Involuntary Spatial Attention

Influences Auditory Processing:

Evidence from Human Electrophysiology

by

Jennifer A. Schneider

B. A. (Hons., Psychology), University of Manitoba, 2006

Thesis Submitted in Partial Fulfillment

of the Requirements for the Degree of

Master of Arts

in the

Department of Psychology

Faculty of Arts and Social Sciences

Jennifer A. Schneider 2012

Simon Fraser University

Summer 2012

All rights reserved. However, in accordance with the Copyright Act of Canada, this work may

be reproduced, without authorization, under the conditions for “Fair Dealing.” Therefore, limited reproduction of this work for the

purposes of private study, research, criticism, review and news reporting is likely to be in accordance with the law, particularly if cited appropriately.

ii

Approval

Name: Jennifer A. Schneider

Degree: Master of Arts (Psychology)

Title of Thesis: Involuntary Spatial Attention Influences Auditory Processing: Evidence from Human Electrophysiology

Examining Committee:

Chair: Thomas Spalek

John McDonald Senior Supervisor Associate Professor

Richard Wright Supervisor Associate Professor

Matthew Tata External Examiner Associate Professor, Department of Neuroscience University of Lethbridge

Date Defended/Approved:

July 20, 2012

iii

Partial Copyright Licence

Ethics Statement

The author, whose name appears on the title page of this work, has obtained, for the research described in this work, either:

a. human research ethics approval from the Simon Fraser University Office of Research Ethics,

or

b. advance approval of the animal care protocol from the University Animal Care Committee of Simon Fraser University;

or has conducted the research

c. as a co-investigator, collaborator or research assistant in a research project approved in advance,

or

d. as a member of a course approved in advance for minimal risk human research, by the Office of Research Ethics.

A copy of the approval letter has been filed at the Theses Office of the University Library at the time of submission of this thesis or project.

The original application for approval and letter of approval are filed with the relevant offices. Inquiries may be directed to those authorities.

Simon Fraser University Library Burnaby, British Columbia, Canada

update Spring 2010

iv

Abstract

The appearance of spatially non-predictive auditory cues can attract attention resulting in

facilitation or inhibition of responses to subsequent targets at short or long cue-target

intervals, respectively. With most research focusing on visual and crossmodal spatial

attention, little is known about the neural mechanisms associated with auditory cue

effects. The present study used ERPs to investigate the consequences of involuntary

auditory spatial attention on the neural processing of sounds in spatial and non-spatial

go/no-go tasks. The negative-difference component – which is known to reflect

attentional enhancement of target processing – was observed in both experiments,

indicating that salient, spatially non-predictive auditory cues captured attention. A

subsequent positive difference was observed only in the spatial task, suggesting this

component corresponds with the presence or absence of RT cue effects in auditory

spatial cueing tasks. In both tasks, auditory sounds activated occipital regions,

suggesting that visual regions are involved in processing auditory stimuli.

Keywords: Event-related potentials; auditory spatial attention; difference waveforms; visual cortex

v

Dedication

To everyone who has supported me

through thick and thin

vi

Acknowledgements

I would like to thank my supervisor, Dr. John McDonald, for welcoming me into

the lab and providing a productive and inspiring environment to work in. Also, thank you

for the wonderful guidance, ideas, feedback, and discussions that made this research

possible.

I would like to thank Greg Christie for the many discussions that contributed to

this work and for his assistance with coding, trouble-shooting, analysis, and defense

preparations. Thanks to John Gaspar and Ali Jannati for their assistance with equipment

trouble-shooting. Also, thanks to Ashley Livingstone for her support and assistance with

defense preparations. Thank you to Ulrich Anglas, T.J. Radonjic, Maksim Parfyonov,

and Christina Hull for their help with data collection. I would also like to thank the staff in

the Department of Psychology for all their assistance throughout the semesters.

Finally, thank you to my family and friends who have encouraged me throughout

the process. I am very grateful for the unwavering support of my husband Patrick who

kept me focused on my goals and got me back on track when I veered off course. Also, I

greatly appreciate the continuous support of Mom, Dad, Janelle, Stef, and Chris, even

though they still don’t fully understand what I do. Thank you!

vii

Table of Contents

Approval.......................................................................................................................... ii Partial Copyright Licence ................................................................................................iii Abstract.......................................................................................................................... iv

Dedication ....................................................................................................................... v

Acknowledgements ........................................................................................................ vi Table of Contents...........................................................................................................vii List of Tables.................................................................................................................. ix

List of Figures.................................................................................................................. x

1. Introduction ..........................................................................................................1

1.1. Early Studies of Covert Spatial Orienting in Audition ..............................................1

1.2. Spatial Relevance Hypothesis ................................................................................6

1.3. Neuroimaging Recording........................................................................................9

1.4. Auditory Attention and ERPs ................................................................................10

1.5. Present Studies ....................................................................................................12

2. Experiment 1.......................................................................................................14

2.1. Methods ...............................................................................................................14

2.1.1. Participants ...............................................................................................14

2.1.2. Apparatus .................................................................................................14

2.1.3. Stimuli .......................................................................................................15

2.1.4. Design and Procedure ..............................................................................15

2.1.5. Electrophysiological Recording .................................................................17

2.1.6. Data Analysis ............................................................................................17

2.2. Results and Discussion ........................................................................................19

2.2.1. Behaviour..................................................................................................19

2.2.2. Target-elicited ERPs .................................................................................20

2.2.3. Cue-elicited ERPs.....................................................................................24

3. Experiment 2.......................................................................................................28

3.1. Methods ...............................................................................................................28

3.1.1. Participants ...............................................................................................28

3.1.2. Apparatus .................................................................................................28

3.1.3. Stimuli .......................................................................................................28

3.1.4. Design and Procedure ..............................................................................29

3.1.5. Electrophysiological Recording .................................................................29

3.1.6. Data Analysis ............................................................................................29

3.2. Results and Discussion ........................................................................................29

3.2.1. Behaviour..................................................................................................29

3.2.2. Target-elicited ERPs .................................................................................30

3.2.3. Cue-elicited ERPs.....................................................................................34

viii

4. General Discussion ............................................................................................37

5. References ..........................................................................................................43

ix

List of Tables

Table 2-1. Inter-Participant Averages of Median Response Times (RT, in Milliseconds) and Standard Errors (SE) as a Function of Cue-Target Stimulus Onset Asynchrony (SOA) and Cue Type in Experiment 1. ............20

Table 3-1. Inter-Participant Averages of Median Response Times (RT, in Milliseconds) and Standard Errors (SE) as a Function of Cue-Target Stimulus Onset Asynchrony (SOA) and Cue Type in Experiment 2. ............30

Table 4-1. Summary of Behavioural and Electrophysiological Effects in the Spatial and Non-Spatial Tasks.....................................................................39

x

List of Figures

Figure 2-1. Trial Sequences for Valid-Cue Trial (Left) and Invalid-Cue Trial (Right). These Illustrations are Examples of Go Trials. ................................16

Figure 2-2. Trial Sequences for a No-Go Trial (Left) and a Catch/No Target Trial (Right)..........................................................................................................16

Figure 2-3. Grand-Averaged ERP Waveforms for Validly- and Invalidly-Cued Targets in Experiment 1...............................................................................21

Figure 2-4. Topographical Voltage Maps of the Nd and the Pd Elicited by Auditory Target Stimuli in Experiment 1. ....................................................................22

Figure 2-5. Topographical Voltage Maps of Positive Deflections over Occipital Scalp in Experiment 1..................................................................................23

Figure 2-6. Grand-Averaged Event-Related Lateralization (ERLs) Elicited by Auditory Cues in Experiment 1. A. Ipsilateral and Contralateral Waveforms for T7/8 and PO7/8. B. Collapsed Ipsilateral and Contralateral Difference Waveforms for T7/8 and PO7/8. ............................25

Figure 2-7. Topographical Voltage Maps of the ACOP Elicited by Auditory Cue Stimuli..........................................................................................................26

Figure 3-1. Grand-Averaged ERP Waveforms for Validly- and Invalidly-Cued Targets in Experiment 2...............................................................................31

Figure 3-2. Topographical Voltage Maps of the Nd Elicited by Auditory Target Stimuli in Experiment 2. ...............................................................................32

Figure 3-3. Topographical Voltage Maps of Positive Deflections over Occipital Scalp in Experiment 2..................................................................................33

Figure 3-4. Grand-Averaged Event-Related Lateralization (ERLs) Elicited by Auditory Cues in Experiment 2. A. Ipsilateral and Contralateral Waveforms for T7/8 and PO7/8. B. Collapsed Ipsilateral and Contralateral Difference Waveforms for T7/8 and PO7/8. ............................35

Figure 3-5. Topographical Voltage Maps of the ACOP in Experiment 2. ........................35

1

1. Introduction

“Everyone knows what attention is. It is the taking possession by the mind, in

clear and vivid form, of one out of what seem several simultaneously possible objects or

trains of thought. Focalization, concentration, of consciousness are of its essence. It

implies withdrawal from some things in order to deal effectively with others, and is a

condition which has a real opposite in the confused, dazed, scatterbrained state which in

French is called distraction, and Zerstreutheit in German" (p. 403-404). ~ William James

Every day, objects in the surrounding environment are competing for people’s

attention. For example, busy streets are lined with countless signs that are meant to

attract the driver’s attention and, if successful, their business. According to the attention

and psychology literature, attention is the ability to attend selectively to relevant stimuli

and ignore, or filter out, all irrelevant stimuli in the surrounding environment. A common

example of this phenomenon is the “cocktail party effect”, where an individual is required

to focus on a single person’s voice and disregard all surrounding conversations and

noise. This orienting of attention is important for extracting information from potentially

important objects in the environment while avoiding distractions by less relevant objects.

Although humans often direct their eyes and ears toward objects to which they attend,

they can also direct their attention covertly, that is, without any corresponding eye or

head movements. The latter is most often studied in research on spatial attention.

1.1. Early Studies of Covert Spatial Orienting in Audition

In order to examine the attentional effects of covert auditory spatial attention,

studies typically use a cueing paradigm, which involves an auditory cue that directs

attention to the left or right of fixation, followed closely by an auditory target presented

either in the same location as the cue (valid trial) or a different location as the cue

(invalid trial). The cue can be predictive or non-predictive of the location of the target.

