Dissociated Representation of Binaural Cues in Single-Sided Deafness: Implications for Cochlear Implantation.

Congenital single-sided deafness (SSD) leads to an aural preference syndrome that is characterized by overrepresentation of the hearing ear in the auditory system. Cochlear implantation (CI) of the deaf ear is an effective treatment for SSD. However, the newly introduced auditory input in congenital SSD often does not reach expectations in late-implanted CI recipients with respect to binaural hearing and speech perception. In a previous study, a reduction of the interaural time difference (ITD) sensitivity has been shown in unilaterally congenitally deaf cats (uCDCs). In the present study, we focused on the interaural level difference (ILD) processing in the primary auditory cortex. The uCDC group was compared with hearing cats (HCs) and bilaterally congenitally deaf cats (CDCs). The ILD representation was reorganized, replacing the preference for the contralateral ear with a preference for the hearing ear, regardless of the cortical hemisphere. In accordance with the previous study, uCDCs were less sensitive to interaural time differences than HCs, resulting in unmodulated ITD responses, thus lacking directional information. Such incongruent ITDs and ILDs cannot be integrated for binaural sound source localization. In normal hearing, the predominant effect of each ear is excitation of the auditory cortex in the contralateral cortical hemisphere and inhibition in the ipsilateral hemisphere. In SSD, however, auditory pathways reorganized such that the hearing ear produced greater excitation in both cortical hemispheres and the deaf ear produced weaker excitation and preserved inhibition in both cortical hemispheres.