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The Journal of Neuroscience, September 1, 2000, 20(17):6631-6639
Right-Hemisphere Dominance for the Processing of Sound-Source
Lateralization
Jochen
Kaiser1,
Werner
Lutzenberger1,
Hubert
Preissl1,
Hermann
Ackermann2, and
Niels
Birbaumer1, 3
1 Magnetoencephalography Center, Institute of
Medical Psychology and Behavioral Neurobiology and
2 Department of Neurology, University of Tübingen,
72076 Tübingen, Germany, and 3 Department of
Psychology, University of Padua, 35131 Padova, Italy
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ABSTRACT |
Cortical processing of change in direction of a perceived sound
source was investigated in 12 human subjects using whole-head magnetoencephalography. The German word "da" was presented either with or without 0.7 msec interaural time delays to create the impression of right- or left-lateralized or midline sources,
respectively. Midline stimuli served as standards, and lateralized
stimuli served as deviants in a mismatch paradigm. Two symmetrically
linked dipoles fitted to the mismatch fields showed stronger moments in
the hemisphere contralateral to the side of the deviant. The right
dipole displayed equal latencies to both left and right deviants,
whereas left dipole latencies were longer for ipsilateral than
contralateral deviants. Frequency analysis between 20-70 Hz and
statistical probability mapping revealed increased induced gamma-band
activity at 53 ± 2.5 Hz to both types of deviants. Right deviants
elicited spectral amplitude enhancements in this frequency range,
peaking at latencies of 160 and 240 msec. These effects were localized bilaterally over the angular gyri and posterior temporal regions. Coherence analysis suggested the existence of two separate
interhemispheric networks. For left-lateralized deviants, both spectral
amplitude enhancements at 110 and 220 msec and coherence increases were restricted to the right hemisphere. In conclusion, both mismatch dipole
latencies at the supratemporal plane and gamma-band activity in
posterior parietotemporal areas suggested a right hemisphere engagement
in the processing of bidirectional sound-source shifts. In contrast,
left-hemisphere regions responded predominantly to contralateral
events. These findings may help to elucidate phenomena such as
unilateral auditory neglect.
Key words:
sound-source lateralization; magnetoencephalogram (MEG); mismatch response; dipole latency; gamma-band activity (GBA); coherence; posterior parietal cortex; auditory dorsal stream; human
subjects
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INTRODUCTION |
The ability to detect the location of sounds in
space represents a remarkable property of the auditory system. Within
the tactile or visual domains, the encoding of spatial information relies on the topographic organization of the receptor sheet and the
projection targets. In contrast, the brain must compute the direction
of a sound source on the basis of cues arising at both ears, such as
interaural intensity and time differences (Middlebrooks and Green,
1991 ; King and Carlile, 1995), as well as spectral differences
attributable to localization-dependent filtering properties of
head and outer ear (Kulkarni and Colburn, 1998). Structures at
different levels of the auditory pathway, e.g., the superior colliculus, respond to auditory space cues (King and Hutchings, 1987). The ability to localize sounds nevertheless depends on the
integrity of the auditory cortex, especially in higher mammalian species (Heffner and Heffner, 1990; cf. Furukawa et al., 2000), because
unilateral temporal dysfunctions impair sound localization within the
contralesional hemifield (Jenkins and Masterton, 1982; Heffner,
1997). Similarly, electroencephalographic (EEG) and
magnetoencephalographic (MEG) evoked responses to both monaural (Reite
et al., 1981; Mäkelä et al., 1993) and lateralized binaural
stimuli revealed increased auditory cortex activations to contralateral
stimuli (McEvoy et al., 1993; Sams et al., 1993; Loveless et al.,
1994; but see Woldorff et al., 1999).
However, deficits in the localization of sounds, particularly in the
left hemifield, may also arise as sequelae of right parietal lesions in
patients with unilateral neglect (Bisiach et al., 1984; De Renzi et
al., 1989; Pinek et al., 1989; Petersen et al., 1994; Griffiths et al.,
1996; Soroker et al., 1997). Accordingly, both positron emission and
functional magnetic resonance tomographic studies in humans
demonstrated conscious perception of sound lateralization and movement
to involve cortical areas outside the primary auditory cortex, such as
the right insula (Griffiths et al., 1994), the right posterior planum
temporale (Baumgart et al., 1999), and the right posterior parietal
cortex (Griffiths et al., 1998; Bushara et al., 1999; Weeks et al.,
1999). These latter findings suggest predominant activation of right
parietal association cortex during auditory spatial processing.
The present study investigated the time course and topography of
magnetoencephalographic responses to changes in sound-source direction.
First, moments and latencies of mismatch dipoles at the bilateral
auditory cortices on the superior temporal gyrus were assessed in
response to left and right lateralized deviant sounds (Paavilainen et
al., 1989; Näätänen, 1990, 1992;
Näätänen and Alho, 1995). Stronger dipole moments
were expected contralaterally to the side of the deviant. Second,
because mental representations may be related to oscillatory neuronal
cell assembly responses that are not phase-locked to stimulus onset
(Jokeit and Makeig, 1994; Singer, 1995; Tallon-Baudry and Bertrand,
1999), we investigated induced gamma-band activity (GBA) to lateralized
deviants compared with midline standards using spatiotemporal
statistical probability mapping and coherence analysis. Because the
coding of lateralized auditory events may depend on networks in right
posterior parietal cortex, we expected GBA enhancements in this region
in response to changes in sound-source direction.
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MATERIALS AND METHODS |
Subjects
Twelve paid healthy adult volunteers (four females, eight males;
mean ± SD age, 29 ± 2 years) participated in the
present investigation. Audiometry before the experiment showed normal hearing in all subjects with thresholds below 30 dB (sound pressure level) at 1000 Hz for each ear. All participants were right-handed as
measured with the Edinburgh Handedness Inventory (Oldfield, 1971). The
study had been approved by the ethics committee of the University of
Tübingen Medical Faculty.
