Biases of spatial attention in vision and audition.

Abstract

Neurologically normal observers misperceive the midpoint of horizontal lines as systematically leftward of veridical center, a phenomenon known as pseudoneglect. Pseudoneglect is attributed to a tonic asymmetry of visuospatial attention favoring left hemispace. Whereas visuospatial attention is biased toward left hemispace, some evidence suggests that audiospatial attention may possess a right hemispatial bias. If spatial attention is supramodal, then the leftward bias observed in visual line bisection should also be expressed in auditory bisection tasks. If spatial attention is modality specific then bisection errors in visual and auditory spatial judgments are potentially dissociable. Subjects performed a bisection task for spatial intervals defined by auditory stimuli, as well as a tachistoscopic visual line bisection task. Subjects showed a significant leftward bias in the visual line bisection task and a significant rightward bias in the auditory interval bisection task. Performance across both tasks was, however, significantly positively correlated. These results imply the existence of both modality specific and supramodal attentional mechanisms where visuospatial attention has a prepotent leftward vector and audiospatial attention has a prepotent rightward vector of attention. In addition, the biases of both visuospatial and audiospatial attention are correlated.

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Biases of spatial attention in vision and audition
Yamaya Sosa, Wolfgang A. Teder-Sälejärvi, Mark E. McCourt*
Center for Visual Neuroscience, Department of Psychology, North Dakota State University, USA
a r t i c l e i n f o
Article history:
Accepted 25 May 2010
Available online 20 June 2010
Keywords:
Line bisection
Pseudoneglect
Visuospatial attention
Audiospatial attention
a b s t r a c t
Neurologically normal observers misperceive the midpoint of horizontal lines as systematically leftward
of veridical center, a phenomenon known as pseudoneglect. Pseudoneglect is attributed to a tonic asym-
metry of visuospatial attention favoring left hemispace. Whereas visuospatial attention is biased toward
left hemispace, some evidence suggests that audiospatial attention may possess a right hemispatial bias.
If spatial attention is supramodal, then the leftward bias observed in visual line bisection should also be
expressed in auditory bisection tasks. If spatial attention is modality specific then bisection errors in
visual and auditory spatial judgments are potentially dissociable. Subjects performed a bisection task
for spatial intervals defined by auditory stimuli, as well as a tachistoscopic visual line bisection task. Sub-
jects showed a significant leftward bias in the visual line bisection task and a significant rightward bias in
the auditory interval bisection task. Performance across both tasks was, however, significantly positively
correlated. These results imply the existence of both modality specific and supramodal attentional mech-
anisms where visuospatial attention has a prepotent leftward vector and audiospatial attention has a pre-
potent rightward vector of attention. In addition, the biases of both visuospatial and audiospatial
attention are correlated.
? 2010 Elsevier Inc. All rights reserved.
1. Introduction
1.1. Hemineglect
Hemineglect refers to a deficit of attention towards stimuli lo-
cated within contralesional (typically left) hemispace, defined in
retinocentric, egocentric or allocentric coordinates (Bisiach, 1996;
Bisiach, Bulgarelli, Sterzi, & Vallar, 1983; Bisiach, Capitani, Colum-
bo, & Spinnler, 1976; Driver, Baylis, Goodrich, & Rafal, 1994; Heil-
man & Valenstein, 1979; Karnath, Schenkel, & Fischer, 1991). Left
hemispatial neglect occurs most commonly after lesions to right
inferior parietal or temporoparietal cortex, but may also result
from lesions to frontal or cingulate cortex, or to subcortical struc-
tures (Heilman & Valenstein, 1972a; Mesulam, 1981; Vallar & Pera-
ni, 1986; Watson, Valenstein, & Heilman, 1981). Line bisection
tasks are commonly employed to assay asymmetries of spatial
attention. Neglect patients bisect horizontal lines of moderate
length significantly rightward of veridical center, as though ignor-
ing the left-hand side of the stimulus or, alternatively, being hyper-
attentive to the right-hand side. Hemispatial neglect has also been
reported to occur for auditory stimuli (Heilman & Valenstein,
1972b; Hugdahl, Wester, & Asbjornsen, 1991).
