CHARACTERIZATION OF WAVE-I OF THE ELECTRICALLY-EVOKED AUDITORY BRAIN-STEM RESPONSE IN THE GUINEA-PIG

This paper examines the first component of the electrically evoked aud itory brainstem response (EABR) of the guinea pig. Short (20 mus/phase ) and long (4000 mus/phase) duration rectangular current pulses were a pplied through a bipolar intracochlear electrode in acute preparations . Short-duration pulses evoked a synchronized response relatively free of stimulus artifact; long pulses facilitated examination of the inte grative capacities of nerve fibers at relatively low current levels. I n deafened control subjects, wave I of the EABR consistently demonstra ted two positive peaks having different latency-level and adaptation r ecovery functions. The early component (wave Ia) showed less decrement in latency with increasing stimulus level and recovered faster in a f orward-masking paradigm. Non-monotonicities in the adaptation recovery curves were also observed, more consistently in the wave Ib data. It is proposed that wave la arises from stimulation of the axons proximal to the spiral ganglion while wave Ib is initiated at the peripheral d endritic processes. Implications for human cochlear implant research a re discussed.