Electrically evoked auditory brainstem response to stimulation of different sites in the cochlea

It is often assumed that stimulation of the eighth nerve with brief current pulses results in a highly synchronous discharge of action potentials from the ensemble of excited nerve fibers. Latency data derived from the electrically evoked auditory brainstem response (EABR) of the guinea pig and human indicate that there is a baso-apical gradient of response latencies in both species. Place-specific stimulation of different regions of the guinea pig cochlea was achieved by positioning a narrowly spaced bipolar electrode into each of the four spiral turns. Peak latencies of EABR waves I and III both revealed longer latencies with increasing distance from the base. Different amplitude growth functions were obtained from each turn. Place-specific stimulation in humans implanted with the Nucleus 22-electrode device was achieved by presenting current pulses across electrodes spaced 1.5 mm apart. A gradient of wave V latencies was observed in all subjects studied. The presence of a latency gradient would influence results obtained in efforts to correlate evoked potential amplitude measures with nerve survival. It may also provide information useful in characterizing the surviving fiber population in human implant subjects.