RESPONSE OF THE PRIMARY AUDITORY-CORTEX TO ELECTRICAL-STIMULATION OF THE AUDITORY-NERVE IN THE CONGENITALLY DEAF WHITE CAT

Neural activity plays an important role in the development and mainten ance of sensory pathways. However, while there is considerable experie nce using cochlear implants in both congenitally deaf adults and child ren, little is known of the effects of a hearing loss on the developme nt of the auditory cortex. In the present study, cortical evoked poten tials, field potentials, and multi- and single-unit activity evoked by electrical stimulation of the auditory nerve were used to study the f unctional organisation of the auditory cortex in the adult congenitall y deaf white cat. The absence of click-evoked auditory brainstem respo nses during the first weeks of life demonstrated that these animals ha d no auditory experience. Under barbiturate anaesthesia, cortical pote ntials could be recorded from the contralateral auditory cortex in res ponse to bipolar electrical stimulation of the cochlea in spite of tot al auditory deprivation. Threshold, morphology and latency of the evok ed potentials varied with the location of the recording electrode, wit h response latency varying from 10 to 20 ms. There was evidence of thr eshold shifts with site of the cochlear stimulation in accordance with the known cochleotopic organisation of AI. Thresholds also Varied wit h the configuration of the stimulating electrodes in accordance with c hanges previously observed in normal hearing animals. Single-unit reco rdings exhibited properties similar to the evoked potentials. Increasi ng stimulus intensity resulted in an increase in spike rate and a decr ease in latency to a minimum of similar to 8 ms, consistent with laten cies recorded in AI of previously normal animals (Raggio and Schreiner , 1994). Single-unit thresholds also varied with the configuration of the stimulating electrodes. Strongly driven responses were followed by a suppression of spontaneous activity. Even at saturation intensities the degree of synchronisation was less than observed when recording f rom auditory brainstem nuclei. Taken together, in these auditory depri ved animals basic response properties of the auditory cortex of the co ngenitally deaf white cat appear similar to those reported in normal h earing animals in response to electrical stimulation of the auditory n erve. In addition, it seems that the auditory cortex retains at least some rudimentary level of cochleotopic organisation.