Evidence of a Tonotopic Organization of the Auditory Cortex in Cochlear Implant Users

Abstract

Deprivation from normal sensory input has been shown to alter tonotopic organization of the human auditory cortex. In this context, cochlear implant subjects provide an interesting model in that profound deafness is made partially reversible by the cochlear implant. In restoring afferent activity, cochlear implantation may also reverse some of the central changes related to deafness. The purpose of the present study was to address whether the auditory cortex of cochlear implant subjects is tonotopically organized. The subjects were thirteen adults with at least 3 months of cochlear implant experience. Auditory event-related potentials were recorded in response to electrical stimulation delivered at different intracochlear electrodes. Topographic analysis of the auditory N1 component (ϳ85 ms latency) showed that the locations on the scalp and the relative amplitudes of the positive/negative extrema differ according to the stimulated electrode, suggesting that distinct sets of neural sources are activated. Dipole modeling confirmed electrode-dependent orientations of these sources in temporal areas, which can be explained by nearby, but distinct sites of activation in the auditory cortex. Although the cortical organization in cochlear implant users is similar to the tonotopy found in normal-hearing subjects, some differences exist. Nevertheless, a correlation was found between the N1 peak amplitude indexing cortical tonotopy and the values given by the subjects for a pitch scaling task. Hence, the pattern of N1 variation likely reflects how frequencies are coded in the brain.

What can be expected from a late cochlear implantation?

“…This hypothesis is supported by experimental studies showing that the destruction of the cochlea during the early phase of development in young animals elicits the establishment of new innervating patterns in the auditory pathway [20,21] and the organization of a new tonotopic distribution in the cerebral cortex [22,23].…”

The maturation of human evoked brain potentials to sounds presented at different stimulus rates

“…This raises some fundamental questions about what crucial perceptual processes are indexed by N1, given the ease with which we decode complex sound information in rapidly occurring acoustic environments typical of speech and music. Although N1 has been suggested to represent multiple perceptual processes, such as pitch (Guiraud et al, 2007;Hirose et al, 2005;Seither-Preisler et al, 2006), stream segregation (Gutschalk et al, 2005), or consciousness (Kotchoubey, 2005;Plourde and Picton, 1991), these relationships may not serve as key indices of the underlying neural events relevant for perception.…”

Music Training Improves Pitch Perception in Prelingually Deafened Children With Cochlear Implants

“…Such reorganization, probably coupled with essential changes in neurotransmission or neuromodulation, might assist in reducing additional deterioration in the nervous system that results from cessation of electrical input as a result of cochlear damage. 22,23 This might reverse the disrupted tonotopic maps toward a relatively "normal" organization, 24 which in turn may lead to better development of frequency tuning in the auditory cortices. In children with normal hearing, improved music perception via music education has been revealed by increased auditory evoked fields, possibly as a result of a greater number and/or synchronous activity of neurons.…”