Functional responses from guinea pigs with cochlear implants. I. Electrophysiological and psychophysical measures

We examined electrophysiological and psychophysical measures of the electrically stimulated auditory system of guinea pigs implanted with chronic intracochlear electrodes. Guinea pigs were trained to detect low-level acoustic stimuli and then unilaterally deafened and implanted with one extracochlear and two intracochlear electrodes. Electrically evoked auditory brainstem responses (EABRs) and psychophysical detection thresholds were obtained from the same animals using pulsatile stimuli. Supplementary EABR data were obtained from additional, untrained, animals. Thresholds were obtained as a function of stimulus phase duration and monopolar and longitudinal-bipolar electrode configurations. The slopes of the EABR and psychophysical functions for bipolar stimulation, averaged across subjects within 1 month after implantation, were -5.25 and -6.18 dB per doubling of pulse duration, respectively. These slopes were obtained with pulse durations ranging from 20 to 400 microseconds/phase; slope was reduced at longer pulse durations. Strength-duration slope also varied as a function of electrode configuration: monopolar stimulation produced steeper functions than did bipolar stimulation. Differences between EABR and psychophysical strength-duration measures suggest the existence of central mechanisms of stimulus integration in addition to that occurring at the level of the auditory nerve. Differences observed with variation of stimulus parameters (e.g., monopolar vs. bipolar stimulation modes) suggest that the specific mode of intracochlear electrical stimulation can influence stimulus integration. Such observations may be useful in the design of prosthetic devices and furthering our understanding of electrical excitation of the auditory system.