirmed by the Edinburgh Handedness Inventory (Oldfield, 1971) (see Appendix 1). All

participants reported no history of neurological disorder or serious head trauma and had normal or

corrected to normal vision.

2.2. Stimuli and Apparatus

The experimental paradigm partially replicates Benwell CSY et al. (2012) alteration of a

computerised landmark task which was performed for approx. 1 hour to induce a time-on-task effect

resulting in a rightward shift of spatial bias in line bisection. This was preceded and followed by a

simple reaction time task, in which participants responded as fast as possible, using their index

finger, to lateralized, unilateral visual stimuli (henceforth referred to as the general spatial attention

task).

The stimuli used for the landmark task were black and white lines of 100% Michelson

contrast presented on a grey background (luminance = 179, hue = 179). Examples of lines used in the

experiment can be found in Figure 1. All the lines measured 24.3cm in length by .5cm in height

which at a viewing distance of 90 cm subtended 17.3 (width) and 0.36 (height) of visual angle.

Each line was transected at 1 of 17 possible points ranging from 4% of absolute line length

to veridical centre at steps of 0.5% of absolute line length. This represents a range of -0.696 to

0.696 of visual angle with a difference of 0.087 of visual angle between transection locations. All

lines were displayed with the transection point horizontally aligned with the fixation cross which was

located at the midline of the screen.

The lateralized visual stimuli used during the general attention task consisted of a small

white square - 1 (height) 1 (width) of visual angle on black background - presented on the left or

right side from the fixation cross at an eccentricity of 7 along the horizontal meridian .


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Stimuli were presented on a CRT monitor with a 1280 1024 pixel resolution and 85 Hz

refresh rate using E-prime software (Schneider et al., 2002).

Figure 1 - Example of transected line stimuli used in the experiment. Lines A and B are transected in the opposite

direction from the veridical centre. Note that the transection points of the lines are aligned at the centre. Line A and

line B differ in contrast polarity. All trials contained a 1:1 ratio of lines with reversed contrast polarity and were

presented at a random order.

2.3. Procedure

At the beginning and the end of the experiment the participants were asked to fill out a 10 point

Likert-style sleepiness scale, providing a subjective estimate of alertness on a scale ranging from 0

(almost asleep) to 100 (fully alert). Participants were then seated in front of a computer screen at a

viewing distance of 90 cm so that their midsagittal plane was aligned with the centre of the monitor.

A chin rest was used to maintain the participants head in position and prevent distortions to viewing

distance and viewing angle.

In the general attention task each trial began with presentation of a fixation cross [0.39

(height) 0.39 (width) of visual angle] for 1 sec followed by a brief presentation of the lateralised

stimulus. The position of the stimulus (left or right) was randomised but was distributed equally

across all trials in each block. The duration of stimulus presentation was 50 msec and the interval

between stimuli was randomised for 1400 400 msec. Subjects were asked to fixate on the cross at

the middle of the screen and press a button on a computer keyboard as quickly as possible using the

index finger on a given hand upon seeing the stimulus. A total of 2x200 trials were sampled in four

blocks of 100 trials each. These blocks differed in terms of the hand participants used to indicate

their responses. In order to prevent learning effects half of the participants was asked to begin the

first block using their right hand and then switch to using their left hand during the second block

while the second half was instructed to begin the first block using their left. The participants

reaction time was measured for each hand and stimulus location.

In the line bi-section task each trial began with presentation of a fixation cross [0.39

(height) 0.39 (width) of visual angle] for 1 sec followed by presentation of the transected line. The

transection mark was always aligned with the fixation cross in order to prevent the relative position

of the bi-section mark to the fixation cross as a visual aid during bi-section judgements. Participants

were asked to fixate on the cross in the middle of the screen throughout the experiment. The


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transected line was presented for 150ms, followed by a blank grey screen and a response period

during which participants indicated which part of the line appeared shorter (whether the perceived

mid-point of the line was located to the left or to the right of the transection point) by pressing one

of two buttons on a computer keyboard. The participants indicated all their responses using the

index and middle finger on their dominant (right) hand. A new trial began as soon as a response was

made. A total of 1360 trials were presented over 10 blocks of 136 trials.

Each participant completed a practice block of 6 trials of the general spatial attention task

and a practice block of 12 trials of the line bi-section task prior to commencing the experimental

trials to familiarise himself/herself with the task.

Each participant completed two blocks of the general spatial attention task (one using each

hand) followed by 10 blocks of line bi-section task followed again by a general spatial attention task.

Each block of the general spatial attention task lasted roughly 5 minutes (cca. 3 sec per trial) and

each block of the line bi-section task lasted roughly 4 minutes (cca. 2 sec per trial). Participants were

allowed to take short self-paced breaks in between each block. The whole experiment lasted 60-70

minutes.

Figure 2 - A schematic representation of the experimental procedure. Half of the participants were asked to complete

first block of the general spatial attention task (GSA) using their right hand and then use their left hand for block two

(top) while the second half was asked to do the opposite. The general spatial attention task was followed by 10 blocks of

line bi-section task followed by another two blocks of the general spatial attention task. The order of hands in the

second general spatial attention task was kept the same as during the first task.

