REFRACTORY PROPERTIES OF AUDITORY BRAIN-STEM RESPONSES EVOKED BY ELECTRICAL-STIMULATION OF HUMAN COCHLEAR NUCLEUS - EVIDENCE OF NEURAL GENERATORS

In this study of electrically-evoked auditory brain-stem responses (EA BRs) elicited by cochlear nucleus stimulation, 3 waves were identified after the initial wave that is directly initiated by the electric sti mulus. Varying the rate of periodic stimulation or the interval betwee n pairs of stimuli revealed that the shorter the latency of a wave, th e faster it recovered from activation (i.e. shorter refractory period) . The slow recovery of the third wave and an accompanying contribution to the second wave could be accounted for by postsynaptic generation in the two medial superior olivary nuclei (MSO); the faster recovery o f another contribution to the second wave by generation in an axonal t ract bending around the contralateral MSG; and the fastest recovery of the first wave by another axonal pathway having larger axons. Compari son with the relative latencies and spatial distribution of an acousti cally-evoked auditory brain-stem response (AABR) indicated that the th ird wave corresponds to wave V, the second to wave IV (called IVb), an d the first to a wave that precedes wave IV (called Na). The anatomica l interpretations for the two later waves of the EABR are consistent w ith most of the extant data on the neural generators of AABR waves N a nd V. Thus, the present data and analysis strengthen the identificatio n of the electrically evoked responses as EABRs and provide a firmer f oundation for intra-operative EABR monitoring to assist auditory brain -stem implant placement. (C) 1998 Elsevier Science Ireland Ltd. All ri ghts reserved.