Stimulus material
The German monosyllabic word "da" (English, "there"),
synthesized by means of commercially available software (Computerized Speech Lab CSL 4300; Kay Elemetrics, Lincoln Park, NJ) with a frame length of 10 msec, a sampling rate of 12,500 Hz, and pulse excitation for voiced portions, served as auditory stimulus. The duration of this consonant-vowel syllable was 190 msec (voice onset
time, 10 msec). The stimulus comprised the following linear transitions
extending across 30 msec: onset, 548, 1834, and 3441 Hz; steady states,
816, 1182, and 2631 Hz. To obtain natural-sounding stimuli, two
additional stationary formants at 4300 and 4900 Hz were added, and
formant bandwidths were manually adjusted. The monotonous fundamental
frequency of the vocalic portion of the syllable amounted to 128 Hz.
For technical reasons, i.e., adaptation to the personal computer used
for syllable presentation, the stimulus had to be resampled at 11,127 Hz (Ackermann et al., 1999). Sound intensity amounted to 75 dB (sound
pressure level).
The stimulus was presented binaurally with either no time delay between
ears, giving the impression of a source in the midsagittal plane or
interaural time delays of 0.7 msec, yielding the perception of events
lateralized at ~70°. There were two versions of lateralized presentations, with the leading stimulus on either the left or the
right side, giving the impression of left- and right-lateralized sounds, respectively. Note that midline and lateralized stimuli only
differed in interaural onset time delay and phase shift, lacking any
differences in the power of the signals.
Procedure
Subjects were seated upright in a magnetically shielded room
(Vakuum-Schmelze, Hanau, Germany). They were instructed to sit still
and keep their eyes open, looking at a fixation cross in the center of
their visual field ~2 m in front of them. Stimuli were presented via
binaural air-conducting tubes with ear inserts. A total of 900 midline
and lateralized stimuli were presented in an oddball manner with 80%
of presentations without interaural time delay and 10% each of right-
and left-leading lateralized sounds. The sequence of deviants and
standards was randomized with the constraint that there were no more
than two consecutive deviants. The onset-to-onset interstimulus
interval was 805 msec.
MEG Recordings
MEG was recorded with a whole-head system (CTF Inc., Vancouver,
Canada) comprising 143 hardware first-order magnetic gradiometers distributed with an average distance between sensors of 2.5 cm. The
subject's head position was determined with localization coils fixed
at the nasion and the preauricular points at both the beginning and end
of each recording. Recordings with head movements exceeding 0.5 cm were
repeated. The signals were sampled at a rate of 250 Hz with an
anti-aliasing filter at 80 Hz. To minimize artifacts, trials with
signals exceeding 1.3 pT in frontal channels were rejected. Epoch
length was 600 msec, including a 48 msec prestimulus baseline.
Data analysis
The following trials were entered in both evoked magnetic field
and induced GBA analyses described below. Only responses to a single
deviant in a train of standards or to the first of two consecutive
deviants were included. Moreover, trials contaminated with artifacts
were rejected, leaving an average of 72 deviants of each type per
subject. Trials for standards were selected as follows. First, 130 standard trials were chosen at random from the middle 500 trials of the
complete stimulus sequence. Then, of these 130 trials, the first 72 artifact-free trials of standards not directly following a deviant were
selected for analysis. The number of selected standard trials was
matched to the number of deviants to ensure that equal numbers of
trials entered the averages for each type of stimulus. The same set of
standards was used for comparisons with both left and right deviants.
Mismatch magnetic fields
After baseline correction and artifact rejection, MEG data were
digitally filtered between 1 and 40 Hz. The averaged evoked magnetic
fields for the difference between standard and deviant sounds were
assessed for both types of lateralized deviants. To evaluate strength,
latency, and origin of these magnetic equivalents to the
electrocortical mismatch negativity (Paavilainen et al., 1989), a
dipole localization technique was used. Two symmetrically linked
dipoles (one in each hemisphere) were fitted to the grand average
across subjects of the difference fields between standards and both
deviants in a time window comprising the mismatch peaks. Symmetrically
linked, fixed dipoles were used to allow for direct comparison of
latencies and moments. The time course of source strength of each of
these dipoles was computed for both deviants during an interval of 50 msec prestimulus to 220 msec poststimulus onset. Latencies were
determined by means of an interactive graphical computer program.
Statistical analysis relied on separate ANOVAs for dipole
moments and peak latencies, with hemisphere (right vs left) and side of
deviant (right vs left) as within-subject factors.
Induced GBA
Frequency analysis of broad-band signals (20-70 Hz).
Frequency analysis of MEG was performed in the range of 20-70 Hz over each complete epoch of 48 msec prestimulus to 552 msec poststimulus onset for each of the deviants and standards. Selecting a 600 msec time
window resulted in records of 150 points, which were zero-padded to
obtain 256 points. To reduce the frequency leakage for the different
frequency bins, the records were multiplied by Welch windows, as
recommended by Press et al. (1992). Subsequent Fast Fourier Transform
yielded 50 spectral power values in the frequency range of 20-70 Hz.
