pattern. A third area in the right superior temporal sulcus also showed a significant
response to the spectral parameter but showed no change to the temporal parameter. Thus,
the data supported the hypothesis that corresponding regions of the auditory cortex in the
two hemispheres have different sensitivity to temporal and spectral information.
Neurophysiological recordings in macaque monkeys have shown that auditory cortical
neurons are highly sensitive to both spectral and temporal features of sounds simultane-
ously, and that they exhibit complex response functions.^37 –^39 It has also been shown that
neurons in the belt region of the macaque auditory cortex are optimally responsive to stim-
uli of specific bandwidths, hence demonstrating sensitivity to a spectral parameter.^40 As
well, Eggermont^41 noted that in two primary fields of the cat there was an inverse relation
between bandwidth and temporal resolution, consistent with our proposal.
Additional evidence that left auditory cortical units have higher temporal resolution comes
from recent depth-electrode recordings within Heschl’s gyri^42 showing that the responses
within the left auditory cortex encode the voice-onset time of a consonant, whereas the right
auditory cortex did not show sensitivity to this temporal parameter. Even more striking con-
sistency is offered by additional data from the same investigators (Chapter 10, this volume),
who showed that intracortically recorded auditory evoked potentials were more sharply tuned
to frequency in the right auditory cortex than in the left, as predicted by the hypothesis that
resolution in the frequency and time domains are different in the two hemispheres.
This hypothesis has also received support from a PET study by Belin et al.,^43 in which the
rate of formant transitions in a pseudospeech sound—not perceived as speech—was var-
ied from 40 to 200 ms. CBF changes in the left auditory region were found for both types
of stimuli, indicating a capacity for processing spectral change over a wide range of dura-
tions; by contrast, regions in the right auditory cortex responded only to the slower rates,
and not to the faster rate. Thus, the right auditory cortex seems unable to respond to fast
formant transitions, whereas the homologous area on the left is better able to track rapidly
changing acoustic information that would be relevant for speech processing. Finally, it is
also relevant to recall the findings of Robin et al.,^9 who studied auditory discrimination in
brain-damaged patients. They too found that damage to association cortices in the right
hemisphere resulted in spectral but not temporal processing deficits, while the converse
was observed after left-hemispheric damage.
Anatomical considerations
One advantage of the hypothesis presented above is that it offers a unifying framework to
understand some of the functional characteristics of the auditory nervous system that allow
us to be able to process speech and tonal patterns. This model raises an important question,
however: How are these functional differences instantiated in the brain? That is, can we find
any evidence in the structure of the auditory cortex that would not only support the existence
of processing differences but would also suggest how these differences are implemented?
Some answers to these questions are provided by recent anatomical data from our labo-
ratory^44 and from other investigators. Unlike the functional techniques described above, the
aim of this research was to characterize the structural features (shape, volume, and position)
of the human primary auditory cortical region in vivo. The approach taken was to use MRI
scans from groups of normal right-handed volunteer subjects and to label the region of HG
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