2.4. Design and Analysis

The experiment followed a within subject repeated measures design. For evaluating the object

specific attention the dependent variable used was the individuals spatial attention bias on the line

bi-section task. Various blocks served as the independent variable to assess the time-on-task effect

on the participants biases. Psychometric functions were derived from the data recorded during the

line bi-section task in order to establish an objective measure of perceived line midpoint which was

then used to calculate individual bias. The functional variables used in the model were the

proportion of all trials in which the participants indicated that the perceived transection point was

GSA 1RH

GSA 1LH

10 blocks of LBSGSA 2

RHGSA 2

LH

GSA 1LH

GSA 1RH

10 blocks of LBSGSA 2

LHGSA 2

RH


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located to the left of the perceived midpoint (left side of the line appeared shorter) and the actual

transection point on the line (based on one of 17 possible transection points). Non-linear least-

squares regression was used to fit a cumulative logistic function to the data for each subject in each

block and to the averaged proportion of left responses for each block. The function used can be

described by the following equation:

( ) ( (

)

where x are the tested transector locations, corresponds to the x-axis location with a 50% left

and 50% right response rate and s is the estimated width of the psychometric curve. The value

corresponding to 50% location on the function is the point of subjective equality (PSE). Individual

PSE was used as an objective measure of participants spatial attention bias. The width of the PF

provides a measure of the precision of participants line midpoint judgements per block. The

direction of the initial bias (estimated based on positivity or negativity of participants PSE) was used

as another independent variable (between subjects) to take into consideration individual variation in

bias effects.

The average reaction time on the general spatial attention task was used as a dependent

measure of general spatial attention. The difference between the average reaction times per each

subject was investigated prior and post completing the line-bisection to assess whether time-on-task

effect in landmark task would also affect visual field advantages in terms of average reaction time in

general spatial attention task (stimulus location was used as the independent variable to analyse

differences in reaction times for left and right visual field). Furthermore the effect of initial bias was

used to investigate individual differences in general spatial attention. Inferential statistical analyses

were performed to investigate relationships between the fitted PSE values, time-on-task, average

reaction time on the general spatial attention task and measures of initial bias as well as Stanford

Sleepiness Scale ratings.


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3. Ethics

Participants were fully informed about the purpose of the study and were debriefed at the end of

the experiment. It was made clear to them that their participation in the study is voluntary and they

have the right to withdraw from the study at any time and all of their data will be removed from the

analysis, if they choose to do so. All of the subjects data was kept confidential and any files

containing participants data were kept in a locked filing cabinet or on a password protected hard -

drive (in case of digital files). There were no risks associated with the testing and every effort has

been made that the participants are as comfortable as possible throughout the experimental

procedure. Due to relatively long duration of the experiment participants were given opportunities

to take brief rest between experimental blocks. An example of the consent form, information sheet

and debriefing sheet used in the experiment can be found in Appendix 2.

The experiment was carried out within the School of Psychology at the University of Glasgow and

was approved by the local ethics committee.

4. Results

It was hypothesised that there will be significant differences between right shifters and left shifters

in terms of [1.1] magnitude as well as direction of recorded spatial attention bias and [1.2] the time-

on-task effect in modulating the magnitude and direction of the bias in the landmark task, in line

with the role of individual factors in determining spatial bias in line bisection. It was further

hypothesised that *2.1+ there will be significant differences between right shifters and left shifters

in visual field advantages (characterised by reaction time differences in the general spatial attention

task) and [2.2] that there will be a significant effect of time-on-task in modulating this difference, in

line with both object-based and space-based processes being at play during line bisection.

4.1. General fatigue and alertness

The sleepiness scale ratings confirmed an overall drop in subjects alertness over the course of the

experimental procedure with the mean alertness score dropping from 71.25 (SD=17.76) at the

beginning of the experimental procedure to 53.75 (SD=17.69) at the end. A one way analysis of

variance (ANOVA) of sleepiness ratings pre- and post-experiment shows a significant time-on-task

effect on sleepiness rating [F(7,12) = 5.944, p=0.004].


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4.2. Landmark task, time-on-task effect and individual differences

Figure 3 shows the average proportion of times participants have indicated that the left side of the

line is shorter as a function of the location of the transection point. left 24 represents a stimulus

type where the transection point is located most to the left whereas right 24 represents a stimulus

type where the transection point is located most to the right. It is expected that the ratio of left

responses would increase as the veridical leftward offset of the transector location from the centre

increases, as this should make it easier for the participant to detect the difference between the two

parts of the transected line. The transector (x-axis) corresponding to a 50% response (y-axis) in

relation to the veridical centre represents the participants spatial bias (negative values for leftward

bias, positive values for rightward bias). In order to observe a time-on-task effect resulting in a

rightward shift of the bias, similar to that found by previous studies, there should be a shift of thefunction to the right along the horizontal axis over the course of the experimental procedure. When

all participants are taken into consideration together (left and right biased participants collapsed)

there seems to be no shift of the functions over the course of the experiment (Figure 3).