The frequency resolution corresponded to 0.98 Hz (maximum frequency,
125 Hz divided by 128 power values). Then, square roots of power values
were computed to obtain more normally distributed spectral amplitude
values. These values were averaged across epochs to obtain measures of
nonphase-locked spectral activity for each of the two deviants and for
standards. Nonphase-locked or "induced" oscillatory activity is
distinguished from phase-locked or "evoked" activity in that the
former is obtained by conducting spectral analysis of each recording
epoch and subsequent averaging of the resulting frequency power values,
whereas the latter is computed by subjecting the average across the raw
signals in each epoch to a subsequent frequency analysis. Gamma-band
oscillations of distributed cells found in the visual cortex of both
anesthetized and awake animals were not phase-locked to the onset of a
stimulus (Eckhorn et al., 1988; Gray and Singer, 1989; Gray and Viana
di Prisco, 1997). Similarly, induced, but not evoked, GBA
recorded from humans with EEG or MEG has been related to feature
binding and object representation (Lutzenberger et al., 1995, 1997;
Tallon et al., 1995; Müller et al., 1996, 1997; Keil et al.,
1999). Therefore, we restricted our frequency analysis to
nonphase-locked, induced oscillatory activity.
Statistical probability mapping of broad-band signals (20-70
Hz). Differences in spectral amplitude between standards and each
deviant were assessed with paired, two-sided t tests for each frequency bin and MEG sensor across the whole subject sample, resulting in 50 (frequency bins) × 143 (sensors) = 7150 tests for each of the two comparisons. t values were
converted to p values. p values from two adjacent
frequency bins had to meet the criterion of p = 0.005 to be considered significant. This was done as an approximation to the
problem of obtaining false positives. The number of two consecutive
p values was chosen as a tradeoff between the aims to (1)
reduce the risk of chance findings and (2) keep the frequency
resolution as high as possible. In addition, a confirmatory statistical
analysis was conducted based on randomization tests suggested by Blair
and Karniski (1993), which were extended to multichannel data. First,
the maximum t value was determined for the observed data
across all sensors and across the frequency bins in which the frequency
analysis had yielded significant effects. Then, the sign of the
task-related spectral amplitude difference was changed for selected
frequency bins and for all recording channels per subject. This was
done for all 212 possible permutations
across subjects. For each permutation, the maximum t value
was identified. Finally, the significance of the observed maximum
t value was tested relative to the distribution of maximum
t values across all 212 tests.
Time course of narrow-band signals (53 ± 2.5 Hz). The
broad-band frequency analysis described above was based on the full recording interval of 600 msec and thus gave no indication of the time
course of the observed gamma spectral amplitude increases. To explore
the time course of GBA increases, the data records were again padded to
obtain 256 points, multiplied with cosine windows, and filtered in the
frequency range in which the broad-band frequency analysis had yielded
significant gamma-band spectral amplitude increases to deviant compared
with standard stimuli. A noncausal, gaussian curve-shaped Gabor filter
in the frequency domain was applied to the signals for each epoch in
each of the three conditions. Based on the spectral analysis results
across the whole epoch, a filter was used with a center frequency of 53 Hz. The width of this filter was chosen at ±2.5 Hz to reduce potential
contamination with 50 Hz background noise. The filtered data were then
amplitude-demodulated by means of a Hilbert transformation (Clochon et
al., 1996). The filtered and amplitude demodulated spectral amplitude
data were then averaged across epochs for each of the three stimulus
types, i.e., midline standards and both left and right deviants. Figure
1 illustrates the present methodological approach; the raw data records, the signals after Gabor filtering and
complex demodulation, and the results of single-trial frequency analysis are shown for representative single trials in one subject and
MEG sensor to both a standard and a left deviant.
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Figure 1.
Illustration of the initial steps of single trial
analysis shown for one sample trial each of a midline standard
(left column) and a left deviant (right
column) in one subject and one MEG sensor over the region of
the right posterior parietal cortex. The deviant trial was selected
because of its particularly large amplitude of the 53 ± 2.5 Hz
filtered signal. The top row shows the raw MEG record,
and the middle row shows the signal after Gabor
filtering in the 53 ± 2.5 Hz range (oscillatory signal) and
subsequent complex demodulation via Hilbert transformation (envelope
curve). The bottom row gives the result of Fast Fourier
Transform of the single trials (amplitude spectrum). Magnetic field
amplitudes (in femtoTesla) are plotted on the ordinates of the
top two rows, and spectral amplitudes (in femtoTesla)
are plotted in the bottom row. The abscissas show
time (in seconds) in the top two rows and frequency (in
Hertz) in the bottom row. Note the different scaling in
each row.
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Differences in the narrow-band amplitudes to midline standards versus
left and right deviants were assessed by a statistical mapping
procedure similar to the evaluation of spectral amplitude differences
in the different frequency bands described above. Note that we were
primarily interested in the time course of GBA increases in those MEG
sensors in which the broad-band frequency analysis had yielded
significant differences. However, for exploratory purposes, it was
decided to conduct the narrow-band analysis for all sensors. Thus,
paired, two-sided t tests were calculated for amplitudes at
every sampling point and sensor for (1) standards compared with left
deviants and (2) standards compared with right deviants for the whole
subject sample. p values from three adjacent time points
(corresponding to a time window of 12 msec) had to meet the criterion
of p = 0.05 to be considered significant.
Topographic mapping of induced GBA. To assess the
topographic localization of significant amplitude values for the whole
group, the sensor positions for each subject were assigned to common spatial coordinates ("common coil system"). To estimate the error that is introduced by this procedure, we used a single dipole localization of the first auditory-evoked component generated by the
midline standards. The spatial coordinates of this dipole were
determined twice for each subject: (1) for the individual sensor
locations and (2) for the representative head model. The comparison of
both sets of coordinates enabled the estimation of the error introduced
by using the common coil system while disregarding the natural
variation across subjects. The differences between individual
localization of the dipole source and common coil system ranged between
0.1 and 2.0 cm in the anteroposterior direction (mean ± SD
absolute deviation, 0.6 ± 0.2 cm), between 0.0 and 1.2 cm in the
left-right direction (mean ± SD, 0.3 ± 0.1 cm) and between
0.0 and 2.3 cm in the superior-inferior direction (mean ± SD,
0.8 ± 0.2 cm). Because no interpolation between sensors was
performed, the spatial resolution was determined by the sensor spacing
(2.5 cm). The results above thus justified the application of a common
coil system. The sensor locations were then projected onto a map with
the major anatomical landmarks based on the magnetic resonance image
from one representative subject of a previous study.
