Plasticity of tonotopic maps in humans: influence of hearing loss, hearing aids and cochlear implants.

Conclusion: This paper reviews psychoacoustical and electrophysiological evidence for reorganization of the human central auditory system in case of auditory deprivation and rehabilitation.

Objective: To investigate the plasticity of cortical tonotopic maps in cochlear-damaged subjects.

Methods: Frequency discrimination scores were analysed in subjects with high frequency hearing loss to test for potential perceptual correlates of auditory deprivation- and rehabilitation-induced plasticity. In cochlear implant patients, electrically evoked auditory cortical responses were obtained using EEG to study scalp potential maps.

Results: Perceptual changes in frequency discrimination were observed at the lesion-edge frequency of steeply sloping hearing loss. Although these results are not direct proof of cortical plasticity, no peripheral phenomenon has been found to explain them. The reversal of such auditory deprivation-induced plasticity, a phenomenon that may be termed rehabilitation plasticity, can be studied in hearing-impaired subjects fitted with a hearing aid. Cochlear implant subjects provide another interesting model for studying rehabilitation plasticity in that even profound to total deafness is made partially reversible by cochlear implantation. We found that the auditory cortex of deaf subjects with at least 3 months of cochlear implant experience is organized in a way similar to the tonotopy described in normal-hearing subjects.