Loudness perception with pulsatile electrical stimulation: the effect of interpulse intervals.

The effect of interpulse intervals on the perception of loudness of biphasic current pulse trains was investigated in eight adult cochlear implantees at three different stimulus levels encompassing the psychophysical dynamic range. Equal-loudness contours and thresholds were obtained for stimuli in which two biphasic pulses were presented in a fixed repetition period (4 and 20 ms), and also for single-pulse/period stimuli with rates varying between 20 and 750 Hz. All stimuli were of 500-ms duration, and the phase durations of each pulse were 100 microseconds or less. The results of these experiments were consistent with predictions of a three-stage model of loudness perception, consisting of a peripheral refractory effect function, a sliding central integration time window, and a central equal-loudness decision device. Application of the model to the data allowed the estimation of neural refractory characteristics of the subjects' remaining peripheral neural population. The average neural spike probability for a 50-Hz stimulus was predicted to be about 0.77, with an associated neural refractory time of 7.3 ms. These predictions did not vary systematically with level, implying that the effect of increasing current level on loudness results more from recruitment of neurons than from any increase in average spike probability.