The perceptual effects of current pulse duration in electrical stimulation of the auditory nerve

In cochlear implants employing pulsatile stimulation, loudness is controlled by current amplitude and/or pulse duration. Five experiments were conducted with cochlear implantees to investigate the hypothesis that perceptual effects other than loudness result from changes in pulse duration for durations from 50 to 266 microseconds. In experiment 1, five subjects' ability to discriminate equally loud pulse trains employing differing pulse durations was measured at four electrode positions. In 11 of the 20 cases, subjects could significantly discriminate these stimuli. In experiments 2 and 3, discrimination was measured of dual-electrode stimuli which differed in overall temporal pattern but had an equal temporal pattern on each of the individual electrodes (separated by 0 to 9 mm). Discrimination was compared for stimuli employing short or long pulse durations and, in experiment 3, employing different pulse durations on each electrode. When the pulse duration was longer, six out of seven subjects could either combine temporal information across electrode positions at wider electrode separations (experiment 2) or had better discrimination at the same electrode separation (experiment 3). This result was consistent with the hypothesis that longer pulse durations result in a greater spread of excitation than equally loud stimuli using shorter pulse durations. In experiment 4, pulse rate discrimination was compared for stimuli with differing pulse durations, and in four out of five subjects, there was no effect of pulse duration. Finally, the dB change in current per doubling of pulse duration for threshold and equally loud stimuli was calculated for nine subjects (52 electrodes). Values ranged from -5.9 to -2.0 dB/doubling, and were significantly correlated with the absolute intensity of the stimulus. This result was hypothesized to be due to a relationship between the neural charging characteristics and the distance of the excited neural elements from the electrode.