Cumulative logistic functions were fitted to the data using non-linear least squares

regression and points of subjective equality (PSEs) (indicating the 50% value of the fitted function)

were calculated to provide a quantitative measure of participants bias. Table 1 shows individual PSE

values averaged across all participants for each block. All the average PSEs across all blocks have

negative values indicating leftward bias and there seems to be no shift in the direction of the bias

over time. It is also important to note relatively high values of standard deviation which suggests

considerable inter-individual differences between the participants in terms of the direction and

magnitude of bias.


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A repeated measures analysis of variance (ANOVA) for time-on-task (within subjects) on

individual PSEs for each subject shows no significant effect of time-on-task across different blocks

[F(1,19)=0.118, p=0.735]. This confirms the original assumption that in cases of samples with high

inter-subject variability when individual differences between participants are not controlled for

there is no statistically significant shift of participants bias over the course of the experimental

procedure.

Individual differences were then taken into consideration by using individual PSE values at

the starting block 1 as an indicator of participants initial bias (negative PSE = leftward bias, positive

PSE = rightward bias). The group average PSE value for starting block for participants with initial

block 1 block 2 block 3 block 4 block 5 block 6 block 7 block 8 block 9 block 10

Average

PSE -1.5056 -1.4398 -1.7206 -1.5976 -2.8698 -2.1972 -2.4432 -1.8747 -1.0109 -2.1546

Standard

deviation 4.1367 3.7340 3.5365 4.1179 4.8970 5.3215 4.9432 4.5007 3.9721 4.4818

Table 3 Average PSE values for each block and standard deviation.

0

10

20

30

40

50

60

70

80

90

100

le ft2 4 le ft2 1 le ft1 8 le ft1 5 le ft1 2 le ft9 le ft6 le ft3 v er r igh t3 r igh t6 r igh t9 r igh t1 2 r igh t1 5 r ig ht1 8 righ t2 1 r ig ht2 4

block 1

block 2

block 3

block 4

block 5

block 6

block 7

block 8

block 9

block 10

Figure 3 - Functions showing the average proportion of left responses based on the location of transection

point for each block.


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rightward bias was 2.861 (SD=1.428) and -3.377 (SD=3.414) for participants with initial leftward bias.

A Mann-Whitney test on the individual PSE values for starting block reveals a significant difference

between the two groups [U(1)=26.325, p=0.001], as would be expected by design. Note that there is

a relatively high proportion of participants with initial rightward bias in the group (30%, 6 of 20).

Together with the lack of an overall leftward bias (see above), this suggests that there are significant

differences between individuals in both the direction and magnitude of bias [hypothesis 1.1].

When considering the time-on-task effect separately for right biased and left biased

individuals, would these groups show different shift of biases across experimental blocks indicating

different time-on-task effect? Figure 4 illustrates the group averaged PSE values over the course of

the experimental procedure based on the participants initial bias. It is possible to observe th at in

right shifters (positive initial bias) the rightward bias gradually decreases with time spent on task

and eventually reverses to the left. In left shifters (negative initial bias) there is no reversal in the

direction of bias however there is a stable decrease in the magnitude of the bias throughout the

experimental session. That is, while the groups have different initial bias both in direction and

magnitude the differences in direction and magnitude of bias decreases throughout the

experimental procedure. A 2 (initial bias) x 10 (experimental block) factorial ANOVA (mixed design)

on individually fitted PSE values revealed no significant main effect of the initial bias [F(1,1)= 1.740,

p=0.204]; no significant main effect of block [F(9,1)=1.187, p=0.306]; and a marginally significant

initial bias block interaction [F(9,1)=1.982, p=0.044] suggesting different time-on-task effects forright shifters and left shifters *hypothesis 1.2+. A follow up analysis of the initial bias block

interaction using linear regression to look for linear trends across time-on-task per group shows no

significant correlation between initial bias and experimental block for the group of left

shifters(n=14) *Pearsons r=0.476, p=0.164+ but reveals a significant correlation for the group of

right shifters (n=6) *Pearson r=-0.721, p=0.019].


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Figure 4 Mean PSE values for each experimental block based on participant's initial bias. Error bars indicate 95%

confidence intervals.