Participants are generally faster and more accurate when responding to targets on valid

2

trials than invalid trials (Buchtel & Butter, 1988; McDonald & Ward, 1999; Posner, 1980;

Quinlan & Bailey, 1995). Such cue effects have been largely interpreted in terms of the

orienting of spatial attention. Specifically, participants are hypothesized to orient

attention to the cued location prior to the onset of the target (Taylor & Klein, 1998).

Spatial cueing studies have found that cues can have different effects on

responses depending on the cue-target interval. If an auditory target is presented after a

short time interval (within about 200 ms) and at the same location of an auditory cue, the

response will be facilitated. This facilitation is considered to be under exogenous control

since the observed cue effects are present even when the cues do not predict the

location of the target. This control of attention is driven by a particular stimulus and is

involuntary. However, if the cues predict the target location, the control of attention is

said to be endogenous since the cues influence attention to shift voluntarily towards the

correct target location, which in turn affects the response time to the target. This control

of attention is goal-driven and voluntary (Posner & Cohen, 1984). Unlike the facilitation

effect at short time intervals, if the cue-target duration is longer (about 700ms) the

response to the target that appears at the same location as the cue is inhibited. This

effect has been coined inhibition of return (IOR). IOR in an inhibitory component of

covert orienting that hinders the ability to return attention to a location that had been just

attended to and is measured by the delayed response to the later stimulus at the original

cued location (Klein, 2000). Therefore, responses to valid trials are longer than to invalid

trials.

Although there is now general agreement that sudden sounds capture attention

exogenously, there has been considerable debate about this in the past. Posner (1978)

conducted several cueing studies in vision, audition, and touch and found that, unlike

vision and touch, there were no spatial cueing effects for auditory targets when followed

by informative central cues. This null effect occurred for both a simple detection task and

target intensity discrimination go-no-go task. Buchtel and Butter (1988) reported that

auditory or visual informative peripheral spatial cues did not affect latency detection for

auditory targets but did affect detection of visual targets. Posner suggested the reason

for the null effect is because auditory frequency detection occurs before sound

localization. His rationale is that auditory receptors do not map spatial locations

topographically, as they do in vision. The structural organization of the auditory pathway

is designed to encode different sound frequencies along the structure. Therefore, he

3

suggested that sound location is determined by specialized, location-sensitive neurons

thought to be in the colliculus (a structure in the midbrain that is involved in activating

eye movements) or the auditory cortices.

Buchtel and Butter (1988) took Posner’s (1978) research further and investigated

within- and between-modality spatial cueing effects using different combinations of visual

and auditory cues and targets in a simple detection task. Cueing effects were elicited

when visual cues (large cost-benefit effects) and auditory cues (small cost- benefit

effects) preceded visual targets; however, when visual and auditory cues preceded

auditory targets, no cueing effect was observed. This one-way influence was also found

in other studies (e.g., Spence & Driver, 1997). Buchtel and Butter suggested covert

spatial orienting occurs only when participants were required to respond to visual stimuli,

because (1) overt movements of the eyes and head improve visual identification but not

auditory identification (because the auditory system lacks an analog of the fovea), and

(2) covert spatial orienting is linked to overt orienting systems. According to this

proposal, covert orienting will occur in vision and touch but will not occur in audition,

because accurate identification of sounds does not require spatial orienting.

Rhodes (1987) revisited Posner’s (1978) null effect by investigating the spatial

properties of auditory attention. She claimed that covert spatial orienting would not be

apparent in simple reaction time (detection) tasks since this task could be performed

solely on the basis of non-spatial representations. Therefore, a localization task was

used to ensure the use of spatial auditory representations, where participants responded

verbally, using a previously learned label (e.g., 1, 2, 3, etc), to the location of the target

sound. The locations of the sounds were spaced evenly around the participant and each

had a number associated with it. Rhodes found that verbal response times increased

linearly with the distance between the location of the target from a given trial and the

location of the target from the preceding trial. This increase was argued to reflect the

time attention took to shift between target locations at a constant rate. This explanation

was adapted from vision research conducted by Tsal (1983), who provided evidence that

it takes more time to shift attention across larger distances than shorter distances in the

visual field. Although Rhodes’ results suggest that participants can orient attention

covertly in auditory space, other researchers (e.g., Spence & Driver, 1994) have

questioned whether the results reflect response priming rather than shifts of auditory

attention. Specifically, Spence and Driver (1994) pointed out that because the semantic

4

distance between the learned labels also increased linearly as the distance between

target sounds increased, participants simply might have counted up to the speakers that

were numbered consecutively to get to the correct response.

Following Rhodes’ (1987) lead to use spatial representation rather than simple

detection to study covert spatial attention in audition, Spence and Driver (1994) used

spatially predictive or non-predictive cues and cued participants in one direction (left or

right) but required them to discriminate the target in an orthogonal direction (up or

down). Therefore, participants discriminated the elevation of the target rather than its

laterality. Spence and Driver found that participants were faster to respond to targets

when the preceding cue was on the same side of the target as opposed to when the

preceding cue was on the opposite side of the target. They also found that predictive

spatial auditory cues elicited spatial orienting in both localization tasks as well as

frequency discrimination tasks. The findings are consistent with the hypothesis from

Rhodes that spatial orienting in audition will occur if the task requires auditory spatial

representations but contradicts Buchtel and Butter’s (1988) hypothesis that covert spatial

orienting will never occur in audition because there is no sensitive receptor, such as a

fovea. By using an orthogonal cueing paradigm, the cue effects cannot be explained in

terms of response-priming by the cue. Amidst these general strengths of the orthogonal-

cueing task, there is one potential weakness: cues and targets never appear at the same

location; and thus the orthogonal-cueing paradigm might underestimate the size of the

attentional effects or miss attentional effects entirely because cues are always invalid to

some degree (Prime, McDonald, Green, & Ward, 2008).

Spence and Driver’s (1997) explanation for this more rapid response to same-

side cued targets is that less information is required to respond to ipsilateral cued targets

than to contralateral cued targets. They reasoned that participants compared the

location of the target with the location of the preceding cue. This comparison is much

easier if the cue and target occurred on the same side. Another possible explanation is

that there are different attentional neural mechanisms that are involved in the response

to a target when the preceding cue is on the same side versus when the cue is on the

other side of the target. Early research on visual attention suggested that attention is

filtered from the receptive fields of specific neurons and only have enhanced processing

when the cue and target were in the receptive field than when the cue was outside the

receptive field (Moran & Desimone, 1985). Therefore, the location of the cue and target

5

has to be quite close in order for these mechanisms to activate. Yet another possible

explanation for Spence and Driver’s (1994) findings is that the attention effects

(attentional facilitation and inhibition of return) are present only when the location of the

cue and target are far apart, which has been hypothesized to be due to oculomotor

preparation or suppression instead of activating receptive fields of specific neurons

(Tassinari, Aglioti, Chelazzi, Marzi, & Berlucchi, 1987).

Contrary to Rhodes’ (1987) claim that spatial representations need to be used in

order to orient covert attention in audition, some studies have found that covert spatial

orienting can occur in auditory detection and non-spatial discrimination tasks (Buchtel,

Butter, & Ayvasik, 1996; Mondor & Zatorre, 1995; Mondor, Zatorre, & Terrio, 1998;

Quinlan & Bailey, 1995; Roberts, Summerfield, & Hall, 2009). Quinlan and Bailey (1995)

claimed that covert auditory attention effects occur at peripheral, non-spatial stages of

the auditory system. Also, Mondor and Zatorre (1995) found that the time required to

shift attention is independent of the distance of the shift, and therefore is not a linear

increase as Rhodes found. Localization performance was dependent on the azimuthal

(i.e., horizontal arc) location; however, these effects were not found for covert orienting

of attention. This finding led the researchers to propose that auditory localization and

auditory covert orienting depend on separate neural mechanisms, as well as auditory

and visual covert orienting depend on different subcortical systems (inferior colliculus

and superior colliculus, respectively; see also Buchtel, Butter, & Ayvasik, 1996, and

Thompson & Masterton, 1978). There have been others (e.g., Farah, Wong, Monheit, &

Morrow, 1989; Hillyard, Simpson, Woods, Van Voorhis, & Münte, 1984; Posner, 1987;

Woods, 1990) who have speculated that the parietal lobe is involved in spatial attention

in audition and in vision; however, the evidence for subcortical systems involved in

auditory covert spatial orienting is incomplete.

Another area in audition that is unclear is the spatial attentional effect of inhibition

of return (IOR). Some studies have not found any evidence of an IOR in audition

(Spence & Driver, 1994, 1997), while others only have found an IOR in audition when

participants either prepared (Schmidt, 1996) or made overt (Reuter-Lorenz, Jha, &

Rosenquist, 1996) eye movements to the cued location. These studies suggest that

oculomotor programming is important in causing IOR in audition. This suggestion is

found in several visual studies as well (Kingstone & Pratt, 1999; Posner & Cohen, 1984;

Posner, Rafal, Choate, & Vaughan, 1985; Rafal, Calabresi, Brennan, & Sciolto, 1989).

6

However, still others have found an inhibitory effect in auditory attention (Facoetti et al.,

2003; Mondor, Breau, & Milliken, 1998). This IOR has been found in different types of

tasks, such as in location-based and frequency-based tasks (Mondor, Breau, & Milliken,

1998) as well as simple detection tasks (Tassinari & Berlucchi, 1995), without

oculomotor preparation or execution. Therefore, it is unclear whether or not IOR is

generated by the oculomotor system.

1.2. Spatial Relevance Hypothesis

There have been some conflicting results regarding the necessary conditions for

spatial covert orienting in audition to occur, as presented above. McDonald and Ward

(1999) sought to clarify these contradictory results by introducing the spatial relevance

hypothesis (SRH). The SRH revisits Rhodes’ (1987) claim that localization tasks are

needed to ensure the use of auditory spatial representations to produce evidence for

auditory covert spatial attention. More specifically, the SRH makes two main predictions

about the spatial-orienting costs and benefits in auditory cueing tasks. The first

prediction is that an auditory spatial cue effect will occur when space is relevant to the

task, irrespective if the cue is predictive or non-predictive to the location of the target.

