Unilateral hearing during development: hemispheric specificity in plastic reorganizations

Abstract

The present study investigates the hemispheric contributions of neuronalreorganization following early single-sided hearing (unilateral deafness).The experiments were performed on ten cats from our colony of deafwhite cats. Two were identified in early hearing screening as unilaterallycongenitally deaf. The remaining eight were bilaterally congenitallydeaf, unilaterally implanted at different ages with a cochlear implant.Implanted animals were chronically stimulated using a single-channelportable signal processor for two to five months. Microelectroderecordings were performed at the primary auditory cortex under stimulationat the hearing and deaf ear with bilateral cochlear implants. Localfield potentials (LFPs) were compared at the cortex ipsilateral andcontralateral to the hearing ear. The focus of the study was on themorphology and the onset latency of the LFPs. With respect to morphologyof LFPs, pronounced hemisphere-specific effects were observed. Morphologyof amplitude-normalized LFPs for stimulation of the deaf and thehearing ear was similar for responses recorded at the same hemisphere.However, when comparisons were performed between the hemispheres,the morphology was more dissimilar even though the same ear was stimulated.This demonstrates hemispheric specificity of some cortical adaptationsirrespective of the ear stimulated. The results suggest a specificadaptation process at the hemisphere ipsilateral to the hearing ear,involving specific (down-regulated inhibitory) mechanisms not foundin the contralateral hemisphere. Finally, onset latencies revealedthat the sensitive period for the cortex ipsilateral to the hearingear is shorter than that for the contralateral cortex. Unilateralhearing experience leads to a functionally-asymmetric brain withdifferent neuronal reorganizations and different sensitive periodsinvolved.