4.3. General attention task, time-on-task effect and individual differences

Figure 5 shows group average times for right shifters and left shifters to stimuli presented to the

left and to the right visual field before and after completing the landmark task. The average reaction

time (msec) of left shifters to stimuli presented to the left versus right visual field prior to

completing the landmark task is 223.554 (SD=10.234) versus 228.705 (SD=12.287). In comparison

the average reaction time (msec) of right shifters to stimuli presented to the left versus right visual

field prior to completing the landmark task is 213.776 (SD=7.904) versus 213.781 (SD=9.069). The

data indicate that prior to completing the landmark task the left shifters may exhibit a slight

hemispheric advantage (equal to 5 msec difference in average reaction time) in favour of the right

hemisphere as indicated by lower reaction times to stimuli presented to the left visual field. The

right shifters demonstrate roughly equal reaction times to stimuli presented to the right and to the

left visual field. After completing the landmark task the reaction time (msec) of left shifters for

stimuli presented to the left versus right visual field increased to 227.859 (SD=9.529) versus 233.036

(SD=10.035). In comparison the reaction times of right shifters for stimuli presented to the leftversus right visual field increased to 218.178 (SD=8.506) versus 218.810 (SD=8.810). While there is a


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general increase in reaction times for both groups to stimuli presented to both right and left visual

field (possibly due to general fatigue) it seems to increase at approximately the same rate per visual

field indicating no change in hemifield advantages over the course of the experimental procedure.

A 2 (initial bias) x 2 (visual field) x 2 (pre vs. post landmark task) factorial ANOVA (mixed design) on

participants reaction times revealed no significant main effect for initial bias *F(1,1)=0.567, p=0.461+,

no significant main effect of visual field [F(1,1)=1.634, p=0.217] and no significant visual field x initial

bias interaction [F(1,1)=1.277, p=0.273] suggesting there are no significant differences between left

shifters and right shifters in terms of visual field advantages (as characterised by reaction time

differences) [hypothesis 2.1]. There was no significant main effect pre vs. post landmark task

[F(1,1)=1.731, p=0.205], no significant pre vs. post landmark task initial bias interaction

[F(1,1)=0.003, p=0.954] and no significant effect for pre vs. post landmark task initial bias

interaction x visual field interaction [F(1,1)=0.011, p=0.920] indicating that there is no significant

time-on-task effect in modulating visual field advantages for either group [hypothesis 2.2].

Left shifters Right shifters

200

205

210

215

220

225

230

235

240

245

250

pre post

LVF

RVF

200

205

210

215

220

225

230

235

240

245

250

pre post

LVF

RVF

Figure 5 -Group average reaction times (msec) for 'right shifters' and 'left shifters' to stimuli presented in the left and

right visual field recorded pre and post completing the landmark task.


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5. Discussion

The results of the current study highlight the importance of inter-individual differences in spatial bias

in line bisection. There are considerable differences between participants in both the direction and

magnitude of bias on a landmark task. As a result statistical methods show no shifts in bias displayed

on a landmark task when individual differences are not controlled for. When dividing participants

based on their initial bias it is possible to observe two distinctively different trends for the shifts in

spatial bias over time. Participants with an initial rightward bias show a gradual shift of the bias to

the left as a result of time-on-task. Participants with initial leftward bias maintain the direction of

bias however its magnitude increases as a result of time-on-task. This suggests that the leftward bias

in line bisection, as reported by previous studies is not systematically present in the entire

population but is modulated by inter-individual differences. In addition the shifts of spatial bias inline bisection resulting from time spent on task differ based on the participants initial bias

suggesting inter-individual variability for the time-on-task effect. The results are in line with those

found by Halligan et al. (1990a-b) clearly identifying right shifters and left shifters based on

differences in subjective line midpoint estimation in a line bisection task. In addition it is shown that

the effect of individual differences translates to the time-on-task shifts in spatial bias demonstrated

by previous research (Manly et al., 2005; Dufour et al., 2007; Benwell CSY et al., 2012). In addition

the results demonstrate lack of influence of space-based attention processes in production of

attentional biases in line bisection by showing no difference in reaction times to stimuli presented to

the left and right visual field, even when accounting for individual differences. These findings

contradict models assuming distinct involvement of space-based processes such as those proposed

by Kinsbourne (1987, 1993) and Heilman (1979). Furthermore there were no distinct changes in the

difference between stimuli presented to the right versus left visual field over time questioning the

involvement of hemispheric differences in generating the time-on-task effect in line bisection when

no object is present in the scene.

5.1. Individual differences in bias on landmark task and variations in time-on-task effect

Most models of pseudoneglect emphasise a leftward bias resulting from right hemispheric

dominance in spatial attention and associated neural network connectivity (Jewell and McCourt,

2000; Foxe et al., 2003; Siman-Tov et al., 2007). The current findings show that 30% of participants in

the sample exhibit a reversed rightward bias that is significantly different both in magnitude and

direction from participants exhibiting a leftward bias. It is questionable whether these participants

possess different connectivity patterns of underlying neurological mechanisms than participants

exhibiting an initial leftward bias causing a left hemispheric advantage rather than the right


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hemispheric advantage proposed by most studies. An alternative explanation would be that the

spatial bias in line bisection is indeed modulated by both space-based and object-based

representations (Tipper and Behrman, 1996; Egly et al., 1994; Post, 2001; Nicholls and Orr, 2005)

and there are individual differences in how the attention orienting processes within the visual

system assign weight to space-based and object-based information to produce midpoint line

judgements on line bisection task. In relation to the time-on-task effect majority of studies propose

an initial leftward advantage facilitated by activity in alerting and orienting networks in right

hemisphere (Corbetta and Shulman, 2002; Sturm et al., 2004) which is reduced or even reversed due

to increased fatigue producing a rightward shift in bias on line bisection task (Manly et al., 2005;