The easiest way to make space relevant to the task is to use a spatial-discrimination

task, although care should be taken to avoid the possibility of response priming by the

cue. Such tasks require participants to localize sounds on the basis of spatial

representations, which means that a spatial representation must be made available for

spatial orienting to occur. This prediction is supported by several studies, when the cues

are informative (Bédard, El Massioui, Pillon, & Nandrino, 1993; Quilan & Bailey, 1995;

Spence & Driver, 1994) or uninformative (Quinlan & Bailey, 1995; Roberts et al., 2009;

Spence & Driver, 1994, 1997; Ward, 1994; Ward, McDonald, & Lin, 2000) of the location

of the target. The second prediction is that “reflexive activation of location-sensitive

auditory neurons is not sufficient to produce attentional facilitation or IOR” (p.1236).

Attentional facilitation or IOR depends on whether the task is spatially relevant.

To test the SRH, McDonald and Ward (1999) conducted several spatial auditory

attention experiments, using a modified spatial cueing paradigm. Their experiments

consisted of different combinations of the spatial relevant and irrelevant tasks, as well as

frequency discrimination tasks. For most of their experiments, cue and target tones were

presented from either the centre speaker placed directly in front of the participant or

7

peripheral speakers placed to the right or left of the centre speaker. Depending on the

experiment, participants were asked to respond to the peripheral targets and to withhold

a response to the centre target (spatial go/no-go task) or to respond to high- and low-

frequency tones and withhold responses to middle-frequency tones (non-spatial go/no-

go task). McDonald and Ward called the former task the implicit spatial discrimination

task because, although participants were not asked to discriminate locations explicitly,

processing of the spatial location was still necessary to perform the task well. Moreover,

since the same response was required on valid and invalid trials, response priming was

not a problem in the implicit spatial discrimination task. McDonald and Ward found that,

at short SOAs, spatially uninformative auditory cues facilitated responses to auditory

targets, whereas, at long SOAs, they inhibited the responses in the implicit spatial

discrimination task. Critically, these effects were absent when the implicit spatial

discrimination task was replaced by the analogous non-spatial task. This pattern of

results – spatial cue effects present in the spatial task but absent in the non-spatial task

– is consistent with the first prediction of the SRH.

To provide further support for the SRH with respect to IOR, a final experiment

was conducted, which used a target-target paradigm. Participants responded to each

successive target and later responded to the spatially relevant targets. This experiment

found that, even in the absence of cues, the spatial information of the previous target

aided performance in the spatially relevant task but not when the task was spatially

irrelevant. With the use of the implicit spatial discrimination paradigm, the cue effects

observed in these experiments were not caused by response priming because space

was made relevant to the participant’s response without requiring participants to select a

different response to different target locations from where the cue effects occurred.

Therefore, McDonald and Ward (1999) concluded that participants activated location-

sensitive neurons to perform the tasks, which in turn produced auditory covert spatial

orienting effects.

In regards to negative cueing effects, no speed-accuracy trade-off (i.e., more

errors made on invalid trials than valid trials) was found in implicit spatial discrimination

tasks (McDonald & Ward, 1999). The presence of IOR was found in spatial tasks but not

in non-spatial tasks. When using identical cues in both types of tasks, it can be assumed

that they would activate the oculomotor system equally. However, IOR occurred in the

absence of oculomotor activation in spatial tasks, indicating that oculomotor preparation

8

or execution is not necessary for IOR to occur. These findings are inconsistent with the

earlier claim that oculomotor activation is necessary to elicit IOR in audition (Reuter-

Lorenz et al., 1996). Through their experiments, they found that spatial relevance is

more important for a negative cueing effect (IOR) than oculomotor activity, which

supports their second prediction.

The SRH is similar to a theory found in visual studies. Folk, Remington, and

Johnston (1992) were interested in investigating the conditions under which involuntary

shifts of spatial attention occur. These researchers conducted a series of spatial and

non-spatial visual experiments using a modified spatial cueing paradigm. Participants

saw either a feature relevant cue (e.g., four dots) or a feature irrelevant cue (e.g., four

red dots) that were non-predictive of the target (“+”) location. They found that the visual

cue captured attention only when the uninformative cue matched (i.e., valid trial) the

dimension that the observer was searching for; therefore, they found no involuntary

orienting of attention in the non-spatial task. These researchers concluded that some

visual spatial attentional processes are contingent on the task. There has been much

support for this finding (Chen & Zelinsky, 2006; Folk, Leber, & Egeth, 2002; Folk,

Remington, & Wright, 1994; Hommel, Pratt, Colzato, & Godijn, 2001; Most, Simons,

Scholl, Jimenez, Clifford, & Chabris, 2001). This finding is similar to the SRH proposed

by McDonald and Ward (1999) because it has the same conclusion as Folk et al. (1992).

The spatial relevance hypothesis posits that for auditory covert spatial orienting to occur,

the task must be spatially relevant to the task (i.e., auditory spatial processes are

contingent on the task). However, these two hypotheses are in different modalities.

The spatial orienting findings presented here are based on behavioural data.

Even though non-spatial tasks did not result in a facilitatory or inhibitory effect in the

behavioural data, studies on primates suggest that oculomotor activation occurs

regardless of the task (Jay & Sparks, 1987a, 1987b). Also, the second prediction in the

SRH states that “reflexive activation of location-sensitive auditory neurons is not

sufficient to produce attentional facilitation or IOR,” (p. 1236; McDonald & Ward, 1999).

This reflexive activation by the cues might have activated the oculomotor system equally

in both the spatial and non-spatial tasks. Even though there were no attentional effects

in the non-spatial tasks, investigating the neural mechanisms behind this equal

activation might provide more information about the underlying processes involved in

non-spatial tasks and also that are contingent on spatial tasks.

9

1.3. Neuroimaging Recording

Researchers have used different neuroimaging techniques to examine the neural

activity involved in spatial attention, focusing mostly on the visual modality.

Hemodynamic methods, such as functional magnetic resonance imaging (fMRI) and

positron emission tomography (PET), provide information about metabolic activity and

blood flow, respectively. These methods are generally used to produce high spatial

resolution images of where the neural activity is occurring. In auditory spatial attention

tasks, these techniques have localized neural activity in the frontal gyri in the prefrontal

cortex, anterior cingulate cortex (ACC), middle cingulate cortex (MCC), superior parietal

lobe (SPL), bilateral anterior insula, and bilateral putamen/caudate nuclei (Smith et al.,

2010; Wu, Weisseman, Roberts, & Woldorff, 2007). A drawback to PET is its invasive

nature since the participant has to ingest a radioisotope tracer in order for the scan to

locate neural activation. Electrophysiological methods, such as electroencephalography

(EEG), are used to record electrical neural activity associated with sensory, motor, and

cognitive process. In a large group of pyramidal cells, simultaneous post-synaptic

potentials summate and create large electrical fields that pass through the skull and

scalp. The electrical fields are recorded non-invasively from the scalp during an

experimental task. EEG has excellent temporal resolution of 1 ms or better, which is

excellent since an action potential can take about 0.5-130 ms (depending on the type of

neuron) to travel down a single neuron, whereas PET and fMRI are limited to a

resolution of several seconds because of the slow tendency of the hemodynamic

response (Luck, 2005). The excellent temporal resolution of EEG allows researchers to

investigate the stages of information processing. With EEG, researchers are able to

study the effect a cue has on a target stimulus, especially if they occur several 100 ms

apart. In addition, this allows researchers to investigate the cue and target activity within

the same stimulus range, which allows the comparison between different sets of cues

and targets. Other neuroimaging techniques are not able to see these effects with their

inferior temporal resolution.

A disadvantage to using EEG methods is that it is unable to provide precise

estimates of the locations of participating neurons. However, spatial distribution can be

inferred from summations of large electrical fields that are recorded from several scalp

electrode sites. A top-down approach can be utilized with certain software (e.g., BESA),

which calculates the source that produced the electrical activity recorded at the scalp.

10

1.4. Auditory Attention and ERPs

To investigate the neural correlates involved in auditory attention, epochs of EEG

activity that are time-locked to a specific event are averaged together to create event-

related potentials (ERPs). By averaging this activity from many trials, it reduces the

activity that is not time or phase locked to the event of interest, which results in a

waveform that represents the neural response of that specific event. The most common

approach to investigate the effects of spatial attention processing is to compare ERPs

elicited by the stimuli at attended locations and unattended locations. Several visual and

auditory studies have found that when target (and non-target) stimuli are presented at

attended locations, the elicited ERPs are more negative than when the stimuli appear at

unattended locations. This negative difference (Nd) contains two phases (early and late),

which suggests that there are multiple stages of processing when voluntarily shifting

attention to a spatial location. The early Nd has been reported to begin as early as 60

ms and usually has a maximum deflection at the fronto-central electrode sites. Woldorff

and colleagues (1993) suggested that the early Nd is generated in the auditory cortex on

the supratemporal plane, just lateral to Heschl’s gyrus. The early Nd is typically thought

to reflect processing of low-level stimulus features (e.g., frequency), which helps to

determine if the stimulus matches the target.

In sustained-attention tasks, a subsequent negative difference called the late Nd

typically occurs from approximately 250 ms to 500 ms post-stimulus. This waveform is

thought to reflect the processing of the many features of the stimuli (Woods & Alain,

2001) and the maintenance of an attentional trace of the stimuli (Shelley et al., 1991).

The amplitude of the late Nd is thought to represent the amount of attention allocated to

the task (Gomes, Duff, Barnhardt, Barrett, & Ritter, 2007). The onset latency might

reflect the duration of the processing required to determine the characteristics of the

stimuli, and the peak latency might reflect time for processing to determine the

significance of the stimulus (e.g., target) and the subsequent decision of the required

action (e.g., button press) for that stimulus (Gomes et al., 2007).

