ReviewInjury-induced reorganization in adult auditory cortex and its perceptual consequences
Introduction
Evidence from studies in a range of species over the last decade has demonstrated that restricted cochlear lesions in adult animals result in plastic changes in the representation of the cochlea in primary auditory cortex (see Irvine and Rajan (1995) for review). The general form of this plasticity is that the cortical region in which the lesioned section of the cochlea is normally represented is occupied by an expanded representation (or expanded representations) of adjacent undamaged (peri-lesion) parts of the cochlea. Analogous dynamic reorganization of cortical ‘maps’ of receptor surfaces has also been described in primary visual and somatosensory cortex after restricted peripheral lesions in adults (see Gilbert (1998) and Kaas, 1991, Kaas, 1995 for reviews). The occurrence of such injury-induced reorganization in different sensory systems and in a wide range of species (including simian primates) suggests that it would also occur in humans with peripheral sensory lesions comparable to those studied in animals.
As a consequence of such reorganization, natural stimuli that activate the peri-lesion areas of the receptor produce activation over a much greater expanse of the cortical sheet than would normally be the case. One of the many questions raised by this form of plasticity in adult sensory systems therefore concerns the perceptual consequences of these dramatically changed patterns of cortical activation. In this review, evidence relating to this question will be examined. Electrophysiological evidence for the occurrence of auditory cortical reorganization in animals, and the more limited imaging evidence for the occurrence of such reorganization in humans, will be reviewed as background to examination of the evidence bearing on the perceptual consequences of injury-induced reorganization. Attention will be restricted to cortical reorganization, although an important issue concerns the contribution of subcortical changes to the plasticity seen in cortex (see, e.g., Rajan and Irvine (1998)).
Section snippets
Injury-induced reorganization in primary auditory cortex of animals
A restricted lesion of the cochlea in an adult animal results in a partial hearing loss (i.e., a loss of hearing over the frequency range affected by the cochlear lesion). Electrophysiological mapping of the cortex 1–2 months after such a lesion reveals that the region of cortex deprived of its normal input by the lesion is occupied by an expanded representation (or representations) of adjacent regions of the cochlea, and thus of the frequencies represented at those cochlear loci. This change
Evidence for the occurrence of injury-induced reorganization in humans
Although a number of perceptual phenomena consequent on restricted receptor lesions in adult humans have been attributed to cortical reorganization, the only direct evidence of such reorganization in humans is provided by a relatively limited number of recent studies using magnetoencephalographic (MEG) or other functional imaging techniques. Thus, injury-induced reorganization has been described in somatosensory cortex of amputees (Elbert et al., 1994, Yang et al., 1994, Flor et al., 1995,
Possible perceptual correlates of injury-induced auditory cortical reorganization
Although it is tempting to think of the cortical reorganization that occurs in animals consequent on restricted receptor damage as a central compensation for the peripheral loss, it seems unlikely that this is an appropriate interpretation. In the case of the auditory system, it is obvious that sensitivity to the frequencies that would normally produce activation in the part of the cochlea damaged by the lesion is in no way restored by central reorganization. Nor is it at all clear that there
Acknowledgments
Financial support for this work was provided by The National Health and Medical Research Council, The Co-operative Research Centre for Cochlear Implant, Speech, and Hearing Research, The Bionic Ear Institute, and The University of Melbourne. The assistance of Margaret Lech and Michelle Kornblum, the technical support of John Cassell, Moyra Farrington, Val Park, and Rosemary Williams, and the helpful comments of Brian Moore and two anonymous referees on an earlier version of the manuscript, are
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