To elucidate the relationship between the results of the present
statistical probability mapping method and possible source structures,
this method was applied to the magnetic fields elicited by a single
dipole source. Data from right median nerve electric stimulation were
used as an example for magnetic fields with a single dipole source
structure. Here, spatiotemporal mapping of 40-60 Hz activity in the
time range of the early somatosensory-evoked fields (latency, 20 msec)
yielded two areas of increased spectral amplitude, corresponding to the
regions with highest magnetic field power (Fig.
2). However, as will be described below,
in the present paradigm, we observed only singular areas of gamma-band activity. This suggested the existence of a different source structure. Whereas a single dipole source elicits a magnetic field with equally strong maxima and minima (ratio of 1), simulating a quadrupolar structure by adding a second dipole with the opposite polarity at 1 cm
distance yields a strong maximum between both dipoles but considerably
weaker outer field minima (ratio of 7.5) (Fig. 3). In further approximation to a
circular current, an octopole increases this ratio to 30 (Fig. 3). This
means that only the inner maximum circumscribed by the four dipoles
remains detectable, whereas the outer minima no longer reach
statistical significance. Thus, octopoles or even circular currents may
be a parsimonious model to describe the source structure of the present
findings. This would imply that the sources are located close to the
area below the sensor with the highest GBA.
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Figure 2.
Example of the present statistical probability
mapping approach applied to the magnetic fields evoked by right median
nerve stimulation. Isocontour lines are plotted onto a head model seen
from above (nose up). The dark arrow represents the
fitted single dipole. Superimposed dark areas show two regions of
significant spectral amplitude increases in the 40-60 Hz range as
yielded by the statistical probability mapping procedure used in the
present study.
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Figure 3.
Models of magnetic fields elicited by different
source structures. Head models (top) illustrate fields
of single dipoles (moments, 10 nA) with opposite orientations
(top left and center). Combination of
those two dipoles arranged parallel at a distance of 1 cm creates a
quadrupole, with the field distribution depicted on the top
right. Whereas single dipoles elicit equal field maxima and
minima with 160 and  160 fT, respectively (ratio of 1), the quadrupole
shows a strong inner field maximum (150 fT) and weaker outer minima
(20 fT; ratio of 7.5); see bottom of the figure. In
further approximation to a circular source structure, an octopole
consisting of four dipoles (bottom) enhances the
imbalance between a strong inner maximum (300 fT) and a weak outer
field (10 fT; ratio of 30) even more.
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Coherence analysis. As feature binding and mental
representations are thought to be related to the activity of coherently oscillating cortical networks (Singer, 1995; Miltner et al., 1999), gamma-band coherence was investigated to elucidate the relationships between the areas with significant GBA increase to deviant compared with midline stimuli. Coherence between two waveforms x and
y is defined as
 xy2(f) = (Gxy(f))2/(Gxx(f)
Gyy(f)),
where Gxy(f)
is the mean cross-power spectral density, and
Gxx(f) and
Gyy(f) are
the respective mean autopower spectral densities (Glaser and Ruchkin,
1976). Coherence was computed in frequency bins between 50 and 55 Hz
for all pairwise combinations of those MEG sensors with a significant
GBA increase to lateralized deviants compared with standards and all
other sensors. The computational procedure to obtain coherence involved
first computing the mean power spectra for x and
y and then the mean normalized cross-spectra. Because
complex analyses are involved, this produced the cospectrum (r for real) and the quadspectrum (q for
imaginary). Then, coherence was computed as
xy2 = (rxy2 + qxy2)/(Gxx
Gyy). Differences in coherence between
standards and deviants were evaluated statistically in the whole
sample, following again the principles described above; t
tests were calculated for each sensor pair, and t values
were converted to p values. p values from three
adjacent frequency bins had to meet the criterion of p = 0.05 to be considered significant.
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RESULTS |
Mismatch magnetic fields
Figure 4a shows the
mismatch evoked magnetic fields in isocontour plots at time points 110 and 150 msec poststimulus, and Figure 4b shows superimposed
field amplitudes for left and right deviants. The symmetrically linked
dipoles fitted to the mismatch fields were localized at the
supratemporal plane (Fig. 4c). These two dipoles explained
>95% of the variance. The time course and magnitude of both left and
right dipoles to left and right deviants are depicted in Figure
5. A hemisphere × side of deviant
interaction emerged for dipole moment
(F(1,11) = 16.7, p = 0.002), in the absence of any main effects. As expected, the left
dipole was stronger for deviants lateralized to the right than to the
left (right deviant, 27.2 ± 5.2 nA; left deviant, 19.7 ± 3.8 nA; t(11) = 2.60;
p = 0.025). Conversely, the right dipole was stronger for left than for right deviants (left deviant, 28.8 ± 4.3 nA; right deviant, 21.9 ± 3.9 nA;
t(11) = 3.18; p = 0.009).
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Figure 4.
a, Magnetic difference fields
evoked by left and right deviants compared with standards
(left and right columns, respectively)
are depicted as isocontour plots at 110 and 150 msec poststimulus onset
(top and bottom rows, respectively). Note
that, for right deviants, the ipsilateral mismatch field was already
present at 110 msec (top right isocontour plot), whereas
for left deviants, it emerged only at ~150 msec (bottom left
isocontour plot). Also, similar isocontour plots were found at
both time points for right, but not left, deviants. b,
Magnetic field amplitudes for the difference activity between left
deviants and standards and between right deviants and standards
(left and right graphs, respectively)
superimposed for all 143 MEG sensors during the recording interval.