Most early ERP spatial attention experiments have focused on sustained

attention paradigms, whereas Schröger and Eimer (1993) used a trial-by-trial paradigm

where central cues were predictive of the target’s location. They examined whether

auditory spatial attention ERP effects in the trial-by-trial cueing paradigm are similar to

11

those found in a sustained attention paradigm. They found that the early Nd and late Nd

onsets were around 125 ms and 200 ms, respectively. They called these two difference

waves Nd1 and Nd2 (Schröger & Eimer, 1997). The Nd1 had a parietal scalp

distribution, which contrasts the fronto-central distribution found in sustained attention for

the early Nd. The scalp distribution for the Nd2 was fronto-central, which is slightly more

posterior than the late Nd found in the sustained attention paradigm. This difference in

scalp distribution indicated that sustained attention and transient spatial attention are

generated by different neural processing.

With most focus being on the visual modality, there have not been any ERP

studies on exogenous cue effects in audition. Therefore, little is known about the neural

mechanisms of these effects. However, there have been a few ERP studies on

exogenous crossmodal attention effects, which provide some insight into the brain

mechanisms involved in auditory spatial tasks and how these mechanisms might be

linked. McDonald and Ward (2000) conducted a crossmodal experiment involving

spatially non-predictive auditory cues and task-relevant visual targets. Participants were

required to respond to peripheral targets by pressing a single button and to withhold

responses to centrally presented targets – that is, the standard implicit spatial

discrimination task was used. McDonald and Ward found that participants responded

faster to validly cued targets than to invalidly cued targets when the SOA was short (i.e.,

cueing benefit or attentional facilitation), with no difference for longer SOAs (i.e., no IOR

effect). The voltage topographies showed two negative peaks during the shorter SOAs,

with one over the contralateral occipital region and the other over the central region. This

suggests that the Nd occurred in two different regions in the brain, with one in the

modality-specific region, the visual cortex, and the other outside of the modality-specific

region. These findings support the hypothesis that involuntary spatial attention orienting

involves linked or shared brain mechanisms (McDonald & Ward, 1999; Spence & Driver,

1997).

In another study, McDonald and colleagues provided further support for the

hypothesis of linked or shared brain mechanisms (McDonald, Teder-Sälejärvi, Heraldez,

& Hillyard, 2001). This time, spatially non-predictive visual cues were presented before

task-relevant auditory targets. A modified implicit spatial discrimination task was used:

participants were once again asked to withhold responses to centrally presented targets,

but rather than making the same simple response to all peripheral targets, they were

12

asked to press different buttons depending on the frequency of the target sound.

Behaviourally, participants were faster to respond on valid trials than on invalid trials.

Electrophysiologically, the target ERPs were more negative on valid trials than invalid

trials in the time ranges of the early Nd1 and later Nd2 components. They also found

that the Nd occurred in two different regions of the brain, with the Nd1 peaking over

parietal scalp ipsilateral to the target and the Nd2 peaking symmetrically over fronto-

central scalp. These findings provide evidence that non-predictive visual cues aid

participants to respond to nearby auditory targets and are consistent with the hypothesis

that involuntary spatial attention orienting involves linked or shared brain mechanisms.

1.5. Present Studies

The present experiments utilized intramodal auditory cueing paradigms and

investigated the spatial cue effects on electrophysiological responses as well as

behavioural responses, in both spatial and non-spatial tasks. By using

electrophysiological recording methods, such as EEG, the neural mechanisms

underlying the processes that are contingent on spatial tasks could be investigated. This

will compliment the existing literature on auditory spatial attention that examined only

behavioural effects (e.g., McDonald & Ward, 1999; Roberts et al., 2009).

In Experiment 1, spatial relevance was established by requiring a spatial go/no-

go response to the auditory targets. To do so, responses were made when the target

occurred in the peripheral speakers but withheld to the targets presented in the centre

speaker. This ensures that the target location is task relevant without having an explicit

location-based discrimination task. Experiment 2 utilized the exact same cue and target

tones as Experiment 1; however participants made non-spatial (frequency-based) go/no-

go responses rather than spatial ones.

These experiments were conducted with two primary goals. The first goal was to

examine the behavioural attentional effects (i.e., attentional facilitation and IOR) in the

first two experiments. By replicating the finding that attentional facilitation and IOR occur

only in spatially relevant tasks, it can be certain that any neural activity time-locked to

these effects is due to orienting of attention. The second goal was to compare the

behavioural attentional effects with the target-elicited ERP waveforms, namely the Nd.

The timing and topography of the Nd can help determine when, and what brain regions,

13

attention modulates auditory processing. If attentional effects are present when the task

is spatially relevant, then corresponding neural activity would be expected in Experiment

1 but not in Experiment 2. Based on previous research (e.g., McDonald et al., 2001) and

the spatial relevance hypothesis (McDonald & Ward, 1999), this hypothesis is expected.

However, it might be the case that the behavioural data does not necessarily reflect

attention processing. Therefore, if an Nd is also present in the non-spatial task, then this

would indicate that non-predictive cues captured spatial attention even though it is

irrelevant to the task. Because the orienting of attention is different in spatial and non-

spatial tasks, it is possible that their scalp topographies would also be different. The

presence of an Nd in a non-spatial task would be a novel finding.

14

2. Experiment 1

Experiment 1 is an auditory spatial task, where participants were asked to attend,

and respond, to all tones from the peripheral speakers and to ignore the tones from the

centre speaker.

2.1. Methods

2.1.1. Participants

Twenty healthy undergraduate students, between 18 and 32 years of age, from

Simon Fraser University participated in this experiment. Data from one participant were

excluded from the analyses as more than 30% of trials were rejected due to blink or eye

movement artifacts. Of the remaining 19 participants (12 females, mean age = 22.9, 16

right-handed), all reported normal hearing and normal or corrected-to-normal vision.

Written informed consent was received from all participants, as per the protocol of the

ethics board at Simon Fraser University. The participants received course credit or

payment for their participation.

2.1.2. Apparatus

A sound-attenuated, electrically shielded chamber was used for the experiment.

The chamber contained three speakers (Creative Inspire T6160 5.1 speaker system)

aligned horizontally in front of the participant. A Windows-based, Pentium-IV PC running

Presentation (Neurobehavioral Systems Inc., Albany, CA, USA) presented the stimuli

and recorded participants’ responses. Another Windows-based PC controlled EEG

acquisition using Acquire (custom software). The acquisition PC was connected to a 64-

channel analog-to-digital board (a 12-bit data-acquisition board; PCI-6071e; National

Instrumentation, Austin, TX, USA), which was in turn connected to SA Instrumentation

EEG amplifiers (SA Instrumentation Co., San Diego, CA, USA).

15

2.1.3. Stimuli

Three target tones differing in frequency (1 kHz, 1.73 kHz, and 3 kHz), with a

duration of 50 ms and a rise and fall time of 5 ms, were used. The target tones were

presented at 75 dB, measured at the ears. The noise (cue) tone was a 70 ms noise burst

(30 ms noise burst followed by a 10 ms blank interval followed by another 30 ms noise

burst), with a rise and fall time of 5 ms. The cue tone was presented at 70 dB, measured

at the ears.

2.1.4. Design and Procedure

The centre speaker was positioned directly in front of the participant, whereas the

left and right speakers were positioned 38° to the left and right of the centre speaker,

respectively. The speakers were raised to ear level. The participant sat in a dimly lit

experimental chamber 57 cm from the centre speaker. To encourage participants to

keep their eyes still, a fixation sticker measuring 1 cm x 1cm was placed in the centre of

the centre speaker. Each trial began with a 70 ms noise-burst cue randomly from the

left, centre, or right loudspeaker (40%, 20%, 40%, respectively). On most trials, a target

was then presented to the left, centre, or right loudspeaker after a short silent interval.

On 14% of trials, no target was presented. On the remaining trials, the cue-target

stimulus-onset asynchrony (SOA) was either 150 ms (71% probability) or 900 ms (14%

probability). Participants were instructed to press either a left or right button with their

index finger on a gamepad when the onset of a target tone from the peripheral speakers

(go trials) and asked to ignore target tones presented from the centre speaker (no-go

trials). The response hand was randomly selected at the beginning of the experiment

and then counterbalanced after the halfway point (i.e., after block 17) of the experiment.

The cue was not predictive of the target's location. All tones were randomly presented

from the speakers with a probability of 40% from the left, 20% from the centre, and 40%

from the right speaker. The intertrial interval (ITI) was between 1,500 ms and 2,000 ms

for all trials. Examples of the stimulus sequences on different trial types are displayed in

Figures 2-1 and 2-2.

At the end of three blocks, participants received verbal feedback on their

performance from the experimenter. A mandatory 10-second rest break was taken after

every block (about 1.5 minutes). Participants received a 5-minute break after 12 blocks

(after about 18 minutes) where participants were able to play computer games or relax

16

Figure 2-1. Trial Sequences for Valid-Cue Trial (Left) and Invalid-Cue Trial (Right). These Illustrations are Examples of Go Trials.

Figure 2-2. Trial Sequences for a No-Go Trial (Left) and a Catch/No Target Trial (Right).

17

their eyes. Data were collected in a single test session consisting of 35 blocks of 35 trials

each block, resulting in 560 peripheral target trials (go trials), with 280 valid-cue trials

and 280 invalid-cue trials.

2.1.5. Electrophysiological Recording

EEG was recorded from 63 tin electrodes and was placed at FP1, FPz, FP2,

AF3, AF4, F1, F3, F5, F7, Fz, F2, F4, F6, F8, FC1, FC3, FC5, FCz, FC2, FC4, FC6, C1,

C3, C5, Cz, C2, C4, C6, T7, T8, CP1, CP3, CP5, CPz, CP2, CP4, CP6, P1, P3, P5, P7,

Pz, P2, P4, P6, P8, PO3, PO7, POz, PO4, PO8, O1, Oz, O2, I3, I5, Iz, I4, I6, SI3, SIz,

and SI4 (Electro-Cap International Inc.). Five electrodes were not positioned on the 10-

10 system; they were placed inferior to the occipital electrodes. The ground was placed

on the midline between Cz and CPz. External electrodes were placed on both mastoids.

All EEG signals were referenced to the right mastoid. Horizontal electrooculogram

(HEOG) tracked eye movements using bipolar external electrodes that were placed 1 cm

lateral to the right and left outer canthi. Blinks were monitored using the FP1 electrode.