Dufour et al., 2007). Benwell CSY et al. (2012) distinguish between the effect of general fatigue and

time-on-task effect by proposing that shifts in line bisection over time are better explained by

uneven neuronal fatigue in right and left hemisphere (depleting neuronal resources in right

hemisphere faster) than general fatigue. The current study has demonstrated an overall increase in

general fatigue across all participants but different time-on-task effects in terms of shifts in

magnitude and direction of bias based on the direction of participants initial bias. The observed

trend for participants with initial leftward bias is in line with previous research showing a steady

decrease in leftward bias however no rightward reversal of bias was found. For participants with

initial rightward bias the trend is reversed showing a steady shift of the bias to the left. This speaks

against theories proposing a rightward shift as a result of decreased activity of alerting and orientingnetworks in right hemisphere due to increased fatigue. An altered version of the neuronal fatigue

explanation proposed by Benwell CSY et al. (2012) could be used to account for these findings

assuming that participants showing an initial rightward bias have different underlying neuronal

connections than participants showing an initial leftward bias and that these connections are

engaging the left hemisphere more than the right hemisphere thus depleting the neuronal resources

in the left hemisphere faster than in the right hemisphere causing a leftward shift in spatial bias on

line bisection task. This assumption, however, remains to be tested.

5.2. Role of object-specific versus spatial (egocentric) information in influencing spatial

biases

Nicholls and Orr (2005) show that both space-based and object-based representations affect visuo-

spatial bias and operate independently on one another. The results from the general spatial

attention task demonstrate that when individual differences are accounted for the general

(egocentric) location of stimulus within the visual field and associated hemispheric activation has no

distinctive effect on producing biases in line bisection. This serves as further evidence against


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models based purely on spatial attention as proposed by Kinsbourne (1987, 1993) and Heilman

(1979). In addition there was no time-on-task effect in modulating visual field advantages as

characterised by reaction times on the general spatial attention task. Considering that there was a

significant time-on-task effect present in the line bisection task when an object (a transected line)

was present in the scene it would seem that location based representations play no role in the time-

on-task effect on line bisection task. Relating this to the neuronal fatigue explanation proposed by

Benwell CSY et al. (2012) it would seem that the differential activation of hemispheres and

associated depletion of neuronal resources occurs only when an object is present. In contrast if an

object is not present in the scene both hemispheres are depleted equally as a result of general

fatigue resulting in no change in hemispheric advantages thus not producing any shift in spatial bias

on line bisection task. This is in line with Benwell CSY et al. (2012) conclusion that the type of

presented stimuli determines the shift of spatial bias over the course of the experimental procedure.

This means that the time-on-task effect is best explained in relation to the object-specific attention

processes.

5.3. Conclusion, Methodological concerns and avenues for further research

Current study emphasises the importance of inter-individual differences for researching the

mechanisms underlying biases in spatial attention in healthy individuals explaining the inability of

some studies to replicate leftward bias in line bisection. In particular it demonstrates the influence of

individual differences on mediating the time-on-task effect in line bisection. The results provide

further support for the role of object-based in addition to space-based representations in producing

spatial biases and suggest that that the time-on-task effect is best explained in relation to the object-

specific attention processes.

The main methodological concern is the uneven distribution of right shifters (n=6) and left

shifters (n=14) within the study with relatively small sample sizes for both groups. It is possible that

the sample might not properly reflect the inter-individual variability in spatial bias in general

population. It is possible to utilise pre-screening methods to identify participants initial bias to

create equally sized groups while also considering larger sample sizes. It is also questionable to what

degree can the differences in reaction times to stimuli presented to the left and right visual field be

taken as indicative of the hemispheric advantages in spatial attention. Neuroimaging methods might

be more suitable for investigating underlying neurological mechanisms than a behavioural paradigm.

Further research can focus on uncovering differences in neurological connectivity underlying

spatial biases between right shifters and left shifters using brain imaging methods. Repeated


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transcranial magnetic stimulation (rTMS) (Kim et al.,2005) and transcranial direct-current stimulation

(tDCS) (Giglia et al., 2011) have been shown to be particularly effective in uncovering neurological

mechanisms associated with biases in line bisection. Testing the applicability of the neuronal fatigue

model proposed by Benwell CSY et al. (2012) for explaining leftward shifts in spatial bias found for

participants showing an initial rightward bias would be particularly useful for providing further

understanding of the role of individual factors in spatial bias in line bisection as well as the

underlying neurological processes. Another avenue for further exploration is the involvement of

object vs. space-based representation in producing special biases particularly the necessity of object

presence in generating the time-on-task effect and the potential role of individual differences in

weighting object and space-based representations in producing subjective line midpoint judgements.

