Temporal response patterns of auditory nerve fibers to electrical stimulation in deafened squirrel monkeys

Electroneural response patterns of single auditory-nerve neurons were studied in aminoglycoside-deafened squirrel monkeys. The electrical stimuli were delivered through bipolar electrodes implanted in the scala tympani. The effects of pulse width, shape, frequency, and intensity on neural adaptation, phase locking, and spectral content were evaluated. Our results did not demonstrate the characteristic adaptation seen in auditory-nerve neurons in response to acoustic stimulation. Phase locking to a broad stimulus pulse (3200 microseconds/phase) was found to a very restricted phase angle of the electrical stimulus which was broader for square wave than for sine wave stimulation. The latency of the phase locked response varied inversely with stimulus intensity with greater variation for square wave stimulation than for sine wave stimulation. Auditory neurons were capable of a very high degree of phase locking to a 200-microseconds/phase pulse presented at 156 pulses per second (PPS) and to the first pulse of a 2500-Hz pulse burst. Phase locking was much poorer for the subsequent 200-microseconds/phase pulses comprising the 2500-Hz pulse burst where the neuron's response was determined by its relative recovery status. These findings can be explained by an interaction between the neuron's relative refractory status and its integration of charge over the stimulatory half cycle of the electrical stimulus. These two factors also appear to determine the interspike interval of the neural response. This interval decreased monotonically with increasing stimulus intensity. The neural spike rate (150-500 Hz) producing this interval increased with intensity and may be a source of periodicity information which the central auditory nervous system could interpret as pitch. This may account for the proportional relationship between pitch and stimulus intensity seen in some cochlear implant patients. Our study demonstrates that auditory-nerve neurons comply with basic neurophysiological principles in their responses to electrical stimulation. These principles should be incorporated into the cochlear prosthesis stimulator if more normal neural response patterns are desired in the cochlear prosthesis patient.