All impedances were kept below 10 kOhms. All signals were amplified with a gain of

20,000, recorded with a bandpass of 0.1-100 Hz (-3 dB point; -12 dB per octave) and

digitized at 500 Hz using a SA Instrumentation amplifiers (a 12-bit data-acquisition

board; PCI-6071e; National Instrumentation, Austin, TX, USA) and Acquire (custom

Windows software).

ERPSS (University of California, San Diego, CA, USA) was used to process the

data offline. The data were epoched from 200 ms before target onset to 800 ms post-

target. All data were visually inspected offline for blinks, eye movements, and amplifier

blocking and the trials containing these artifacts were excluded. Participants with

excessive noise due to these artifacts were removed from further analysis. Trials that

participants incorrectly responded to the centre speaker target were discarded. A

digitally low-pass filter (-3 dB cutoff at 25 Hz) was applied to remove high-frequency

noise caused by muscle movements and external electrical sources. Averaged ERP

waveforms were created from the artifact-free data.

2.1.6. Data Analysis

Behavioural analysis. Trials with response times less than 100 ms, more than

1,500 ms, or with no response on a go trial were removed from further analysis. All tones

18

from the centre speaker were excluded from further analysis. Mean response times and

false alarms were analysed in a 2 x 2 repeated-measures analysis of variance (ANOVA),

with SOA (150 ms and 900ms) and Cue Type (valid and invalid) as the within-subject

factors. Central (i.e., “neutral”) cues were not included as a third level of the Cue Type

factor because of the inherent difficulty of interpreting “neutral" cues (cf. Jonides & Mack,

1984; Wright, Richard, & McDonald, 1995). Planned Bonferroni t-tests between valid-cue

and invalid-cue conditions at both SOAs (two tests) were performed using the MSe from

the experiment's SOA X Cue Type interaction to calculate the critical difference for the

cue effects. These tests were two-tailed, with the familywise error rate set at 0.05.

ERP analysis. The difference waveforms were calculated by subtracting invalid-

cue trials from valid-cue trials. In order to remove overlapping cue ERPs and thus isolate

the target-elicited ERPs for the short SOA, cue ERPs were averaged using the 900-ms

SOA trials and no-target trials and were then subtracted from the target-elicited ERPs

obtained on the 150-ms SOA trials. Target-elicited ERPs were collapsed across target

location (left, right) and cue type (valid, invalid) to reveal ERP waveforms recorded

contralateral and ipsilateral to the target location.

For the analysis of the cue- and target-elicited waveforms, mean amplitudes

were measured relative to a 100 ms pre-stimulus baseline. Separate t-tests were

performed to compare the mean amplitudes of the target ERPs on valid and invalid trials

in the time intervals of the N1 (80-120 ms, at FCz), Nd (200-325 ms, at FCz and PO7/8),

and a subsequent positive difference (Pd) observed over the fronto-central scalp (400-

500 ms, at FCz). Three distinct positive deflections over occipital scalp (330-430 ms)

were analysed in a repeated-measures ANOVA, with Electrode as the sole within-

subject factor across the sites of interest. Planned comparisons were performed to

compare the amplitudes of the sites of interest

The mean voltages for the Nd and Pd amplitudes, as well as for a posterior

positive deflection, were mapped for their latency ranges in order to estimate its neural

generators. Voltage maps were used to visualize the distribution of the electrical fields

across the scalp. To examine the topography of lateralized ERP activity, contralateral-

minus-ipsilateral voltage differences were calculated for homologous left and right

electrodes (e.g., PO7 & PO8), the resulting voltage differences were assigned to

electrodes on the right side of the head and were copied to electrodes on the left side of

19

the head after inverting the voltage polarities (so-called “anti-symmetric” mapping;

Praamstra, Stegeman, Horstink, & Cools, 1996). Voltages at midline electrodes were set

to zero for the now symmetrical maps (Green, Teder-Sälejärvi & McDonald, 2005).

2.2. Results and Discussion

2.2.1. Behaviour

False alarms were made to only 5% of the centre (no-go) targets, which

demonstrates that participants largely withheld responses to targets presented from the

centre location. The ANOVA on median RTs revealed a significant main effect for SOA,

F(1, 18) = 19.1, p < .001, with shorter RTs in the 150-ms SOA condition than in the 900-

ms SOA condition (Table 2-1). This SOA effect, which is opposite to the typical SOA

effect associated with alertness (shorter RTs at longer SOAs), was probably due to the

relatively high proportion of short-SOA trials. That is, participants probably expected the

target to occur after a 150-ms SOA, and thus responses were delayed when the target

appeared after an unexpectedly long SOA. The main effect of Cue Type was not

significant, F(1, 18) < 1, but there was a significant SOA x Cue Type interaction, F(1, 18)

= 24.5, p < .001. At the 150-ms SOA, the RTs were significantly shorter (by Bonferroni

comparison) on valid-cue trials (491 ms) than on invalid-cue trials (514 ms). This 23-ms

cueing effect indicates that participants directed their attention to the cued location and

that this deployment of spatial attention facilitated the processing of targets appearing at

the cued location. As Table 2-1 shows, this cue-validity effect reversed at the 900-ms

SOA – that is, RTs were significantly longer on valid-cue trials (568 ms) than on invalid-

cue trials when the SOA was 900 ms (547 ms; by Bonferroni comparison). This -21-ms

difference indicates that IOR occurred when the cue-target SOA was long, possibly

because attention was inhibited from returning to the cued location.

20

Table 2-1. Inter-Participant Averages of Median Response Times (RT, in Milliseconds) and Standard Errors (SE) as a Function of Cue-Target Stimulus Onset Asynchrony (SOA) and Cue Type in Experiment 1.

SOA

150 ms 900 ms

Cue Type RT SE RT SE

Valid 491 20.1 568 15.5

Centre 542 23.3 553 14.9

Invalid 514 20.9 547 15.9

As expected, the behavioural results of Experiment 1 replicate the pattern of cue-

validity effects observed in McDonald and Ward’s (1999) experiments that used similar

implicit-spatial-discrimination tasks (Experiments 1, 3, and 4). This is in line with the first

prediction of the spatial-relevance hypothesis outlined in Section 1.4 above. As noted by

McDonald and Ward, the biphasic pattern of cue effects – with facilitation at the short

SOA and inhibition at the long SOA – is strikingly similar to the biphasic pattern of cue

effects observed in visual cueing studies that employed spatially non-predictive

peripheral cues (for recent reviews, see Klein, 2000; Klein, 2004). Such similarity has

been interpreted as evidence for a shared attention-control system that mediates

exogenous shifts of attention in visual as well as auditory space (e.g., Spence &

McDonald, 2004; Spence, McDonald, & Driver, 2004).

2.2.2. Target-elicited ERPs

Figure 2-3 shows the grand-averaged ERP waveforms for validly- and invalidly-

cued targets at ipsilateral and contralateral fronto-central and parietal- occipital

electrodes (denoted iFC1, cFC2, iPO7, and cPO8, respectively). Generally, the target-

21

Figure 2-3. Grand-Averaged ERP Waveforms for Validly- and Invalidly-Cued Targets in Experiment 1.

elicited ERPs for valid and invalid trials had similar morphologies. Both waveforms

consisted of a fronto-central N1 peak 100–110 ms after target onset as well as a

prolonged positive peak over the posterior scalp between 300–450 ms. The waveforms

overlapped in the early phase of the N1, but the N1 peak appeared to be smaller on

valid trials than on invalid trials over the midline fronto-central scalp. Statistical analysis

revealed only a marginally significant validity effect on the N1 amplitude, t(18) = -1.75, p

= .097.

Following the N1, the target ERPs became more negative on valid trials than on

invalid trials in the 200–325 ms time range. This negative difference, which was isolated

by subtracting the invalidly-cued target ERP from the corresponding validly-cued target

ERP (at each electrode), corresponds to the fronto-central Nd2 reported in previous

auditory cueing studies. Unlike typical Nd2 waves associated with the voluntary

deployment of auditory spatial attention (in response to symbolic cues; see Tata & Ward,

2005), however, the Nd2 observed in Experiment 1 extended posteriorly to the

contralateral occipital scalp (Figure 2-4). Statistical analysis confirmed that the ERP was

22

Figure 2-4. Topographical Voltage Maps of the Nd and the Pd Elicited by Auditory Target Stimuli in Experiment 1.

significantly more negative on valid trials than on invalid trials in the Nd2 time range, at

FCz, t(18) = 2.85, p < .05, as well as contralateral PO7/8, t(18) = 2.67, p < .05.

Following the Nd, the target ERPs became more positive on valid trials than on

invalid trials over a large portion of the scalp (Figure 2-3). To remain consistent with the

nomenclature of the Nd, this difference will be labelled the positive difference (Pd). The

Pd was largest over the fronto-central scalp but extended posteriorly to the occipital

scalp (Figure 2-4). Over the posterior scalp, the Pd was in evidence only ipsilaterally.

The absence of the Pd over the contralateral occipital scalp may have been due to a

lingering Nd in that region. Statistical analysis confirmed that the ERP waveform was

more positive on valid trials than on invalid trials in the 400–500 ms time interval (at FCz,

t(18) = -3.82, p < .01). A late Pd has been reported previously around 500 – 700 ms after

target onset and was maximal over parieto-central scalp in voluntary auditory spatial

23

Figure 2-5. Topographical Voltage Maps of Positive Deflections over Occipital Scalp in Experiment 1.

orienting tasks (Tata & Ward, 2005). However, the functional significance of this late Pd

was unclear.