1.2. Pseudoneglect
It is well established that visuospatial attention in neurologi-
cally normal subjects is asymmetrically distributed as well, result-
ing in a modest but systematic and significant leftward deviation of
perceived line midpoint in line bisection tasks (Bradshaw, Nathan,
Nettleton, Wilson, & Pierson, 1987; Foxe, McCourt, & Javitt, 2003;
Jewell & McCourt, 2000; Leone & McCourt, 2010; McCourt, 2001;
McCourt,Freeman,Tahmahkera-Stevens,
McCourt & Garlinghouse, 2000a, 2000b; McCourt, Garlinghouse,
& Butler, 2001; McCourt, Garlinghouse, & Reuter-Lorenz, 2005;
McCourt, Garlinghouse, & Slater, 2000; McCourt & Jewell, 1999;
McCourt & Olafson, 1997; McCourt, Shpaner, Javitt, & Foxe,
2008), a left hemifield bias in perceived luminance in the grey-
scales task (Nicholls, Bradshaw, & Mattingley, 1999; Nicholls &
Roberts, 2002), a left hemispatial bias in perceived stimulus size
(Charles, Sahraie, & McGeorge, 2007; Nicholls et al., 1999) and
numerosity (Luh, Rueckert, & Levy, 1991; Nicholls et al., 1999),
and a left hemifield advantage in the processing of faces (Levy &
Heller, 1981). This constellation of left-biased asymmetries of spa-
tial attention is called pseudoneglect (Bowers & Heilman, 1980;
Jewell & McCourt, 2000). The phenomena of neglect and pseudone-
glect, as their names suggest, are theorized to be twin manifesta-
tions of a common and fundamental hemispheric asymmetry in
the neural substrates of visuospatial attention (McCourt & Jewell,
1999). Supporting this idea are experiments illustrating that a vari-
ety of stimulus and task-related variables modulate the magnitude
&Chaussee,2001;
0278-2626/$ - see front matter ? 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.bandc.2010.05.007
* Corresponding author. Address: Center for Visual Neuroscience, Department of
Psychology, NDSU Department 2765, PO Box 6050, North Dakota State University,
Fargo, ND 58108-6050, USA. Fax: +1 (701) 231 8426.
E-mail address: mark.mccourt@ndsu.edu (M.E. McCourt).
Brain and Cognition 73 (2010) 229–235
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and direction of both neglect and pseudoneglect in a complimen-
tary manner (Anderson, 1996; McCourt & Jewell, 1999).
1.3. Visual and auditory spatial attention
Most research on spatial attention has focused on visual pro-
cessing, but environmental space is monitored by multiple sensory
modalities (Stein & Meredith, 1993), and there is a burgeoning
interest in developing a comprehensive understanding of multi-
sensory attention and perception (Calvert, Spence, & Stein, 2004).
Pseudoneglect arises due to a prepotent vector of visuospatial
attention deployed into left hemispace by the dominant right cere-
bral hemisphere. There is some evidence for a rightward asymme-
try in the deployment of spatial attention within the auditory
modality (Corral & Escera, 2008; Cusak, Carlyon, & Robertson,
2001; Dufour, Touzalin, & Candas, 2007; see however: Bisiach, Cor-
nacchia, Sterzi, & Vallar, 1984; Kerkhoff, Artinger, & Ziegler, 1999;
Vallar, Guariglia, Nico, & Bisiach, 1995). If the leftward bias ob-
served in visuospatial attention arises from asymmetry in a supra-
modal attentional system, then both visual and auditory spatial
attention should be similarly biased. If, however, a bias in auditory
spatial attention is found which differs from that for visuospatial
attention, then this implies that auditory and visual spatial atten-
tion are governed by modality-specific processes. Using a within-
subjects design we investigate the relationship between biases in
visual and auditory spatial attention using visual line bisection
and auditory interval bisection tasks.
