Pitch matching of amplitude-modulated current pulse trains by cochlear implantees: the effect of modulation depth

An experiment was conducted with four cochlear implantees, which investigated the pitch evoked by amplitude-modulating current pulse trains delivered to a single cochlear location. These stimuli produce a pitch percept which may be similar to that of acoustic stimuli such as modulated noise for modulation frequencies in the range 80-300 Hz, approximately. The experiments investigated the effect of modulation depth on the way pitch was matched to that of unmodulated pulse trains. The method of constant stimuli was used, in which the variable parameter was the rate of the unmodulated stimulus. The modulated stimuli comprised pulses having one of two possible current values, with the higher value occurring once in every modulation period. The results showed that the matched rate fell exponentially from a value close to the carrier rate towards a value equal to the modulation frequency as the modulation depth increased. The results were compared to the predictions of a simple model in which the matched rate corresponded to a weighted average of carrier and modulation frequencies, with the weightings proportional to the number of neurons firing at each of these frequencies. The results agreed with the predictions of the model reasonably well, except in cases where the carrier rate was 700 Hz or higher, and for one subject at the highest intensity level.