Electrical stimulation of the auditory nerve. I. Correlation of physiological responses with cochlear status

The purpose of the present study was to evaluate evoked potential and single fibre responses to biphasic current pulses in animals with varying degrees of cochlear pathology, and to correlate any differences in the physiological response with status of the auditory nerve. Six cats, whose cochleae ranged from normal to a severe neural loss (< 5% spiral ganglion survival), were used. Morphology of the electrically evoked auditory brainstem response (EABR) was similar across all animals, although electrophonic responses were only observed from the normal animal. In animals with extensive neural pathology, EABR thresholds were elevated and response amplitudes throughout the dynamic range were moderately reduced. Analysis of single VIIIth nerve fibre responses were based on 207 neurons. Spontaneous discharge rates among fibres depended on hearing status, with the majority of fibres recorded from deafened animals exhibiting little or no spontaneous activity. Electrical stimulation produced a monotonic increase in discharge rate, and a systematic reduction in response latency and temporal jitter as a function of stimulus intensity for all fibres examined. Short-duration current pulses elicited a highly synchronous response (latency < 0.7 ms), with a less well synchronized response sometimes present (0.7-1.1 ms). There were, however, a number of significant differences between responses from normal and deafened cochleae. Electrophonic activity was only present in recordings from the normal animal, while mean threshold, dynamic range and latency of the direct electrical response varied with cochlear pathology. Differences in the ability of fibres to follow high stimulation rates were also observed; while neurons from the normal cochlea were capable of 100% entrainment at high rates (600-800 pulses per second (pps)), fibres recorded from deafened animals were often not capable of such entrainment at rates above 400 pps. Finally, a number of fibres in deafened animals showed evidence of 'bursting', in which responses rapidly alternated between high entrainment and periods of complete inactivity. This bursting pattern was presumably associated with degenerating auditory nerve fibres, since it was not recorded from the normal animal. The present study has shown that the pathological response of the cochlea following a sensorineural hearing loss can lead to a number of significant changes in the patterns of neural activity evoked via electrical stimulation. Knowledge of the extent of these changes have important implications for the clinical application of cochlear implants.