2. Methods
2.1. Subjects
Subjects were 33 dextral students (18 male, mean age = 22.9 -
years; 15 female, mean age = 23.7 years). Handedness laterality
quotients were assessed using a standard instrument (Oldfield,
1971) on which a composite score of ?100 denotes exclusive
left-handedness, and +100 denotes exclusive right-handedness.
Mean handedness laterality quotients for males and females were
+77.2 and +78.0, respectively. There was no significant difference
in mean age or handedness laterality score between male and fe-
male subjects [F1,31= 0.15, p = .70, and F1,31= 0.02, p = .90, respec-
tively]. Subsequent inferential statistical tests were therefore
conducted on data collapsed across subject sex.
The study was conducted in accordance with The Code of Ethics
of the World Medical Association (Declaration of Helsinki) for
experiments involving human subjects. Prior to their participation
in the study all subjects provided written informed consent, and all
procedures were approved by the Institutional Review Board of
North Dakota State University.
2.2. Stimuli
2.2.1. Auditory interval bisection (AB)
Fig. 1 illustrates a schematic of the horizontal array of 27 speak-
ers used to deliver the auditory stimuli. At a distance of 110 cm the
inter-speaker separation was 1.02? of spatial angle. The spatial
interval to be bisected was defined by two speakers with a spatial
separation of 26.6?. This spatial interval was defined on each trial
by the delivery of two complex tones (200 Hz and 400 Hz square-
waves; 65 dB SPL) of 300 ms duration. The target consisted of a
complex tone (300 Hz squarewave; 65 dB SPL) which was also
300 ms in duration. On a given trial the target tone could appear
at one of 13 spatial locations ranging from ±10.2? with respect to
veridical interval midpoint. Ambient noise level was 45 dB SPL.
Auditory calibration was performed using a sound level meter (Ex-
tech, model 407764).
2.2.2. Visual line bisection (VB)
Fig. 2 illustrates the stimuli used in the visual line bisection
task. Horizontal lines of 100% Michelson contrast were tachisto-
scopically presented for 150 ms. At a viewing distance of 70 cm
the lines subtended 19.06? ? 0.33? of visual angle. Lines were
pre-transected at 29 locations ranging from ±1.66? with respect
to veridical line midpoint. Mean display luminance was 67 cd/
m2. Display resolution was 640 ? 480 pixels (26.52? ? 19.89?),
and the screen refresh rate was 60 Hz. Luminance and contrast cal-
ibration were performed using a spot photometer (Konica Minolta
LS110).
2.3. Procedure
Both AB and VB experiments were conducted in a single session.
The order of presentation of the two tasks was counterbalanced
across subjects. Subjects were seated in straight-backed chairs
with their midsagittal plane aligned with the midpoint of the
speaker array and the visual display. All subjects had normal or
corrected-to-normal vision. Audiometric tests confirmed that all
subjects had normal auditory thresholds. Subjects performed the
AB task with eyes closed. In both the visual and auditory bisection
tasks a programmed microcomputer sensed and collected subject
responses (Presentation: Neurobehavioral Systems, Inc.).
Speaker Array
26.6o
20.4o
t1
t2
t3
Timing
300
300
300
300
t (ms)
100
Fig. 1. A schematic diagram of the horizontal array of 27 speakers used to deliver the auditory stimuli. At a distance of 110 cm the inter-speaker separation was 1.02? of
spatial angle. The spatial interval to be bisected was defined by two speakers with a spatial separation of 26.6?. On a given trial the target tone could appear at one of 13
spatial locations ranging from ±10.2? with respect to veridical interval midpoint.