Magnetic field curves to left deviants showed a second activity peak
for a few, left hemispheric sensors (depicted in the bottom
panel of a), whereas the pattern for right
deviants was stable across the time interval of 110-150 msec.
c, Dipole source localization for linked dipoles fitted
to the evoked magnetic field. All the data shown are based on averages
across the subject sample.
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Figure 5.
Time course of moments of symmetrically linked,
fixed dipoles fitted to the mismatch fields evoked by left and right
deviants compared with midline standards in the left (solid
lines) and right (dotted lines) hemisphere from
50 msec prestimulus to 210 msec poststimulus. Left deviants were
processed later in the left than in the right hemisphere, whereas there
was no latency difference for right deviants. The curves
are based on averages across the subject sample.
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For dipole latency, there were main effects of both hemisphere
(F(1,11) = 20.5; p = 0.001) and side of deviant (F(1,11) = 8.7; p = 0.013), as well as a hemisphere × side of deviant interaction (F(1,11) = 20.1; p = 0.001) (Fig.
6). Left deviants gave rise to mismatch
dipoles that peaked 25 msec faster on the right than on the left
(right, 114 ± 3 msec; left, 139 ± 3 msec;
t(11) = 4.92; p < 0.001), whereas right deviants elicited peak latencies that were only 5 msec shorter on the right than on the left (right, 118 ± 2 msec;
left, 123 ± 2 msec; t(11) = 2.15; NS). Comparing the responses to deviants within each
hemisphere, the left dipole peaked significantly later to left than
right deviants (t(11) = 4.60;
p = 0.001), whereas the right dipole latencies were
only slightly shorter for left than right deviants
(t(11) = 1.69; NS).
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Figure 6.
Peak latencies for left and right mismatch dipoles
(means and SDs) for left and right deviants.
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Induced GBA
Frequency analysis of broad-band signals
Figure 7 shows the results of
spectral amplitude analysis across the whole recording epoch for all
MEG channels. Applying the significance criterion of p = 0.005 for two consecutive p values disclosed that both
right and left deviants were distinguished from midline standards by
enhanced spectral amplitudes in the frequency range of 50-55 Hz.
Mapping onto the common coil system showed that the spectral amplitude
increases to right deviants were located in one posterior
left-hemispheric and one posterior right-hemispheric sensor. In
contrast, spectral amplitude enhancements to left deviants were
restricted to two sensors over right posterior regions. Randomization
tests were conducted across all sensors in the frequency range between
48 and 55 Hz. These tests confirmed the effects for both right and left
deviants (p = 0.042 and 0.030, respectively).
Whereas there were no significant spectral amplitude increases in any
other frequency band between 20 and 70 Hz, decreases in spectral
amplitude were observed both for right and left deviants at 26-27 Hz.
In addition, left deviants were accompanied by a spectral amplitude
suppression at 36-40 Hz. Reductions in both of these frequency ranges
were localized over right primary sensorimotor cortex at the level of
the trunk/shoulder representation areas. The spectral amplitude
decreases were also confirmed by randomization tests
(p < 0.05 and 0.01 for right and left deviants,
respectively). Because we had no hypothesis for GBA reductions, the
subsequent analyses were restricted to the frequency range at which
enhancements of spectral amplitude were found (50-55 Hz).
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Figure 7.
Statistical probability mapping of spectral
amplitude differences between right deviants and standards
(top) and between left deviants and standards
(bottom). a, Distribution of
logarithmically transformed p values for each frequency
bin superimposed for each MEG sensor (shown between 20 and 60 Hz;
resolution of 1 Hz). The p value curves were smoothed
with a sliding average across three log-transformed p
values. The pairs of vertical markers indicate which
frequency ranges were selected for depiction in parts c
of the figure. b, Here only those effects are depicted
that met the significance criterion of p values for two
consecutive frequency bins of 0.005 (p values
were transformed to z values). Each peak stands for a
significant difference between deviants and standards in one MEG sensor
at a particular frequency. The only significant spectral amplitude
increases were in the range of 50-55 Hz. The pairs of vertical markers
indicate which frequency ranges were selected for depiction in parts
d of the figure. c, Isocontour plots of
the complete p value distributions between 52 and 53 Hz
projected onto maps of MEG sensors (seen from above, nose up).
d, Isocontour plots of the significant spectral
amplitude increases at 52-53 Hz projected onto maps of MEG sensors
(seen from above, nose up).
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Time course of narrow-band signals
Based on the results of the broad-band frequency analysis, signals
were subjected to Gabor filtering (central frequency, 53 Hz; width,
±2.5 Hz) and complex demodulation via Hilbert transform to investigate
the time course of energy increases in this frequency range to deviant
compared with standard stimuli. Absolute mean amplitudes amounted to
~13 ± 1 fT for standard stimuli averaged across the latency
window of 100-300 msec poststimulus onset. Results of statistical
probability mapping of amplitude differences between midline standards
and lateralized deviants are depicted in Figure
8. Responses to both types of deviants
could be separated into an earlier and a later component (in the
following sections, termed components 1 and 2, respectively). For right
deviants, component 1 peaked at ~160 msec and component 2 peaked at
~240 msec poststimulus. Both components exhibited a bilateral
topography. The right-hemispheric center of activation of component 2 was located superior to the corresponding area of component 1, whereas the left-hemispheric center of component 2 was posterior to the corresponding area of component 1. Mean amplitude increases for right
deviants minus midline standards averaged in the time window of
100-300 msec poststimulus were 0.9 ± 0.2 fT for the sensors over
the left hemisphere (t(11) = 4.01;
p = 0.002). The mean amplitude enhancement averaged
across both right-hemisphere sensors was 0.8 ± 0.1 fT
(t(11) = 5.96; p < 0.001).
