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The perception of temporal modulations of pulsatile electric stimuli was measured in seven cochlear implant patients using the Cochlear Pty. Limited prosthesis. Four patients were postlingually deafened adults and three patients were young adults who were deafened very early in life. The first study measured detection thresholds for modulated pulse duration for a series of modulation frequencies and pulse rates. The shape of the detection thresholds as a function of modulation frequency, the temporal modulation transfer function, often resembled a low-pass filter with a 50-100-Hz cut-off frequency. Thresholds did not markedly vary across the different pulse rates for most patients. Thresholds were less than 10%-20% of the range of usable hearing for most patients. The second study compared detection thresholds for modulated pulse durations around different reference pulse durations: 50, 100, and 300 microseconds. Detection thresholds were generally proportional to the different reference pulse durations. The third study measured difference limens for the discrimination of modulation depth. The difference limens were similar to the detection thresholds for the same reference pulse duration and pulse rate. The three patients deafened very early in life showed more within-group variation in performance, and their overall levels of performance were poorer than those of the postlingually deafened adults.
Busby et al. (Thu,) studied this question.