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2.3.1. Auditory interval bisection (AB)
Fig. 1 illustrates the stimulus arrangement and experimental
procedure used in the auditory interval bisection task. Subjects
faced the array. Three tones were presented sequentially. The first
two tones (t1and t2) served to define the 26.6? spatial interval to be
bisected. Following the presentation of the third (target) tone (t3),
subjects judged whether the spatial location of the target tone was
to the left or right of the midpoint of the spatial interval defined by
the two fiducial tones. The sequence of fiducial tones was pre-
sented with an interstimulus interval (ISI) of 100 ms. The target
tone was presented 300 ms after the second fiducial tone. Subse-
quent trials began 400 ms after subject response.
Subjects performed two blocks of trials. In one block the left
fiducial tone preceded the right fiducial tone in a left-to-right se-
quence (ABL?R); the second block employed a right-to-left tone se-
quence (ABR?L). The order of presentation of the two sequences
was counterbalanced across subjects. Within a block of trials the
target tone (t3) could appear randomly at one of 13 different speak-
er locations. Subjects made fifteen bisection judgments in conjunc-
tion with each of the 13 target tone locations, such that the
determination of perceived auditory interval midpoint was based
on a total of 195 (13 target tone locations ? 15 judgments per loca-
tion) forced-choice trials. Subjects indicated decisions using the
right hand to depress a left or right mouse button, as appropriate.
2.3.2. Visual line bisection (VB)
Lines were tachistoscopically presented for 150 ms; intertrial
interval varied randomly, with a boxcar distribution, between
500 and 1000 ms following subject response. Lines of each contrast
polarity appeared with equal frequency and the order of appear-
ance of lines with different transector locations was randomized
within blocks of trials. Subjects made ten bisection judgments at
each of the 29 transector locations, such that the determination
of perceived line midpoint was based on a total of 290 (29 transec-
tor locations ? 10 judgments per location) forced-choice trials.
Subjects indicated decisions using the right hand to depress a left
or right mouse button, as appropriate.
2.4. Data analysis
For both VB and AB tasks the dependent measure was the pro-
portion of trials on which subjects judged that the visual line tran-
sector or auditory target was located to the left of the midpoint of
the line (VB task) or spatial interval (AB task). Psychometric func-
tions were derived using the method of constant stimuli. Multidi-
mensional unconstrained nonlinear optimization (Nelder & Mead,
1965) was used to fit logistic functions to the psychometric data
using maximum likelihood optimization. The logistic function is
described by the equation
pðxÞ ¼
1
1 þ expð?x?l
rÞ
where x refers to the spatial location of the visual line transector or
the auditory target, l is the point of subjective equality (PSE), cor-
responding to the inflection point of the sigmoidal function, andr is
the standard deviation whose value is inversely proportional to dis-
crimination precision. Line transector and auditory target locations
corresponding to a 50% probability of ‘‘left” responses (l), and cor-
responding standard deviations (r) were estimated for each subject
in each condition.
Subsequent inferential statistical tests, including one-sample
and paired sample t-tests, were conducted on these optimized val-
ues of PSE and standard deviation. The t-statistics were used to cal-
culate estimates of effect size using the formula, d = 2t/pdf (Cohen,
1988). By convention, an effect size of ±0.2 is considered to be
small, a value of ±0.5 is moderate and a value of ±0.8 or greater
is considered a large effect (Cohen, 1992).