References

Behrmann, M., & Moscovitch, M. (1994). Object-centered neglect in patients with unilateral neglect:Effects of left-right coordinates of objects.Journal of Cognitive Neuroscience, 6, 1-16.

Benwell, C.S.Y., Harvey, M., Gardner, S. & Thut, G. (2012). Stimulus- and state-dependence ofsystematic bias in spatial attention: Additive effects of stimulus-size and time-on-task, Cortex,doi:10.1016/j.cortex.2011.12.007

Bowers, D. & Heilman, K.M. (1980). Pseudoneglect: Effects of hemispace on a tactile line bisectiontask.Neuropsychologia, 18(4-5),491-498.

Cicek, M., Deouell, L. Y., & Knight, R. T. (2009). Brain activity during landmark and line bisectiontasks.Frontiers in Human Neuroscience, 3, 7.

Corbetta, M. & Shulman, G.L. (2002). Control of goal-directed and stimulus-driven attention in thebrain.Nature Reviews Neuroscience, 3(3), 201-215.

Driver, J., & Halligan, P. W. (1991). Can visual neglect operate in object-centered coordinates? Anaffirmative single-case study. Cognitive Neuropsychology, 8, 475-496.

Dufour A, Touzalin P. & Candas V. (2007) Time-on-task effect in pseudoneglect.Experimental BrainResearch, 176(3), 532-537.

Duncan, J. (1984). Selective attention and the organization of visual information.Journal ofExperimental Psychology, 113, 501-517.

Egly, R., Driver, J., & Rafal, R. D. (1994). Shifting visual attention between objects and locations:Evidence from normal and parietal lesion subjects.Journal of Experimental Psychology: General,123, 161-177.

Fierro, B., Brighina, F., Giglia, G., Palermo, A., Francolini, M., & Scalia, S. (2006). Paired pulseTMS over the right posterior parietal cortex modulates visuospatial perception.Journal of theNeurological Sciences, 247(2), 144148.


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Fink, G. R., Marshall, J. C., Shah, N. J., Weiss, P. H., Halligan, P. W., Grosse-Ruyken, M., et al.(2000). Line bisection judgments implicate right parietal cortex and cerebellum as assessed by fMRI.

Neurology, 54(6), 13241331.

Fischer, M. H. (2001). Cognition in the bisection task. Trends in Cognitive Sciences, 5(11), 460462.

Foxe, J. J., McCourt, M. E., & Javitt, D. C. (2003). Right hemisphere control of visuospatial attention:Line-bisection judgments evaluated with high-density electrical mapping and source analysis.Neuroimage, 19(3), 710726.

Gibson, B. & Egeth, H. (1994). Inhibition of return to object-based and environment-based locations.Perception & Psychophysics, 55, 323-339.

Giglia, G.,Mattaliano, P., Puma, A., Rizzo, S., Fierro, B. & Brighina, F. (2011). Neglect-like effectsinduced by tDCS modulation of posterior parietal cortices in healthy subjects. Brain Stimulation, 4,294-299.

Halligan, P.W, Manning, L. & Marshall J.C.(1990a). Individual variation in line bisection: a study offour patients with right hemisphere damage and normal controls. Neuropsychologia, 28, 104351.

Halligan, P.W, Manning, L. & Marshall J.C.(1990b). Individual variation in line bisection: a study ofnormal subjects with application to the interpretation of visual neglect.Neuropsychologia, 28, 64755.

Heber I, Siebertz S, Wolter M, Kuhlen T, and Fimm B. (2010). Horizontal and vertical pseudoneglectin peri and extrapersonal space.Brain and Cognition, 73(3), 160-166.

Heilman, K. M. & Valenstein, E. (1979). Mechanisms underlying hemispatial neglect.Annals of

Neurology, 5(2), 166170.

Heilman, K.M. and Van Den Abell, T. (1980) Right-hemisphere dominance for attention: Themechanisms underlying hemispheric asymmetries of inattention (neglect).Neurology, 30(3), 327-330.

Heilman, K. M., Watson, R. T.,& Valenstein, E. (2002). Spatial neglect. In H. O. Karnath, A. D. Milner,

& G. Vallar (Eds.), The cognitive and neural bases of spatial neglect. Oxford, NY: Oxford University

Press, 3-30.

Jewell, G., & McCourt, M. E. (2000). Pseudoneglect: A review and meta-analysis of performance

factors in line bisection tasks. Neuropsychologia, 38(1), 93110.

Kanwisher, N. & Driver, J. (1992). Object, attributes, and visual attention: Which, what, and where.

Current Directions in Psychological Science, 1, 2631.

Kim, Y. H., Min, S. J., Ko, M. H., Park, J. W., Jang, S. H., & Lee, P. K. (2005). Facilitatingvisuospatial attention for the contralateral hemifield by repetitive TMS on the posterior parietal cortex.

Neuroscience Letters, 382(3), 280285.

Kinsbourne, M. (1970). The cerebral basis of lateral asymmetries in attention.Acta Psychologica, 33,

193201.

