Threshold and loudness functions for pulsatile stimulation of cochlear implants.

Thresholds and loudness estimates were measured for biphasic pulsatile electrical stimulation of the auditory nerve. Measures were collected as a function of the parameters: pulse duration, and pulse rate. The results indicate that the sensations of threshold and loudness are determined by a complex function of the stimulating current waveform. For stimuli with the same charge, maximum loudness is seen at the shortest pulse durations, and a secondary maximum is seen at pulse durations of 2-3 ms/phase. It is possible that the secondary peak in the loudness function and the slow growth of loudness just above threshold for long pulses are indications of dendrite survival near the electrode. If this interpretation is valid, these measures could lead to perceptual tests of peripheral nerve viability. In addition, a speech processor device could use these measures to equalize the loudness of stimuli with different pulse durations and pulse rates.