frequency, as shown on the right in the same figure. For these components, amplitude was
maximal at a range of frequencies between 600 Hz and 2 kHz.
Recordings from the secondary auditory cortex in the left and right hemispheres
The secondary auditory areas studied were confined to an area anterior and lateral to the
primary cortex, as previously identified in physiological25,32and morphological studies.37,38
In this secondary area, frequency-dependent fluctuations in amplitude were observed for
the 80-ms component. Figure 10.5 shows the localization of electrodes implanted in the
right (left-hand side offigure) and left (right-hand side offigure) secondary auditory areas
in 11 patients (six right;five left). The sagittal views (top) show the localization of each
electrode, while the coronal views (below sagittal views) show the localization of those elec-
trode leads yielding the greatest frequency-dependent fluctuations in amplitude. In the
right hemisphere, a mediolateral as well as an anteroposterior distribution of BFs was
observed for the 80-ms component. Neuronal populations sensitive to high frequencies
(graphs A and E) 80-ms poststimulus onset were localized in the posterolateral part of right
HG, while those sensitive to low frequencies (graph B) were localized in the anterolateral
part of right HG. For the 80-ms component, high frequencies were also represented vent-
rally in an area adjacent to the lateral part of the primary cortex in the middle of right HG,
overlapping the area in which low frequencies were represented for the 50-ms component.
The profile of amplitude curves for the right hemisphere was noticeably different from
that for the left hemisphere. In the dorsolateral part of the left HG, the generators of the
80-ms component were most responsive over a range of high frequencies (1–4 kHz;
graph B) or to two isolated sets of frequencies (500 Hz and 1–2 kHz; graph C); in the antero-
lateral part of left HG, this component was sensitive either to a range of low-to-medium fre-
quencies (500 Hz–1 kHz) or to two isolated sets of frequencies (400 Hz and 1 kHz; graph D).
Figure 10.6 summarizes the main results. In the right primary cortex, the 30-ms
component was not clearly tonotopic. A first tonotopic map is observed for the 50-ms
component. A strict organization displaying high frequencies medially and low
frequencies laterally is observed. A second map is observed for the 80- or 100-ms com-
ponents generated in the secondary cortex. Neurons responsive to low frequencies were
lateral to those responding to higher frequencies, but they were less finely tuned (broken
line). Another map was observed for the 100-ms component generated in the planum
temporale. High frequencies seemed to be more represented. On the left hemisphere, in
the primary as well as in the secondary auditory cortex, the neurons were most sensitive
to a range of frequencies (low or high). Nevertheless, the same tonotopic organization of
the frequencies seems to be respected—high frequencies medially and low frequencies
laterally represented.
Discussion
The present study suggests that the auditory cortex is composed of frequency-dependent
tonotopic maps and that these maps are more complex than previously reported in
surface EP studies.20,21,23,39Although the previously reported mediolateral tonotopic
organization was also observed in this study, it was not robust for all AEP components
recorded and was hemisphere specific.
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