The perception of temporal patterns for electrical stimulation presented at one or two intracochlear sites

The question of how well the temporal structure in pulsatile electrical stimulation is perceived, and the nature of the information that may be conveyed by this temporal structure, is of importance to the further development of speech processing strategies for cochlear implants. The two experiments described here investigated the perception of temporal fine structure in amplitude modulated 1-kHz pulse trains, both when a single electrode position was used, and when the pulses alternated between two electrode positions. Five subjects with the Mini System 22 implant took part in these experiments. The amplitude modulations were constructed so that all dual-electrode stimuli had the same temporal pattern on each individual electrode but differed in the aggregate temporal pattern. A hypothesis was investigated that subjects perceive the aggregate temporal pattern rather than the pattern at each individual electrode place, only when the electrodes are less than a critical distance apart. The first of these two experiments used a four-interval forced-choice task to measure the ability of subjects to detect changes in the aggregate temporal pattern. At electrode distances greater than 3 to 4 mm, subjects could no longer perceive the aggregate pattern, confirming the hypothesis. The second experiment used a single-interval pitch estimation task to test the hypothesis that the perceptual differences in temporal patterns measured in the previous experiment were classified similarly to rate pitch differences by the subjects. The results confirmed this hypothesis, and showed that the pitch of the modulated stimuli could be predicted by the expected inter-pulse intervals in the excited neural population. (C) 1996 Acoustical Society of America.