Cochlear Implants and Brain Plasticity

Behavioral data indicate the existence of sensitive periods in the development of audition and language. Neurophysiological data demonstrate deficits in the cerebral cortex of auditory-deprived animals, mainly in reduced cochleotopy and deficits in corticocortical and corticothalamic loops. In addition to current spread in the cochlea, reduced cochleotopy leads to channel interactions after cochlear implantation. Deficits in corticocortical and corticothalamic loops interfere with normal processing of auditory activity in cortical areas. Thus, the deprived auditory cortex cannot mature normally in congenital deafness. This maturation can be achieved using auditory experience through cochlear implants. However, implantation is necessary within the sensitive period of the auditory system. The functional role of long-term potentiation and long-term depression, inhibition, cholinergic modulation and neurotrophins in auditory development and sensitive periods are discussed. Copyright © 2002 S. Karger AG, Basel Humans use symbolic communication, language, for the exchange of information. This communication is based on intact hearing. The ability to acquire language is inborn, but language has to be learned under appropriate acoustic stimulation. Congenital hearing loss therefore strongly affects the development of language competence. This development takes place in sensitive periods, i.e. age spans in which the nervous system requires external stimulation to develop competence for processing the inputs. Deprivation often results in a nonfunctional neural system. The present review is focused on the cortical nervous processes taking place after early and late cochlear implantation in congenitally deaf patients. First, we will review evidence for behavioral deficits following congenital deprivation of the audition-language system, secondly the functional neural processes taking place in primary and higher-order cortical areas in congenital deprivation and cochlear implantation will be addressed. Behavioral Deficits in Congenital Deprivation Behavioral manifestations for sensitive periods can be found both in language-deprived and auditorily deprived children. Language Deprivation and Language Development Language is a human-specific phenomenon [Pinker, 1994]. The cortical areas involved in language processing (here called language system) have been identified by case reports and electrical stimulation of the cortex in unanes-