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Figure 8.
Statistical probability mapping of the time course
of differences in amplitude of the Gabor-filtered and demodulated
signals in the 53 ± 2.5 Hz range between right deviants and
standards (top) and between left deviants and standards
(bottom). a, Here those effects are
depicted that met the significance criterion of p values
for three consecutive time points of 0.05 (p
values were transformed to z values). Each peak stands
for a significant difference between deviants and standards in one MEG
sensor at a particular latency. Significant amplitude increases were
found between 140 and 260 msec for right deviants and between 100 and
130, and 200 and 240 msec for left deviants. The pairs of
vertical markers indicate which latency ranges were
selected for depiction in parts b of the figure.
b, Isocontour plots of the significant 53 ± 2.5 Hz
amplitude increases at latencies of 140-180 and 230-260 msec for
right deviants (top) and at latencies of 100-130 and
200-240 msec for left deviants (bottom) projected onto
maps of MEG sensors (seen from above, nose up).
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For left deviants, the earlier component 1 of the 53 ± 2.5 Hz
amplitude increase peaked at ~110 msec, whereas component 2 reached
its maximum at ~220 msec. Left deviants elicited amplitude enhancements in sensors over the right hemisphere only. Similar to
right deviants, the earlier component showed a more inferior topography
than the later component. The mean amplitude increase to left deviants
averaged across both right-hemisphere sensors in the latency window of
100-300 msec was 0.8 ± 0.2 fT
(t(11) = 4.04; p = 0.002). Amplitudes and log-transformed p value curves for
components 1 and 2 in right-hemisphere sensors are depicted in Figure
9 for both types of deviants in the time
window of 48 msec prestimulus to 400 msec poststimulus onset.
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Figure 9.
Components of significant amplitude differences in
the narrow-band signals (53 ± 2.5 Hz) in right-hemisphere
sensors. Top left, Amplitude difference waves for right
deviants compared with standards in the latency window of 48 to 400 msec poststimulus for components 1 and 2; see Results for a
description of the components. Bottom left, Amplitude
difference waves for left deviants compared with standards. Top
right, Corresponding log-transformed p value
curves for the amplitude difference between right deviants and
standards. Bottom right, Log-transformed
p value curves for the amplitude difference between left
deviants and standards.
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Topographic mapping of induced GBA
The topographic localization of the components identified in the
analysis of the filtered narrow-band signals was investigated using the
superimposition of the MEG sensor map onto a brain surface model
derived from a magnetic resonance image (Fig.
10a,b). The mapping of the earlier component 1 and the later component 2 onto the
brain model is shown for right and left deviants in Figure 10,
c and d, respectively. For right deviants, the
right-hemisphere part of component 1 was localized in the posterior
temporal cortex, whereas the right-hemisphere part of component 2 was
localized over the angular gyrus (see "Coherence analysis"
below for a description of area 2a). The left-hemisphere equivalents
were both localized at the junction of posterior parietal and middle
temporal gyrus. For left deviants, component 1 was localized slightly
more inferior than for right deviants, whereas the topography of
component 2 did not differ between deviants. To demonstrate the
validity of the present topographic mapping approach, a double-dipole
model was applied to the evoked magnetic field response elicited by midline sound onsets at a latency of ~100 msec. The locations of the
first auditory-evoked responses are depicted as green arrows on the head model, showing sources in bilateral supratemporal cortex
(Fig. 10).
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Figure 10.
a, The projection of the MEG
sensor positions (small red circles) onto the
two-dimensional magnetic resonance surface image (seen from above, nose
up). b, Map with anatomical landmarks derived from the
magnetic resonance image. The locations of the MEG sensor positions
averaged across subjects (common coil system) were projected onto this
map to investigate the topography of GBA increases.
ce.s, Central sulcus; po.s,
parieto-occipital sulcus; ca.s, calcarine sulcus;
sm.g, supramarginal gyrus; a.g, angular
gyrus. c, Topographic mapping of the two components of
53 ± 2.5 Hz amplitude enhancements onto maps with anatomical
landmarks for right deviants. The right-hemisphere part of component 1 was localized over the posterior temporal cortex, and the
right-hemisphere part of component 2 was localized over the angular
gyrus. The left-hemispheric counterparts were both localized over the
junction of parietal and posterior temporal cortex. Connecting
lines indicate coherence increases between right- and
left-hemispheric parts of both components. In addition, coherence was
increased between the right-hemisphere part of component 2 and a
neighboring area (2a); see Results, Coherence analysis.
Green arrows indicate the source locations of dipoles
fitted to the magnetic response elicited by midline sound onsets at a
latency of ~100 msec over the supratemporal cortex, demonstrating the
validity of the present mapping approach. d, Areas of
amplitude increase to left deviants compared with standards. Here the
only coherence increase was found between component 2 and area 2a.
e, The blue areas show the topography of
component 2 on the right-hemisphere and left-hemisphere area to which
coherence was increased to right deviants. Here a projection onto a
realistic brain model derived from the same magnetic resonance scan is
used. From left to right, the brain model
is shown from above (nose up), from behind, from the left, and from the
right.
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Coherence analysis
Coherence analysis was conducted to investigate further the
observation of two components in the 53 ± 2.5 Hz response to
changes in sound-source lateralization. Because the exploratory
character of the narrow-band signal analysis did not allow the
conclusion that components 1 and 2 were parts of separate processes,
coherence analysis was used to investigate possible increases or
decreases in coupling between those right-hemisphere sensors in which
significant amplitude increases were found and the remaining sensors.