In addition to the N1, Nd, and Pd components, which were maximal over the

fronto-central scalp, a prolonged positive peak was in evidence in the ERPs recorded

over the posterior scalp (Figure 2-3). This positive peak resembled the P3b in latency,

peaking at approximately 375 ms, but unlike the P3b, it was larger in the occipital ERPs

than in parietal ERPs. Figure 2-5 displays the scalp topography of this positivity,

separately for validly- and invalidly-cued targets. Like the maps of Nd and Pd (Figure 2-

4), these topographical maps were plotted so that the left and right sides of the maps

show activity recorded ipsilateral and contralateral to the target, respectively. On valid

trials, there were three distinct peaks at POz, iPO7, and cPO8. Separate t-tests

performed at each of the three electrodes revealed that the P3-like positivity was

significant at all of these sites: POz, t(18) = 3.49, p < .01, iPO7/8, t(18) = 3.9, p = .001,

and cPO7/8, t(18) = 5.37, p < .001. A follow-up repeated-measures ANOVA with

Electrode as the lone within-subject factor was performed to investigate differences in

the P3 amplitude across the three sites of interest. The ANOVA revealed a significant

main effect of Electrode, F(2, 17) = 11.61, p = .001, and planned comparisons revealed

that the amplitude at the posterior contralateral electrode (cPO7/8) was significantly

larger than the amplitude at the posterior ipsilateral electrode (iPO7/8), t(18) = 4.45, p <

.001. Similar statistical analyses were performed on the target-elicited P3-like amplitudes

obtained on invalid trials. On those trials, the P3 was significant at each of the electrodes

24

in question: POz, t(18) = 3.52, p < .01, iPO7/8, t(18) = 4.46, p < .001, and cPO7/8, t(18)

= 5.14, p < .001. The ANOVA revealed a significant main effect of Electrode, F(2, 17) =

6.16, p = .01. The topography was similar on valid trials with the positivity significantly

larger over the contralateral scalp (cPO7/8) than the ipsilateral scalp (iPO7/8), t(18) =

3.23, p < .01.

In sum, three main findings emerged from Experiment 1. First, and as expected

on the basis of the spatial relevance hypothesis, validly cued targets elicited an Nd2 that

was maximal over the fronto-central scalp. This suggests that salient but spatially non-

predictive auditory cues influenced the processing of nearby auditory targets when the

cue-target SOA was 150 ms. Although no attempt was made to localize the neural

sources of the Nd2 here, previous studies have indicated that other similar Nd waves

originate in bilateral auditory cortices and reflects selective processing of the attended

target’s features (cf. Hillyard, Mangun, Woldorff, & Luck, 1995; Näätänen, 1992;

Schröger & Eimer, 1997). Second, following the Nd, the cue-validity effect reversed –

that is, the ERPs became more positive on valid trials than on invalid trials. The Pd’s

functional significance is unclear at the moment. This issue will be revisited in

Experiment 2 as well as the General Discussion. Third, auditory targets elicited a P3-like

component that appeared to originate from the occipital cortex. This suggests that visual

regions of the brain participated in the processing of the auditory targets.

2.2.3. Cue-elicited ERPs

To investigate further the mechanism by which the salient but spatially non-

predictive auditory cue influenced processing of subsequent target sounds, the ERPs

elicited by the cues themselves were examined. As expected, several typical auditory

ERP components were observed in the initial 200 ms following cue onset, including the

N100 (90–100 ms) over the central scalp and a subsequent N140 (130–150 ms) over

bilateral temporal scalp regions (Figure 2-6). These negative ERP components are

known to reflect modality-specific sensory processing within the auditory cortex (Picton,

2011).

To determine whether there was any spatially specific ERP activity associated

with the presentation of the lateral auditory cue, differences between ERPs recorded

contralateral and ipsilateral to the side of the cue were examined. This event-related

25

Figure 2-6. Grand-Averaged Event-Related Lateralization (ERLs) Elicited by Auditory Cues in Experiment 1. A. Ipsilateral and Contralateral Waveforms for T7/8 and PO7/8. B. Collapsed Ipsilateral and Contralateral Difference Waveforms for T7/8 and PO7/8.

lateralization (ERL) method has been widely used to study location-specific ERP activity

associated with manual responding (e.g., lateralized readiness potential, LRP;

Oostenveld, Stegeman, Praamstra, & van Oosterom, 2003; Praamstra et al., 1996),

visual search (e.g., N2pc; Hickey, Di Lollo & McDonald, 2009; Luck & Hillyard, 1994;

Woodman & Luck, 2003), and voluntary shifts of spatial attention associated with

centrally presented symbolic cues (e.g., LDAP; Harter, Miller, Price, LaLonde, & Keyes,

1989; Hopf & Mangun, 2000; McDonald & Green, 2008). For example, when a centrally

presented symbolic cue indicates that an impending visual target is likely to appear on

the left side of fixation, ERPs recorded over the occipital scalp are often more positive

over the right side (contralateral) of the scalp than the left side (ipsilateral). This posterior

contralateral positivity has been labelled the late directing attention positivity (LDAP)

because it happens relatively late in the cue- target interval (400-800 ms post-cue) and

is believed to reflect attentional modulation of activity in the contralateral visual cortex

(Hopf & Mangun, 2000).

26

Figure 2-7. Topographical Voltage Maps of the ACOP Elicited by Auditory Cue Stimuli.

Two cue-elicited ERLs were evident in the present experiment (Figure 2-6). The

first ERL occurred in the time interval of the N140, which was larger contralateral to the

side of the cue (electrodes T7/T8; t(18) = -5.08, p < .001). This indicates that relatively

early processing within the auditory system was lateralized. The polarity of the N140

reversed at more posterior sites (PO7/8). The second ERL was in evidence over the

posterior scalp 300-450 ms after cue onset. In this interval, the ERPs were more positive

contralateral to the side of the cue than ipsilateral to it (PO7/PO8; t(18) = 7.15, p < .001).

This finding is in line with the results of a recent (unpublished) study, according to which

salient but spatially non-predictive sounds activate visual cortex automatically

(McDonald, Störmer, Martinez, Feng, & Hillyard, 2012). To help determine whether the

posterior contralateral positivity observed in the present experiment stemmed from visual

cortex, its scalp topography was determined using the anti-symmetric mapping method

(see methods for details). Consistent with a source in occipital cortex, the resulting

topographical map was found to have a positive focus over the lateral occipital scalp

(Figure 2-7).

In summary, the auditory cue was found to elicit lateralized ERP activity initially

over the superior temporal scalp (i.e., auditory cortex) and then over the occipital scalp

(i.e., visual cortex). The latter ERL – tentatively labelled the auditory-evoked

contralateral occipital positivity (ACOP) – is consistent with the hypothesis that salient

but spatially non-predictive sounds activate visual cortex automatically, even when the

task requires no visual processing. Since the ACOP has been reported in only one other

27

study (McDonald et al., 2012), the functional significance of this positivity is unclear. This

will be revisited in Experiment 2. However, a possible explanation for a large positivity in

the visual cortex is that since the visual modality has the highest spatial resolution out of

all the senses, the attentional shift from the cue to the target would involve the

processing of the to-be-attended visual space (Green et al., 2005; Ward, 1994).

28

3. Experiment 2

Experiment 2 was identical to Experiment 1, with one exception: Participants

were asked to discriminate between the three frequency tones rather than their spatial

location.

3.1. Methods

3.1.1. Participants

Twenty-four healthy undergraduate students, between 18 and 23 years of age,

from Simon Fraser University participated in this experiment. Data from six participants

were excluded from the analyses as more than 30% of trials were rejected due to blink

or eye movement artifacts. Of the remaining 18 participants (14 females, mean age =

20.3, 17 right-handed), all reported normal hearing and normal or corrected-to-normal

vision. Written informed consent was received from all participants, as per the protocol of

the ethics board at Simon Fraser University. The participants received course credit or

payment for their participation.

3.1.2. Apparatus

The apparatus was identical to those used in Experiment 1.

3.1.3. Stimuli

The stimuli were identical to those used in Experiment 1, with the exception that

only two of the three tones are targets. The tones were the same frequency as

Experiment 1 (i.e., 1 kHz, 1.73 kHz, and 3 kHz). However, the difference is that the

go/no-go decision was based on frequency rather than sound location, with the target

tones being the low (1 kHz) frequency tone and the high (3 kHz) frequency tone (go

trials) and non-target tones being the middle (1.73 Hz frequency tone; no-go trials). Both

target tones had the probability of 40% and the non-target tone occurred at the rate of

20%.

29

3.1.4. Design and Procedure

The design and procedure were identical to Experiment 1, with the exception that

participants were instructed to press either a left or right button with their index finger on

a gamepad to the onset of target tones (i.e., low- or high-frequency tones) presented

from any speaker (go trials) and to ignore non-target tones (i.e., middle-frequency tones)

presented from any speaker (no-go trials). The response hand was randomly selected at

the beginning of the experiment and then counterbalanced after the halfway point (i.e.,

after block 17) of the experiment

3.1.5. Electrophysiological Recording

All electrophysiological recordings were identical to those used in Experiment 1.

3.1.6. Data Analysis

The analysis procedures were identical to those used in Experiment 1 with the

exception that the non-target tone was excluded from the analysis, since participants

were instructed to withhold responses to this tone.

3.2. Results and Discussion

3.2.1. Behaviour

False alarms were made to only 6% of the centre (no-go) targets, which

demonstrates that participants largely withheld responses to the non-target (middle)

tone. The ANOVA on median RTs revealed a significant main effect for SOA, F(1, 17) =

67.3, p < .001, with shorter RTs in the 150-ms SOA condition than in the 900-ms SOA

condition (Table 3-1). This SOA effect, similar to Experiment 1, was likely a

consequence of the high proportion of 150-ms SOA trials relative to 900-ms SOA trials.

Neither the main effect of Cue Type, F(1, 17) < 1, nor SOA x Cue Type interaction, F(1,

17) < 1, was significant. These non-significant results indicate that participants

responded to the target tones with similar speed on valid-cue and invalid-cue trials;

spatial attention neither facilitated nor inhibited processing of targets appearing at the

cued location.

30

Table 3-1. Inter-Participant Averages of Median Response Times (RT, in Milliseconds) and Standard Errors (SE) as a Function of Cue-Target Stimulus Onset Asynchrony (SOA) and Cue Type in Experiment 2.