3. Results
3.1. Auditory interval bisection (AB) accuracy (bias)
The leftmost bars of Fig. 3 plot mean PSE (±1 sem) in the ABL?R
and ABR?Lconditions. Mean PSE was 0.195? (0.73% interval length)
in the ABL?Rcondition and 1.627? (6.12% interval length) in the
ABR?Lcondition. A paired-samples t-test reveals that mean PSE
in the ABR?Lcondition is significantly rightward of the mean PSE
in the ABL?Rcondition [t32= 4.50, p < .001, d = 1.59], indicating a
highly significant effect of directional attentional scanning. Sin-
gle-sample t-tests reveal that mean bisection error was signifi-
cantly rightward of veridical in the ABR?L condition [t32= 5.04,
p < .001, d = 1.78] but not in the ABL?R condition [t32= 0.57,
p = .573, d = 0.20]. The third bar in Fig. 3 plots the average bisection
error in the AB condition collapsed across the two fiducial tone
direction conditions (ABAVE). A single-sample t-test confirms that
the average auditory bisection error (0.911?) deviates significantly
19.06º
A
B
C
D
E
Fig. 2. Examples of line stimuli used in the experiments. The members of the upper
pair (A and B) are transected to the left of veridical line midpoint (by ?0.75? and
?0.08?, respectively). The members of the lower pair (D and E) are transected to the
right of veridical center (by +0.70? and +0.33?, respectively). Line C is veridically
transected. The members of line pairs (A and B) and (D and E) differ in contrast
polarity. Lines of opposite polarity appeared with equal frequency and were
counterbalanced within and across blocks of trials.
Experimental Condition
Mean PSE (deg re veridical) ±1 sem
0.0
0.5
1.0
1.5
2.0
ABL→R
ABR→L
ABAVE
VB
Fig. 3. Leftmost bars plot mean PSE in the ABL?Rand ABR?Lconditions. Mean PSE in
the ABR?L condition is significantly rightward of the mean PSE in the ABL?R
condition, indicating a significant effect of directional attentional scanning. Mean
bisection error was significantly rightward of veridical in the ABR?Lcondition, but
not in the ABL?Rcondition. The third bar plots the average bisection error in the AB
condition. Average auditory bisection error deviates significantly rightward of
veridical interval midpoint. The rightmost bar plots mean bisection error in the VB
task, which deviates significantly leftward of veridical line midpoint.
Y. Sosa et al./Brain and Cognition 73 (2010) 229–235
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rightward of veridical interval midpoint (3.42% interval length)
[t32= 3.12, p = .004, d = 1.10].
3.2. Visual line bisection (VB) accuracy (bias)
The rightmost bar of Fig. 3 plots mean bisection error in the VB
task which was ?0.117? (0.61% line length). A single-sample t-test
shows that visual bisection error is significantly leftward of verid-
ical line midpoint [t32= ?3.51, p = .001, d = ?1.24].
3.3. Auditory interval versus visual line bisection
Fig. 4 plots VB versus ABAVEPSE for the entire sample of 33 sub-
jects. Despite the significant difference in the direction of visual
and auditory hemispatial bias, there is a significant positive corre-
lation between bisection errors across the two sensory modalities
[r32= 0.38, p = .029].
3.4. Auditory interval bisection (AB) and visual line bisection (VB)
precision
Fig. 5 plots mean standard deviation (precision) values for the
AB and VB tasks. There was no significant difference in mean pre-
cision between the ABL?R (1.87?) and ABR?L conditions (2.06?)
[t32= ?1.60, p = .120, d = 0.57]. Average precision in the auditory
interval bisection task (1.97?) is significantly poorer than in the vi-
sual line bisection task (0.25?) [t32= 15.11, p < .001, d = 5.34].
4. Discussion
4.1. Visual versus auditory spatial processing
Whereas the neural basis for visual spatial localization is well
understood, the neural mechanisms for sound source localization
are still a subject of considerable debate (Zatorre, Bouffard, Ahad,
& Belin, 2002). The visual system is organized for spatial localiza-
tion; it possesses numerous spatiotopically mapped low-level cor-
tical areas (e.g., V1, V2, V3) in which the spatial location of stimuli
is mapped explicitly. By contrast, auditory cortex is tonotopically
mapped; sound localization depends primarily on interaural time
and intensity differences, with some contribution from monaural
spectral cues (Blauert, 1996). It is not known with certainty how
these cues are processed by the auditory system to achieve sound
localization (Richter, Schroger, & Rubsamen, 2009). In addition,
central auditory projections have a large ipsilateral component
that is absent in the visual system, and whereas the neural net-
works subserving visuospatial attention are largely housed in the
right hemisphere (Kastner & Ungerleider, 2000; Nobre et al.,
1997), there is substantial evidence for both right and left hemi-
sphere involvement in sound localization and audiospatial atten-
tion (Bellman, Meuli, & Clarke, 2001; Clarke, Bellmann, Meuli,
Assal, & Steck, 2000; Clarke et al., 2002; Richter et al., 2009; Zatorre
et al., 2002).