The results of coherence analysis for frequency bins between 50 and 55 Hz are shown in Figure 11. Absolute
coherence values for standard stimuli were ~0.45 ± 0.02 for
proximal sensor pairs within the right hemisphere, and ~0.20 ± 0.02 for distant, interhemispheric sensor pairs. Values of significant
coherence changes between midline standards and lateralized deviants
and corresponding p values are given in Figure 11. For right
deviants, coherence was increased between the right-hemisphere part of
component 1 and a left parietotemporal area. For the right-hemisphere
part of component 2, coherence was increased to both a closely
neighboring area (2a) (Fig. 10) in the right supramarginal gyrus and to
a left posterior temporal area. For left deviants, both components 1 and 2 showed increased coherence with the same right-hemispheric area
(2a) (Fig. 10). Interestingly, there was no significant coherence change between components 1 and 2 for either type of deviant. This
supported the involvement of two separate processes with distinct
topographies and response latencies in the cortical representation of
changes in sound-source lateralization.
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Figure 11.
Statistical probability mapping of coherence
increases for lateralized deviants compared with standards for the
frequency range of 50-55 Hz (MEG sensor maps seen from above, nose
up). Top, Coherence changes for right deviants compared
with standards. Top left, Coherence for all pairwise
connections involving the sensor at the center of component 1 of
spectral amplitude increase in the 53 ± 2.5 Hz range (depicted as
a bold black circle). Top right,
Coherence increases for all pairwise connections involving the sensor
at the center of component 2 of spectral amplitude increase in the
53 ± 2.5 Hz range (depicted as a bold black
circle). The values of coherence increases are given (in
parentheses, p values) for those sensor pairs with
significant changes (p values for three
consecutive frequencies, 0.05). Bottom, Coherence
changes for left deviants compared with standards. Bottom
left, Coherence increases for all pairwise connections
involving the sensor at the center of component 1 of spectral amplitude
increase in the 53 ± 2.5 Hz range (depicted as a bold
black circle). Bottom right, Coherence for all
pairwise connections involving the sensor at the center of component 2 of spectral amplitude increase in the 53 ± 2.5 Hz range (depicted
as a bold black circle). Note that there were no
significant coherence decreases.
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DISCUSSION |
This study was designed to investigate both evoked magnetic fields
and nonphase-locked oscillatory MEG responses to changes in
sound-source direction in a passive mismatch paradigm. As expected, evoked mismatch fields showed larger amplitudes contralaterally to the
side of the deviant. The dipole at the left supratemporal plane
responded slower to ipsilateral than to contralateral deviants, whereas
its right-hemisphere counterpart peaked equally fast to both deviants.
Induced GBA was enhanced over right posterior parietal and posterior
temporal regions to both deviants, whereas a GBA increase in homologous
left-hemisphere areas was observed to rightward sound-source shifts only.
Mismatch magnetic fields
Computing the difference in evoked MEG fields between lateralized
deviants and midline standards yielded a mismatch response (cf.
Paavilainen et al., 1989; Schröger, 1995, 1996; Winkler et al.,
1998), with the respective sources localized at the supratemporal plane
(Fig. 4). As hypothesized, the dipoles fitted to the mismatch fields
were stronger contralaterally than ipsilaterally to the side of the
deviant (Figs. 5, 6). Although previous studies using dichotic stimuli
have shown bihemispheric mismatch responses that appeared to be more
pronounced in the hemisphere contralateral to the ear in which the
deviant sound occurred (Praamstra and Stegemann, 1992; Deouell et al.,
1998), our findings suggest the existence of preattentive change
detectors with stronger responses to apparent location shifts of
binaural sounds into the contralateral hemifield. This is consistent
with the fact that most neurons in auditory cortex that are sensitive
to spatial information respond to locations in the contralateral
hemifield (Jenkins and Masterton, 1982; Phillips and Irvine, 1983;
Heffner, 1997). The present analysis focused on the supratemporal
generators of the mismatch response, which have been suggested to
signal predominantly changes in the contralateral hemifield (Giard et
al., 1990). Unlike some previous studies (Paavilainen et al., 1991;
Levänen et al., 1996), we found no additional mismatch generator
in the right hemisphere. Interestingly, the detection of
contralaterally larger responses may depend on the presentation of
lateralized sounds in a mismatch paradigm, because lateralized binaural
tone bursts alone did not seem to elicit larger contralateral MEG field
amplitudes (Woldorff et al., 1999).
Looking at latencies, sound lateralization to the left elicited longer
dipole latencies in the left than in the right auditory cortex, with
the delay between both dipole peak latencies (25 msec) exceeding the
transcallosal transmission time of maximally 20 msec (Banich, 1997). In
contrast, a rightward sound displacement elicited only a slight
interhemispheric latency difference (5 msec). The comparison of
responses to deviants within each hemisphere showed that the left
dipole exhibited a slower response to ipsilateral than contralateral
deviants, whereas this difference was not significant for the right
dipole. These findings suggest a dominance of the right auditory cortex
in the detection of change in sound-source lateralization. Shorter
mismatch response latencies over the right compared with the left
hemisphere have been reported for different kinds of monaurally
presented nonlanguage deviants by Levänen et al. (1996), who
concluded that the right hemisphere may be involved more strongly in
acoustic change detection than the left. However, although this may
hold true for simple tones, infrequent binaural linguistic stimuli
identical to the one used in the present study have been found to be
processed earlier at the level of the left than the right auditory
cortex (Ackermann et al., 1999). The fact that there was no such
left-hemisphere advantage when deviants were distinguished from
standards by their perceived lateralization can be interpreted in terms
of a right auditory cortex predominance in the processing of changes in
sound-source lateralization also for language material.