SOA

150 ms 900 ms

Cue Type RT SE RT SE

Valid 567 14.9 657 20.0

Centre 575 14.2 676 20.9

Invalid 574 15.7 657 17.0

These behavioural results replicate the pattern of absent cue-validity effects seen

in previous implicit-frequency-discrimination tasks by McDonald and Ward (1999;

Experiment 2 and 5). This supports the second prediction of the spatial-relevance

hypothesis, which states that no cue effects are found when space is irrelevant in a non-

spatial task. However, as mentioned in the introduction, previous research has found

spatial cueing effects in non-spatial auditory tasks when the cues are informative of the

target location (Buchtel, Butter, & Ayvasik, 1996; Mondor & Zatorre, 1995; Quinlan &

Bailey, 1995). The first prediction of the SRH states that if space is made relevant to the

task, even if the task is inherently non-spatial, spatial cueing effects are present. Thus,

spatially informative cues covertly oriented attention to the spatial location of the targets

even though the response was based on a non-spatial aspect of the target.

3.2.2. Target-elicited ERPs

Grand-averaged ERP waveforms for validly- and invalidly-cued targets at

ipsilateral and contralateral fronto-central and parietal-occipital electrodes are shown in

Figure 3-1 (iFC1, cFC2, cPO7/8, and iPO7/8, respectively). The target-elicited ERPs for

31

Figure 3-1. Grand-Averaged ERP Waveforms for Validly- and Invalidly-Cued Targets in Experiment 2.

valid and invalid trials had similar morphologies. At the early phase of the N1, the

waveforms overlapped at fronto-central scalp, peaking between 100-110 ms after target

onset, but the N1 peak appeared to be larger on valid trials than on invalid trials over

midline fronto-central scalp; however, similar to Experiment 1, this difference was not

statistically significant, t(17) = 1.22, p = .239.

After the N1, the target-elicited ERPs became more negative on valid trials than

on invalid trials in the 200-350 time range. In the valid-invalid difference waves, the

single-peak Nd can be clearly seen over midline central scalp (Figure 3-2), which is

more anterior than the posterior peak seen in Experiment 1. This difference in scalp

topography may suggest that there is enhanced processing in a posterior brain region

that is associated with the attentional facilitation effect found in Experiment 1. The

distribution of this Nd corresponds to the fronto-central Nd2 reported in previous auditory

cueing studies (McDonald et al., 2001; Tata & Ward, 2005). Statistical analysis

confirmed that the ERP was significantly more negative on valid trials than on invalid

trials in the Nd time range, at Cz, t(17) = 4.82, p < .001.

32

Figure 3-2. Topographical Voltage Maps of the Nd Elicited by Auditory Target Stimuli in Experiment 2.

Unlike Experiment 1, following the Nd, no Pd was evident in this frequency-based

go/no-go task (Figure 3-2). The absence of the Pd suggests that this component may be

associated with spatial attention since the only difference in experimental manipulation

between the two experiments was whether space was relevant to the task (Experiment

1) or not (Experiment 2).

The waveforms over posterior scalp (shown in Figure 3-1) had similar

morphologies for valid and invalid trial, with a prolonged positive deflection peaking

between 350-500 ms after target onset. Similar to Experiment 1, this positive peak was

larger in the occipital ERPs than in parietal ERPs for both valid and invalid trials. The

scalp topography of this positivity for valid and invalid trials is shown in Figure 3-3.

These topographical maps were plotted to display recorded activity ipsilaterally and

contralaterally to the target on the left and right sides, respectively. On valid trials, the

33

Figure 3-3. Topographical Voltage Maps of Positive Deflections over Occipital Scalp in Experiment 2.

peak was largest at POz, iPO7/8, and cPO7/8. Separate t-tests performed at each of the

three electrodes revealed that this positivity was significant at each of these sites: POz,

t(17) = 6.78, p < .001, iPO7/8, t(17) = 5.73, p < .001, and cPO7/8, t(17) = 7.13, p < .001.

A repeated-measures ANOVA with Electrode as the only within-subject factor was

performed to examine differences across the three electrode sites. The ANOVA revealed

a significant main effect of Electrode, F(2, 16) = 19.58, p < .001. Planned comparisons

revealed that the amplitude at the posterior contralateral electrode (cPO7/8) and at the

posterior midline electrode (POz) were significantly larger than the amplitude at the

ipsilateral electrode (iPO7/8; t(17) = 4.58, p < .001, and t(17) = 5.69, p < .001,

respectively). For invalid trials, similar statistical analyses were performed for the target-

elicited P3 amplitude. Separate t-tests revealed that the P3 was significant at each of the

electrodes in question: POz, t(17) = 6.85, p < .001, iPO7/8, t(17) = 6.33, p < .001, and

cPO7/8, t(17) = 7.54, p < .001. The ANOVA revealed a main effect of Electrode, F(2, 16)

= 17.26, p < .001. The topography was similar on valid trials with the positivity

significantly larger over the contralateral scalp (cPO7/8) and midline scalp (POz) than

the ipsilateral scalp (iPO7/8; t(17) = 4.53, p < .001, and t(17) = 4.77, p < .001,

respectively). Regardless of the task, a P3-like component was observed over occipital

scalp in Experiment 1 and Experiment 2. This component appeared to be generated in

the visual cortex for both experiments.

In sum, the four main findings from Experiment 2 are as follows. First, the

behavioural data showed that no cueing effects (i.e., attentional facilitation and IOR)

34

were present in this non-spatial task. This finding is in line with the spatial relevance

hypothesis, according to which spatial cueing will not influence behavioural performance

when the task does not require spatial processing. Second, although the absence of the

behavioural cue effect suggested that cues did not affect the spatial distribution of

attention, the target was found to elicit the Nd on valid-cue trials. The presence of the Nd

indicates that non-predictive auditory cues captured spatial attention even though the

task was non-spatial. The Nd observed in Experiment 2 was found over the fronto-

central scalp only, whereas the Nd in Experiment 1 was also observed over the

contralateral occipital scalp. This difference suggests that the auditory cue influenced

target processing in posterior parietal and possibly occipital cortical regions in

Experiment 1 but not in Experiment 2. Third, the Pd that was observed after the Nd

interval in Experiment 1 was not in evidence in Experiment 2. This pattern of results is

consistent with the pattern of behavioural cue effects as well as the predictions

stemming from the spatial relevance hypothesis. The functional significance of the Pd is

unclear but will be revisited in the General Discussion. Fourth, auditory targets once

again elicited a P3-like component that appeared to originate from the occipital cortex.

This suggests that visual regions of the brain participated in the processing of the

auditory targets in Experiment 2, as it did in Experiment 1.

3.2.3. Cue-elicited ERPs

The ERPs elicited by the cues were examined to investigate the mechanism by

which auditory cues influenced processing of subsequent target sounds. The same two

cue-elicited ERLs observed in Experiment 1 – the lateralized N140 and the ACOP –

were once again in evidence in Experiment 2 (Figure 3-4). Over the temporal scalp, the

N140 was larger contralateral than ipsilateral to the side of the cue (electrodes T7/T8;

t(17) = -5.55, p < .001), indicating that early processing within the auditory system was

lateralized. As in Experiment 1, the polarity of the lateralized N140 reversed at more

posterior sites – that is, the contralateral N140 was more positive than the ipsilateral

N140 over the posterior scalp. The ACOP was observed over the posterior scalp,

beginning around 300 ms after cue onset and continuing to about 550 ms. This finding is

similar to that found in Experiment 1, as well as a recent (unpublished) study, which

suggests that salient but spatially non-predictive sounds activate visual cortex

automatically (McDonald et al., 2012).

35

Figure 3-4. Grand-Averaged Event-Related Lateralization (ERLs) Elicited by Auditory Cues in Experiment 2. A. Ipsilateral and Contralateral Waveforms for T7/8 and PO7/8. B. Collapsed Ipsilateral and Contralateral Difference Waveforms for T7/8 and PO7/8.

Figure 3-5. Topographical Voltage Maps of the ACOP in Experiment 2.

36

The anti-symmetric mapping method was used to help determine whether the

posterior contralateral positivity observed in the present experiment stemmed from visual

cortex. Consistent with a source in occipital cortex, the resulting topographical map was

found to have a positive focus over the lateral occipital scalp, and was significant at

PO7/8, t(17) = 4.85, p < .001. The ACOP in this experiment extended more posteriorly

than the ACOP observed in Experiment 1 (Figure 3-5).

In sum, the auditory cue elicited ERLs first over the temporal scalp (N140) and

then later over the occipital scalp (ACOP). As seen in Experiment 1, the ACOP was

observed over occipital scalp, suggesting that the visual cortex was automatically

activated by spatially non-predictive sounds without the presence of visual stimuli.

37

4. General Discussion

The present study investigated the neural activity elicited by spatially non-

predictive auditory cues and subsequent auditory targets in spatial and non-spatial

tasks. The behavioural results in both experiments were consistent with the spatial

relevance hypothesis (SRH), according to which attentional cueing effects shall be found

only in tasks that require spatial processing of the auditory stimuli. As predicted by the

SRH, spatial cue effects were in evidence in Experiment 1, which involved a spatial

go/no-go task, but were absent in Experiment 2, which involved a non-spatial go/no-go

task. This pattern of effects replicates the original pattern of results reported by

McDonald and Ward (1999).

The present behavioural results appear to be inconsistent with results from a

recent study that cast some doubt on the SRH (Roberts et al., 2009). In Roberts et al.’s

study, participants judged whether complex targets tones that were presented through

headphones were tuned or mistuned. Spatially non-predictive auditory cues were found

to facilitate responses to same-ear auditory targets in this task. Such results are clearly

inconsistent with the SRH, so one may ask why such results were obtained by Roberts

et al. but not in the present experiment. In fact, McDonald and Ward (1999) did note that

non-predictive auditory cues tend to produce small but statistically significant cue effects

in non-spatial tasks (detection, frequency discrimination) when sounds are delivered

monaurally but not when delivered from external space. They suggested that monaural

presentation may establish “spatial relevance directly because the sounds are

unambiguously localized to one ear,” (McDonald & Ward, 1999, p. 1250). In other words,

monaurally presented sounds might capture spatial attention reflexively, as appears to

be the case for easy-to-localize visual stimuli (Yantis & Jonides, 1990). By contrast, in

the non-spatial experiments of McDonald and Ward’s study and the present study

(Experiment 2), participants made go/no-go judgements about auditory targets that were

presented in external space. Such stimuli are not as easy to localize and do not tend to

activate the contralateral auditory cortex more than the ipsilateral cortex in the same way

monaurally presented sounds do (Woldorff et al., 1999).

