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.
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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.
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
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
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 &
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),
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
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
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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