Induced GBA
Changes in sound-source direction elicited 50-55 Hz spectral
amplitude enhancements in the latency range of the mismatch response. These nonphase-locked GBA activity increases were localized in right
posterior temporal and posterior parietal regions for both types of
deviants, thus supporting the hypothesized involvement of right
parietal cortex in the processing of auditory spatial information. In
addition, right deviants were accompanied by 50-55 Hz spectral
amplitude increases in homologous left-hemisphere regions. Induced GBA
enhancements in human EEG are thought to reflect cortical networks
involved in the representation of meaningful or gestalt-like stimuli
(Lutzenberger et al., 1994, 1995; Tallon et al., 1995; Tallon-Baudry et
al., 1996, 1997; Müller et al., 1997; Pulvermüller et al.,
1997; Basar et al., 1999; Tallon-Baudry and Bertrand, 1999; Keil et
al., 1999). The present findings thus suggest the participation of
posterior parietal networks in the coding of auditory space.
Interestingly, induced GBA increases in the EEG studies cited above
often showed a wide distribution across recording sites. In contrast,
the present statistical probability mapping approach yielded
circumscribed areas with GBA enhancements (Sokolov et al., 1999; Kaiser
et al., 2000) that may have been generated by a multipolar source
structure in the cortical region below the center of increased GBA (see
Materials and Methods; Figs. 2, 3).
When investigating effects in a large number of frequency bands and MEG
sensors, multiple comparisons represent a major problem. Here we used a
statistical probability mapping approach based on a combination of a
high  criterion for two consecutive frequency bins and randomization
tests. Although setting the criteria was somewhat arbitrary, it ensured
that only the most robust effects were subjected to further
investigation. The subsequent assessment of narrow-band signal time
course and coherence yielded effects in the same regions as the
broad-band frequency analysis, thus confirming the involvement of these
areas in the representation of sound-source lateralization. Because
group studies are necessary to identify effects with a low
signal-to-noise ratio like the gamma-band response, individual
topographic mapping was not possible. Nevertheless, the use of a common
coil system enabled the localization of GBA enhancements over areas
found to exhibit increased hemodynamic activity to perceived sound
movement or lateralization in functional imaging studies (Griffiths et
al., 1998; Weeks et al., 1999).
Both the assessment of the time course of the induced GBA response and
coherence analysis suggested the existence of two separate components:
an earlier component with a posterior temporal topography, and a later,
posterior parietal component. The involvement of parietal areas was
expected on the basis of functional imaging and lesion studies
(Griffiths et al., 1994, 1998; Soroker et al., 1997; Bushara et al.,
1999) and because of their relevance for the representation of space.
The posterior parietal cortex receives input from somatosensory,
visual, and auditory modalities, is closely connected with the
prefrontal cortex (Kolb and Whishaw, 1996; Romanski et al., 1999), and
participates in the control of spatially guided behavior and attention,
as well as in the abstract representation of space (Andersen, 1995).
The increase in posterior temporal GBA is in line with studies in the
monkey showing an involvement of caudal areas of the superior temporal gyrus in auditory spatial processing (Rauschecker et al., 1997, 1999;
Recanzone et al., 2000). The present findings provide further evidence
supporting a role of this area as a part of the auditory dorsal or
"where" stream (Rauschecker, 1998; Romanski et al., 1999).
An unexpected finding was the GBA suppression over right and central
primary sensorimotor areas found at ~26 Hz for both types of deviants
and additionally at ~38 Hz for left deviants. It can be speculated
that, depending on their content, lateralized auditory stimuli may
elicit a tendency to orient toward the perceived sound source. The
activity reduction of motor circuits could thus have reflected the
subjects' suppression of an urge to move. GBA reductions, previously
reported during the perception of pseudowords (Lutzenberger et al.,
1994; Pulvermüller et al., 1996), may indicate suppression of
network activity that is irrelevant for or detrimental to the performance on explicit or, as in this case, implicit tasks.
Conclusions
In summary, the present study showed that a change in sound-source
direction from midline toward the right side elicits both simultaneously peaking evoked magnetic fields at the level of the
bilateral supratemporal planes and increased nonphase-locked spectral
amplitudes in the gamma-band range within bilateral posterior temporal
and posterior parietal areas. In contrast, a leftward sound-source
shift is detected earlier at the level of the right than left
supratemporal plane and leads to GBA enhancements exclusively over
right-hemispheric parietotemporal regions. These findings suggest that
(1) bilateral processing and representation of rightward, but
contralateral processing and representation of leftward, changes in
sound-source direction, and (2) an involvement of right-hemisphere auditory association areas in the representation of bidirectional sound-source direction changes. The fact that this was found for linguistic stimuli underscores the robustness of the processing pattern. Both dipole latency and GBA findings were in keeping with the
hypothesized right-hemisphere dominance for the processing of
sound-source lateralization. Our findings might help to explain why
unilateral auditory neglect is found much more frequently for stimuli
in the left hemifield after right parietal lesions than vice versa
(Pinek et al., 1989; Griffiths et al., 1994, 1996; Soroker et al.,
1997; Driver and Mattingley, 1998).
| |
FOOTNOTES |
Received April 18, 2000; revised June 21, 2000; accepted June 21, 2000.
This research was supported by Deutsche Forschungsgemeinschaft Grant
SFB 550/C1.
Correspondence should be addressed to Dr. Jochen Kaiser, Institute of
Medical Psychology and Behavioral Neurobiology, University of
Tübingen, Gartenstrasse 29, 72074 Tübingen, Germany.
E-mail: jochen.kaiser{at}uni-tuebingen.de.
| |
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ABSTRACT
INTRODUCTION