38

Recently, a study was conducted in our lab to determine whether monaural

stimulus presentation was a crucial factor in Roberts et al. (2009) study (Hull &

McDonald, 2011, unpublished). The stimuli and general methodology were similar to

those used by Roberts et al., but sounds were delivered from either headphones or

external loudspeakers in different groups of participants. When the stimuli were

presented through headphones, behavioural cueing effects were observed, thereby

replicating the results reported by Roberts et al. However, when the same stimuli were

presented through external speakers, no behavioural cueing effects were observed,

thereby replicating the null findings of the present Experiment 2. Such results

demonstrate that the presentation method of auditory stimuli is an important factor for

spatial cue effects in non-spatial tasks. While it is not fully clear why monaurally

presented sounds elicit spatial cue effects in non-spatial tasks, it is clear that when

sounds are presented externally, the RT cue effects are consistent with the predictions

of the SRH.

The primary goals of the present study were to use ERPs to determine (1)

whether the auditory cue captured attention in spatial and non-spatial tasks, and (2)

what ERP effect, if any, closely mirrored the RT effect (and thus were in line with the

SRH). At the outset, two possible outcomes were considered. First, if auditory cues

captured attention only in the spatial task as reflected in the behavioural results, the

main ERP components associated with auditory attention – the Nd and the newly

discovered ACOP – would be in evidence in a spatial task (Experiment 1) but not in a

non-spatial task (Experiment 2). Second, auditory cues might capture spatial attention in

both tasks but yet fail to affect behavioural performance in non-spatial tasks. This latter

prediction was based on the hypothesis that spatial orienting effects might be contained

in a spatial auditory system (e.g., “where” pathway) while non-spatial target processing

might tap into a different, completely non-spatial auditory system (e.g., “what” pathway;

Arnott, Binns, Grady, & Alain, 2004; Lomber & Malhotra, 2008; Sumner, Palmer, &

Moore, 2008; Tata & Ward, 2005).

The ERP results were generally consistent with the second prediction (see Table

4-1). As expected, a target-elicited valid-invalid difference waveform (Nd) was observed

in the spatial task, corresponding to the behavioural results. This suggests that, at the

short cue-target SOA (150 ms), salient but spatially non-predictive auditory cues

influenced the processing of subsequent auditory

39

Table 4-1. Summary of Behavioural and Electrophysiological Effects in the Spatial and Non-Spatial Tasks.

Task

Spatial Non-spatial

RT cue effects � �

Anterior Nd � �

pc Nd � �

Pd � �

ACOP / P3 � �

targets. However, this neural activity was also observed in the non-spatial task, even

though the absence of the behavioural cueing effects suggested that the cues did not

affect the spatial distribution of attention in that task. These results are consistent with

the auditory dual-pathway hypothesis, according to which the auditory cue initiated a

shift of spatial attention but that the processes related to the control and deployment of

attention were contained in a task-irrelevant spatial system.

The scalp topography of the Nd was more posterior in Experiment 1 than in

Experiment 2 suggesting that the auditory cue influenced target processing differently in

the two tasks. The fronto-central Nd in Experiment 2 is consistent with the topography of

the Nd2 in previous auditory cueing studies (McDonald et al., 2001; Schröger & Eimer,

1997; Tata & Ward, 2005). The difference in topography between the two experiments

may reflect the involvement of different attention orienting mechanisms in the two

different tasks. The Nd observed in Experiment 1 had three distinct peaks. The anterior

peaks observed over the fronto-central scalp in Experiment 1 are similar to the midline

central distribution observed in Experiment 2. Perhaps the processing involved for

auditory spatial targets are similar to processing non-spatial targets as well. However,

40

spatial targets may require enhanced processing in the contralateral posterior brain

regions that are associated with the attentional facilitation effect and may reflect target

processing in spatial tasks only. Even though neural sources were not estimated here,

previous studies have indicated that the Nd waves are generated in the bilateral auditory

cortices and may indicate the selective processing of the target’s features (Hansen &

Hillyard, 1984; Hillyard et al., 1995; Näätänen, 1992). Furthermore, a meta-analysis of

38 studies reviewed PET and fMRI results and examined auditory cortical organization in

spatial and non-spatial tasks (Arnott, Binns, Grady, & Alain, 2004). The majority of the

reviewed studies found activation in posterior regions (e.g., posterior temporal lobe and

inferior parietal lobe) when the task was spatially relevant and activation in more anterior

regions (e.g., anterior temporal lobe and inferior frontal gyrus) when the task was

spatially irrelevant. Therefore, spatial and non-spatial information are processed

differently. These results provide evidence for an auditory dual-pathway model, where

spatial information is processed along the dorsal auditory pathway (“where” pathway),

whereas non-spatial information is processed along the ventral stream (“what” pathway).

Following the Nd, a positive deflection in the valid-invalid waveform was

observed in the spatial task only. The target-elicited Pd was observed over fronto-central

midline scalp, similar to the anterior scalp topography of the Nd. The presence of this

positivity suggests that the processing of auditory targets was influenced by spatially

non-predictive auditory cues and may be influenced by the spatial location of the stimuli.

The presence of the Nd and then the Pd indicates multiple stages of processing when

orienting attention to a spatial location. However, the Pd was not evident in the non-

spatial task. This pattern of ERP results mirrored the RT effects – that is, the Pd was

present when the behavioural cue effects were present (in Experiment 1) and was

absent when the behavioural cue effects were absent (in Experiment 2). This is also

consistent with the predictions from the SRH based on the behavioural results.

Therefore, the presence of the Pd may be associated with the presence of behavioural

cue effects as well as in spatially-relevant tasks. A possible explanation of the Pd is that

it might reflect suppression of the neural mechanisms that are irrelevant to the task,

according to the dual-pathway model. In the spatial task, the frequency of the target

(“what” stream) might be suppressed in order to facilitate the processing of the target

location (“where” stream), whereas the opposite is true for the non-spatial task. The

dual-pathway hypothesis was investigated whether similar parallel processing streams in

41

the visual system were also present in the auditory system (Lomber & Malhotra, 2008;

Sumner, Palmer, & Moore, 2008). The results identified that the bilateral posterior

auditory regions are associated with processing spatial information and the bilateral

anterior auditory regions are associated with processing non-spatial information, which

suggests that auditory information is processed in separate streams. A valid-invalid

polarity reversal after the Nd has been observed in a voluntary auditory spatial orienting

study that used symbolic cues (Tata & Ward, 2005). However, their Pd occurred later

and was lateralized and more posterior than the one observed in Experiment 1.

Also keeping with the second prediction that auditory non-predictive cues might

capture spatial attention regardless of the presence of the behavioural cue effects, the

recently discovered auditory contralateral occipital positivity (ACOP; McDonald et al.,

2012) was present in both tasks. This cue-elicited ERP component and its similar scalp

topography between the two experiments provide evidence that auditory cues captured

attention automatically in spatial and non-spatial tasks. In McDonald et al.’s study,

participants judged the orientation of a visual cue (one experiment) or whether a low or

high frequency tone was perceived to be louder (three experiments). Auditory non-

predictive cues elicited a positivity over the contralateral occipital region for all of their

experiments even though participants were not preparing for visual stimuli in the

unimodal auditory tasks. The goal of their experiments was to investigate whether the

non-predictive auditory cues activated the visual cortex; however, the present study

made no attempt to examine the neural generators of the ACOP. McDonald et al.

estimated the neural generators of the ACOP and found the best-fitting dipole to be

located near the fusiform gyrus (Brodmann’s area 19) of the occipital cortex in each

experiment. The fusiform gyrus has been found to be involved in visual stimuli

processing in face and body recognition (Peelen & Downing, 2005; Wojciulik, Kanwisher,

& Driver, 1998) and word recognition (Dehaene, Le Clec’H, Poline, Le Bihan, & Cohen,

2002). McDonald et al.’s study suggests that irrelevant auditory cues automatically

activated the visual cortex. Previous research using voluntary auditory cues found that

directing attention to the location of the auditory targets activated the visual cortex

(Harter et al., 1989; Hopf & Mangun, 2000; McDonald & Green, 2008; Störmer, Green, &

McDonald, 2009). This late directing attention positivity (LDAP) happens relatively late

(approximately 400-800 ms) after the cue onset and is thought to reflect voluntary spatial

attention processing in the contralateral visual cortex. However, the ACOP in McDonald

42

et al.’s study as well as in the present study suggests that non-predictive auditory cues

involuntarily activated the visual cortex automatically.

An unexpected, yet novel, target-elicited ERP component was observed over

occipital regions in both experiments. The latency of this component is similar to the

P3b, peaking around 375 ms in both experiments; however, the scalp topography of this

component is more occipital than the parietal P3b. Similar to the ACOP, this component

appears to originate in the visual cortex. Therefore, the visual areas of the brain might

reflect supramodal processing and are involved in processing auditory stimuli. This

component is present in both the spatial and the non-spatial task suggesting that these

areas are involved in the general processing of auditory targets. In Experiment 1, the

scalp topography for invalid trials was larger over contralateral scalp than over ipsilateral

scalp, which may be due to an underlying negativity over ipsilateral scalp when

participants have to disengage their attention from the cue location and then shift and

engage their attention to the location of the target (Hopfinger & Mangun, 2001).

Disengagement is required only for invalid trials since the auditory cue and target are

presented at different locations. Therefore, this activity might be reducing the positive

ipsilateral activity seen on invalid trials.

In sum, little is known about the neural mechanisms of exogenous cue effects on

electrophysiological responses in audition since most of the research focus has been on

the visual modality. The present intramodal auditory study revealed that the behavioural

effects do not necessarily correspond with the ERPs, with the Nd present in both tasks

regardless of the behavioural results. However, an ERP component (i.e., Pd) mirrored

the behavioural results, which revealed that the presence of the Pd is contingent on the

task being spatially relevant. The difference-wave components are consistent with the

SRH predictions and also supported the dual-pathway hypothesis. The ACOP and the

novel P3-like component were also evident in both experiments, suggesting that the

visual regions are involved in processing auditory stimuli. This reflects that spatial

attention is processed in these supramodal areas.

43

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