4.2. Visuospatial attention
Our results for the visual line bisection task contribute to the
growing consensus that visuospatial attention in neurologically
normal subjects exhibits a small, but significant and consistent
leftward bias, i.e., pseudoneglect (Bowers & Heilman, 1980; Dickin-
son & Intraub, 2009; Jewell & McCourt, 2000; Leone & McCourt,
2010; McCourt, 2001; McCourt & Jewell, 1999; McCourt & Olafson,
1997; McCourt et al., 2005, 2008; Nicholls et al., 1999). The left-
ward bias of normal subjects and the profound rightward bias of
neglect patients are twin manifestations of the specialization of
neural networks in the right hemisphere for the deployment of vis-
uospatial attention (Heilman & Valenstein, 1979; Kastner &
Ungerleider, 2000; Kinsbourne, 1970, 1977, 1993; Mesulam,
2000; Nobre et al., 1997). The emerging consensus is that the (nor-
mally) dominant right hemisphere projects a prepotent vector of
visuospatial attention into contralateral (left) hemispace, differen-
tially increasing the salience of left hemispace in general (in ego-
centric coordinates), and the left-hand portions of visual stimuli
such as lines (in allocentric coordinates), thereby biasing perceived
midpoint leftwards (Anderson, 1996; McCourt & Jewell, 1999).
4.3. Audiospatial attention
Our results for the auditory interval bisection task indicate that
audiospatial attention in neurologically normal subjects exhibits a
significant rightward bias. This finding is consistent with several
previous reports. Cusak et al. (2001) manipulated the interaural
time delay (ITD) of headphone-delivered noise bursts and found
ABAVE PSE (deg re veridical)
-3-2-101234
VB PSE (deg re veridical)
-0.50
-0.25
0.00
0.25
0.50
Fig. 4. VB PSE plotted against ABAVE PSE for the entire sample of 33 subjects.
Bisection error in the two tasks is significantly correlated.
Experimental Condition
Mean SD (deg) ±1 sem
0.0
0.5
1.0
1.5
2.0
ABAVE
VB
Fig. 5. Mean standard deviations of the logistic function fits to the psychometric
data AB and VB tasks. Average bisection precision in the AB task is significantly
poorer than in the VB task.
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that six of seven control subjects perceived sounds with positive
ITDs (consistent with a physical sound source located in right
hemispace) as located on the midsagittal plane. Dufour et al.
(2007) delivered noise bursts from two speakers situated at ±30?
with respect to the midsagittal plane. Subjects judged the location
of the binaurally fused stimuli to be aligned with the auditory mid-
line when the left speaker possessed a greater physical intensity,
and stimuli with an interaural intensity difference of zero were
perceived to be located rightward of the midsagittal plane. Corral
and Escera (2008) embedded novel sounds (distracters) in a repet-
itive stream of auditory stimuli and presented these at various azi-
muthal locations relative to gaze direction while subjects were
engaged in a demanding visual discrimination task. They found a
significant effect of distracting sounds on visual task performance
only for distracters delivered in right hemispace. Sosa, Clarke,
and McCourt (2009) used a tachistoscopic visual line bisection par-
adigm to assess whether exogenous lateral auditory cues can bias
PSE and to characterize the manner in which auditory (A) and vi-
sual (V) cues combine to jointly influence PSE. They found a signif-
icant hemifield asymmetry in the weights assigned to A and V cues,
where V cues were more heavily weighted in left hemispace and A
cues were more heavily weighted in right hemispace.
