(Figure 16.1) using interactive pixel-marking software that permits simultaneous viewing in
all three planes of the section. The result of greatest relevance pertains to the estimates of
the volume of HG, which was found to be significantly greater on the left than on the right
in two independent samples of 20 subjects each.^44 Most interesting of all was that the dif-
ferences in volume were found to be confined to the white matter underlying HG, and not
to the volume of cortical tissue (grey matter) within the structure (this effect may be seen
qualitatively in the MRI shown in Figure 16.1). This finding suggests an anatomical asym-
metry that arises from a difference in the volume offibres that carry information to and
from the primary auditory cortex and surrounding regions.
Although an MRI cannot reveal details of the underlying neuronal organization, the
greater degree of white matter on the left could be consistent with the notion of greater
speed of processing on the left, if the white-matter volume measures are related to degree
of myelination. That is, a greater degree of left-sided myelination could lead to faster trans-
mission of acoustically relevant information, because it is well established in neurophysi-
ology that more heavily myelinated fibres transmit neuronal impulses more quickly.
Confirmation that the left auditory cortical regions are indeed more heavily myelinated
comes from a recent postmortem electron microscopy study showing that the myelin
sheath is thicker in the left than in the right posterior temporal cortex.^45 As well, Hutsler
and Gazzaniga^46 reported larger left- than right-layer IV pyramidal cells in the human
auditory cortex, which would also lead to faster time constants.
In addition to the idea that left auditory cortical fibres are more heavily myelinated, there
is also evidence for other hemispheric differences in the structural organization of the
auditory cortex that are broadly consistent with the hypothesis presented above. For exam-
ple, Seldon^47 reported that cortical columns in the left auditory cortex were more widely
spaced than those on the right. Galuske et al.^48 recently found that clusters of neurons were
spaced more widely in the left posterior temporal cortex, with more long-range intrinsic
connections between these clusters. These features could be compatible with a greater
degree of integration across tonotopically organized cortical areas on the left than on the
right, hence leading to a relative degradation of spectral resolution on the left. Conversely,
neurons in the right auditory cortex would have structural features that would enhance
spectral resolution (much as shown by Liégeois-Chauvel, Chapter 10, this volume) because
they would be smaller, with fewer myelinated inputs, therefore perhaps more tightly packed
together; and would integrate over narrower frequency regions. These characteristics
would also result in poorer temporal resolution.
Concluding comments
We have seen how a variety of different methods and techniques point towards a general
conclusion about the way in which the auditory nervous system processes information
relevant for our ability to process speech and tonal sounds. These two domains of auditory
processing are, of course, at the core of our uniquely human abilities to communicate. It is
perhaps no coincidence that abilities to process both speech and music arise precociously in
development and follow relatively fixed developmental sequences (see Chapter 1, this volume).
It is perhaps also not irrelevant that both speech communication and tonal patterns appear
to be ubiquitous across all human cultures (see Chapters 4 and 5, this volume).
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