Kinsbourne, M. (1987). Mechanisms of unilateral neglect. In: Jeannerod M (ed.)Neurophysiologicaland neuropsychological aspects of spatial neglect. Amsterdam: Elsevier (North-Holland), 6986.


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Kinsbourne, M. (1993). Orientational bias model of unilateral neglect: evidence from attentional

gradients within hemispace. In: Robertson IH, Marshall JC (eds.) Unilateral neglect: clinical

and experimental studies. Hove UK: Lawrence Erlbaum Associates, 6386.

Manly, T., Dobler, V. B., Dodds, C. M. &George, M. A. (2005). Rightward shift in spatial awareness

with declining alertness.Neuropsychologia, 43(12), 1721

1728.

McCourt, M.E, Olafson, C. (1997). Cognitive and perceptual influences in line bisection: Stimulus

modulation of pseudoneglect.Neuropsychologia, 35, 369380.

Mesulam, M.M. (1983). The functional anatomy and hemispheric specialization for directed attention:

the role of the parietal lobe and its connectivity. Trends in Neurosciences, 6, 384387.

Mesulam, M.M. (1990). Large-scale neurocognitive networks and distributed processing for attention,language and memory.Annals of Neurology, 28, 597613.

Nichelli, P., Rinaldi, M., Cubelli, R. (1989). Selective spatial attention and length representation in

normal patients and in patients with unilateral spatial neglect.Brain and Cognition, 9, 5770.

Nicholls, M.E.R., Bradshaw, J.A., Mattingley, J.B. (1999). Free-viewing perceptual asymmetries forthe judgement of brightness, numerosity and size.Neuropsychologia, 37, 307314.

Nicholls, M.E.R., Orr, C.A. (2005). The nature and contribution of space- and object-basedattentional biases to free-viewing perceptual asymmetries.Experimental Brain Research, 162, 384-393.

Oldfield, R. C. (1971). The assessment and analysis of handedness: The Edinburgh inventory.Neuropsychololgia, 9, 97-113.

Posner, M.I. (1980). Orienting of attention. Quarterly Journal of Experimental Psychology, 32, 325

Posner, M. I., & Petersen, S. E. (1990). The attention system of the human brain.Annual Review ofNeuroscience, 13, 2542.

Post, R.B., Caulfield, K.J., Welsh, R.B. (2001). Contributions of object and space-based mechanismsto line bisection errors.Neuropsychologia, 39, 856864.

Schenkenberg, T., Bradford, D.C.,, Ajax, E.T. (1980). Line bisection and unilateral visual neglect inpatients with neuroleptic impairment.Neurology, 30, 509517.

Schmitz, R., Deliens, G., Mary, A., Urbain, C., and Peigneux, P. (2011). Selective modulations ofattentional asymmetries after sleep deprivation. Neuropsychologia, 49(12), 3351-3360.

Schneider, W., Eschman, A. & Zuccolotto, A. (2002). E-Prime reference guide. Psychology SoftwareTools.

Siman-Tov, T., Mendelsohn, A., Schonberg, T., Avidan, G., Podlipsky, I., Pessoa, L., et al. (2007).Bihemispheric leftward bias in a visuospatial attention-related network.Journal of Neuroscience,

27(42), 11271-11278.

Sturm, W., Longoni, F., Weis, S., Specht, K., Herzog, H., Vohn, R., et al. (2004). Functional

reorganisation in patients with right hemisphere stroke after training of alertness: A longitudinal PET

and fMRI study in eight cases. Neuropsychologia, 42(4), 434-450.


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Tipper, S.P, Behrmann, M. (1996). Object-centred not scene-based visual neglect.Journal ofExperimental Psychology: Human., 22(5), 12611278.

Vallar, G. (1998). Spatial hemineglect in humans. Trends in Cognitive Sciences, 2(3), 87-97.

Waberski, T.D., Gobbele, R., Lamberty, K., Buchner, H., Marshall, J.C. & Fink, G.R. (2008). Timing of

visuo-spatial information processing: Electrical source imaging related to line bisection judgements.

Neuropsychologia, 46(5), 1201-1210.


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Appendices

Appendix 1 Edinburgh Handedness Inventory

Edinburgh Handedness Inventory1

Your Initials:

Please indicate with a check () your preference in using your left or right hand in the

following tasks.

Where the preference is so strong you would never use the other hand, unless absolutely

forced to, put two checks ().

If you are indifferent, put one check in each column ( | ).

Some of the activities require both hands. In these cases, the part of the task or object forwhich hand preference is wanted is indicated in parentheses.

Task / Object Left Hand Right Hand

1. Writing

2. Drawing

3. Throwing

4. Scissors

5. Toothbrush

6. Knife (without fork)

7. Spoon

8. Broom (upper hand)

9. Striking a Match (match)

10. Opening a Box (lid)

Total checks: LH = RH =

Cumulative Total CT = LH + RH =

Difference D = RHLH =

Result R = (D / CT) 100 =

Interpretation:

(Left Handed: R < -40)

(Ambidextrous: -40 R +40)

(Right Handed: R > +40)

1

Oldfield, R. C. (1971). The assessment and analysis of handedness: The Edinburghinventory.Neuropsychololgia, 9, 97-113.