4.3.1. Attentional scanning
Our results from the auditory interval bisection task show sig-
nificantly greater rightward error in the ABR?Lversus ABL?Rcondi-
tion. This result is consistent with previous findings. In their
review and meta-analytic treatment of the visual pseudoneglect
literature Jewell and McCourt (2000) noted that directional scan-
ning, as executed either overtly (e.g., occulomotor or limb scan-
ning) or covertly (absent eye or limb movements), exerted a
significant modulatory influence on perceived line midpoint such
that that left-to-right scanning was associated with significantly
larger leftward bisection errors than right-to-left scanning (Brodie
& Pettigrew, 1996; Chokron, Bartolomeo, Perenin, Helft, & Imbert,
1998; Chokron & Imbert, 1993a, 1993b; Halligan, Manning, & Mar-
shall, 1991), which can sometime lead to rightward bisection er-
rors. The asymmetricaleffect
perceptual asymmetries has been studied behaviorally (McCourt
& Jewell, 1999; McCourt et al., 2000; Nicholls & Roberts, 2002)
but the neural basis for this effect is still poorly understood, partic-
ularly since covert shifts of spatial attention leftward or rightward
from fixation are generally associated with increased activation of
contralateral extrastriate and parietal cortical areas (Kelley, Ser-
ences, Giesbrecht, & Yantis, 2008; Yamaguchi, Tsuchiya, & Kobay-
ashi, 1995). Thus, according to activation-orientation theory
(Kinsbourne, 1970, 1977, 1993) the increased left hemisphere acti-
vation which accompanies a left-to-right attentional shift would
predict smaller leftward bisection errors, whereas the opposite
finding is observed. It is nonetheless interesting and potentially
significant that directional scanning has a similar profound influ-
ence on interval midpoint estimation in both visual and auditory
modalities.
ofdirectionalscanningon
4.4. Supramodal spatial attention
Based on findings that the severity and direction of inattention
for visual and auditory stimuli is positively correlated in neglect
patients (Pavani, Husain, Ladavas, & Driver, 2004), and on neuroim-
aging studies of normal subjects which show largely coextensive
regions of increased BOLD signal for shifts of spatial attention to vi-
sual and auditory cues, it has been suggested that spatial attention
is supramodal (Krumbholz, Nobis, Weatheritt, & Fink, 2009). To the
extent that we likewise find a significant positive correlation be-
tween normal attentional biases in visual and auditory interval
bisection tasks our results are consistent with this hypothesis.
However, the quite different distributions of visual and auditory
spatial attention are more difficult to reconcile with a supramodal
mechanism.
4.5. Left hemisphere control of audiospatial attention: a hypothesis
Five experimental/clinical findings that a model of visual and
auditory spatial attention must explain in an integrated fashion
are: (1) the leftward bias of normal visuospatial attention, i.e.,
pseudoneglect; (2) the rightward bias of normal audiospatial atten-
tion; (3) the positive correlation of these biases in normal observ-
ers; (4) the rightward bias of both visuospatial and audiospatial
attention following right hemisphere damage, i.e., hemineglect;
and (5) the positive correlation between errors of visuospatial
and audiospatial attention in patients with hemineglect.
There is strong evidence for a lateralized neural network for vis-
uospatial attention where the right hemisphere deploys attention
into both contralateral (left) and ipsilateral (right) hemispace and
the left hemisphere attends primarily to contralateral (right) hemi-
space (Anderson, 1996; Gitelman et al., 1999; Mesulam, 1981).
