Abstract:
Two experiments were completed with five subjects who have been implanted with the Mini System 22 cochlear implant. In the first experiment, 1000-Hz pulse trains were modulated so that there were two high- and eight low-current pulses in each 10-ms period. Five stimuli were constructed with differing time intervals between the two high-current pulses, and a further six stimuli were unmodulated pulse trains with rates between 100 and 250 Hz. All stimuli were loudness balanced and presented to the subjects in a single-interval pitch ranking task. The unmodulated stimuli showed a monotonic increase in pitch with rate. The pitch of the modulated stimuli was mostly determined by the longer of the two intervals between the high currents. When the high currents were very close, the modulation period was also important in determining the pitch. The second experiment used constant-current pulse trains with two pulses in every 20 ms. The effect on loudness of the interval between the two pulses was measured using loudness balancing. For interpulse intervals less than 4 ms the loudness increased as the interval decreased. This result implies that a facilitatory effect dominates over any refractory effect on loudness at short interpulse intervals.