359of sounds in virtual auditory space via headphones, sighted humans normally activate
parts of their posterior parietal cortex in an area separate from known visual areas involved
in spatial analysis.^18 Blind humans, in addition to activating a much more extended area in
the same region, also activate large parts of the occipital lobe, which is normally used for
seeing^19 (Figure 23.2). The results of cross-correlating activities in different brain regions
suggest that the auditory input to the occipital cortex is relayed via posterior parietal cor-
tex, but a sustenance or strengthening of direct connections from auditory to visual cortex,
as they occur especially in young animals,20–22cannot be excluded. These surprising
results warrant further testing in animals with neurophysiological and neuroanatomical
techniques.
‘What’ and ‘where’ in auditory cortex
Another fundamental tenet of Hebbian models of long-term memory is that information
gets stored in the same places where it is processed. We are far from understanding how
lengthy sequences of musical melodies or even more complex pieces of music are stored in
our brains, although it is clear that music is a powerful stimulus that activates large parts
of auditory cortex in both hemispheres (Figure 23.3). What we are slowly beginning to
understand is that complex sounds are first broken down into their frequency components
in the periphery of the auditory system and in primary auditory cortex. Then, at later stages
of cortical processing this tonal information is integrated into more complex response
properties,23,24and single units respond to more and more complex contents. The hierar-
chical processing of such ‘what’ information seems to take place in more antero-lateral por-
tions of the superior temporal gyrus (STG),25–28all the way to the temporal pole. By
Figure 23.2Results of PET scanning in congenitally blind subjects during localization of sounds presented via
headphones in virtual auditory space (data from Ref. 19). Areas in the occipital cortex that are driven by visual
stimuli in sighted individuals are now activated during sound localization. Note that the right hemisphere seems
to profit more from the expansion of auditory into visual territory. The ‘rewiring’ seems to be mediated by the
parietal cortex, consistent with studies on visually deprived animals.^9 (See Plate 18 in colour section.)