Based on our present findings, as well as on cognate results from
other laboratories (Corral & Escera, 2008; Cusak et al., 2001; Du-
four et al., 2007), we postulate the existence of a left hemisphere
based network governing audiospatial attention. Just as the right
hemisphere is attentive to visual events in both contralateral and
ipsilateral hemispace, there is converging evidence that sounds
confined to left hemispace activate only the contralateral (right)
hemisphere, whereas sounds located in right hemispace cause
bihemispheric activation (Deouell, Bentin, & Giard, 1998; Hine &
Debener, 2007; Kaiser, Lutzenberger, Preissl, Ackermann, & Birbau-
mer, 2000; Krumbholz, Hewson-Stoate, & Schonwiesner, 2007;
Krumbholz et al., 2005; Petit et al., 2007; Schonwiesner, Krumb-
holz, Rubsamen, Fink, & von Cramon, 2007). Further, just as the
right hemisphere’s specialization for visuospatial attention in-
creases the salience of visual stimuli in left hemispace and leads
to tonic leftward error in tasks like visual line bisection, so the left
hemisphere’s specialization for audiospatial attention causes a to-
nic rightward bias in auditory midline and interval judgments. If,
as suggested by the activation–orientation theory (Kinsbourne,
1970, 1977, 1993) the left and right cerebral hemispheres compete
for control of various functions (such as speech production or the
deployment of spatial attention) via a process of mutual inhibition,
then any phasic stimulation of the right hemisphere will up-regu-
late activity in visuospatial networks and so exacerbate the normal
tonic leftward error in line bisection. Due to mutual interhemi-
spheric inhibition the phasic stimulation of the right hemisphere
will also result in the down-regulation of activity in the audiospa-
tial attention networks of the left hemisphere. Since the strongest
vector of audiospatial attention from the left hemisphere is direc-
ted towards right hemispace, the weakening of this vector will
cause a leftward shift of PSE in auditory interval bisection tasks.
The complimentary pattern of altered attentional biases will result
from phasic stimulation of the left hemisphere (i.e., a rightward
shift of both visual line bisection and auditory interval bisection).
Thus, despite the different directions of the prepotent vectors of
visuospatial and audiospatial attention, interhemispheric inhibi-
tion explains the correlated nature of these biases. While phasic
changes of relative hemispheric activity serve to illustrate this
point, it should be noted that any static imbalances in right versus
left hemispheric activation (due to individual differences) will also
result in correlated bisection errors across the visual and auditory
modality.
In hemineglect following right hemisphere lesions the normal
leftward vector of visuospatial attention from the right hemisphere
is greatly weakened; the distribution of visuospatial attention
which remains is dominated by the left hemisphere and is strongly
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right-biased (Anderson, 1996). In addition, according to activation–
orientation theory, right hemisphere lesions will allow the left
hemisphere to be released from inhibition. Such disinhibition in-
creases the normally right-biased distribution of audiospatial
attention. Thus, here too a correlated pattern of increased right-
ward errors is produced in both visual and auditory tasks. Finally,
this theory makes a testable prediction: if audiospatial attention
depends on left hemisphere neural networks, then lesions to the
left hemisphere should cause leftward shifts in audiospatial atten-
tion, and a pattern of greater left bias in auditory bisection tasks.
5. Conclusions
In contrast to the significant leftward bias of visuospatial atten-
tion, a significant rightward attentional bias characterizes mid-
point judgments of spatial intervals defined via the auditory
modality. This dissociation implies that the neural architectures
which underpin spatial attention are modality specific, and that
distinct networks govern the deployment of visuospatial and
audiospatial attention. This dissociation is moderated by a signifi-
cant positive correlation between bisection errors across the visual
and auditory modalities which is consistent with the activation–
orientation theory of mutual interhemispheric inhibition.
Acknowledgments
This work was supported by grants to MEM: NIH P20 RR020151
and NIH R15 EY12267. The National Center for Research Resources
(NCRR) and the National Eye Institute (NEI) are components of the
National Institutes of Health (NIH). The contents of this report are
solely the responsibility of the authors and do not necessarily re-
flect the official views of the NIH, NCRR, or NEI. The authors thank
Aaron Clarke for assistance with data analysis and Dan Gu for assis-
tance with programming.
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