Characterization of wave I of the electrically evoked auditory brainstem response in the guinea pig

This paper examines the first component of the electrically evoked auditory brainstem response (EABR) of the guinea pig. Short (20 microseconds/phase) and long (4000 microseconds/phase) duration rectangular current pulses were applied 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 integrative capacities of nerve fibers at relatively low current levels. In deafened control subjects, wave I of the EABR consistently demonstrated two positive peaks having different latency-level and adaptation recovery functions. The early component (wave Ia) showed less decrement in latency with increasing stimulus level and recovered faster in a forward-masking paradigm. Non-monotonicities in the adaptation recovery curves were also observed, more consistently in the wave Ib data. It is proposed that wave Ia arises from stimulation of the axons proximal to the spiral ganglion while wave Ib is initiated at the peripheral dendritic processes. Implications for human cochlear implant research are discussed.