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Appendix 2.1. Example of consent form

STUDY INFORMED CONSENT(This form must be completed prior to any experiment)

Study title: Testing models of spatial attention through computerized line

bisection and a CUD reaction time task.

I confirm that I have read and understood the Study Information Sheet provided to me for the above studyand have had the opportunity to ask questions.

I understand that my participation is voluntary and that I am free to withdraw at any time, without giving

a reason.

I understand that at all times my personal data will be kept confidential in accordance with data protection

guidelines.

I agree to take part in the study.

I have initialed the above boxes myself and I freely agree to take part in the study.

Signature of Volunteer: ___________________________

Name: ___________________________

Date: ___________________________

Subject ID: ___________________________

Signature of Witness: ___________________________

Name: ___________________________

Date: ___________________________

School of Psychology

University of Glasgow

58 Hillhead Street, Glasgow G12 8QB

Tel: +44 (0)141-330 3395, email: [email protected]


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Appendix 2.2. Information sheet

LETTER OF INFORMATION

Spatial attention and line bisection

Please read this information form carefully. If you have any questions, ask theexperimenter before signing the form.

PURPOSE OF THE STUDY

The purpose of this study is to examine spatial attention and underlying neurologicalmechanisms in healthy subjects.

PROCEDURE

Should you choose to participate, you will be presented with two tasks. During the first taskyou will be briefly presented with a white square on a computer screen. Your task will be toreact as quickly as possible by pressing a button on a keyboard using either your left or righthand. During the second task you will view a series of segmented lines on a computer

screen. Your task will be to indicate which of the two segments presented on the screenappears shorter. After completing the second task you will be asked to repeat the first taskagain using either your left or right hand.

The duration of the experiment is roughly 1 hour, including short self-paced breaks.

The exact predictions being made in this study will be explained to you at the conclusion ofyour participation.

POTENTIAL RISKS AND DISCOMFORTS

The risks associated with testing are no greater than risks you encounter in everyday life.Every effort has been made to ensure that you are as comfortable as possible during testing.You will have opportunities to take brief rests periodically during the experiment.

POTENTIAL BENEFITS

In return for your participation you can choose to be paid 6 or receive 3 e-credits if you area first year Psychology student. You will be debriefed at the end of the experiment, so youmay gain a better understanding of the subject under investigation and enhanced vision offuture scientific avenues in general. In a broader sense, the results from these experimentsmight have implications for the status of scientific knowledge about critical functions of the

visual system and underlying neurocognitive mechanisms in both healthy subjects and,through future extensions of the research into stroke patients, individuals with visual and/or


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attention deficits. Thus, your involvement in this study might benefit not only the scientificcommunity, but also, in terms of potential applications of this knowledge, the society at large.

PARTICIPATION AND WITHDRAWAL

Your participation in this experiment is voluntary. You are under no pressure to participate inthe experiment, and if you choose to participate you are free to withdraw from theexperiment at any time, with no penalty to yourself. All information about the participants inthe study and collected data will remain fully confidential howeveryou may exercise theoption of removing your data from the study after participation.

If you have any questions or concerns about the research, please contact Martin

Bacik ([email protected])

RIGHTS OF RESEARCH PARTICIPANTS

You may withdraw your consent at any time and discontinue participation without penalty.You are not waiving any legal claims, rights or remedies because of your participation in thisresearch study. This study has been reviewed and received ethics clearance through theFaculty of Information and Mathematical Sciences Ethics Committee. If you have questionsregarding your rights as a research participant, contact:

Klaus Kessler - MREB ChairTel. 0141-330-4774E-mail: [email protected]: 0141-330-4606Room 610, Dept of Psychology, 58 Hillhead Street, Glasgow, G12 8QB
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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Appendix 2.3. Debriefing sheet

DEBRIEFING SHEET

Spatial attention and line bisection

Thank you very much for participating in the study!

The aim of the experiment was to investigate a phenomenon called pseudo-neglect and the

underlying neurological mechanisms.

The second task you completed, involving segmented lines is called a line bisection task

and is used to investigate biases in spatial attention. Even when healthy people are asked to

complete the task they tend to exhibit a bias, in other words they tend to see the mid-points

of the lines more to the side than they really are. The interesting thing is that this bias isnt

static but changes depending on how much time we spend on the task and how tired we get

as a result.

It was hypothesised in the current study that these changes in bias might be the result of a

decrease in the ability of the right and left hemisphere to communicate with each other.

The task you have completed at the beginning and end of the experiment was designed to

measure your inter-hemispheric communication.

What was of the interest was the relationship between the changes in inter-hemispheric

communication at the beginning and end of the experiment and the changes in bias

throughout the line-bisection task.

If you have any more questions, please feel free to ask the experimenter.

Documents

The effect of individual differences, time-on-task and object versus space-based representations on production of systematic bias in spatial attention