Pitch percepts associated with amplitude-modulated current pulse trains in cochlear implantees

The percepts elicited by electrical stimulation of auditory neurons by trains of amplitude-modulated current pulses were studied in a group of six cochlear implant users. Modulation frequencies of 100, 150, and 200 Hz were studied, with a range of carrier rates up to 1200 Hz. It was found that all but one subject could consistently rank 150- and 200-Hz modulated stimuli by modulation frequency when the carrier rate was more than 800 Hz, but for lower carrier rates the ranking was greatly affected by the harmonic relationship between carrier and modulation frequency. Pitch matching experiments showed that the subjects generally considered the modulated stimuli to be equal in pitch to unmodulated stimuli with rates the same as, or somewhat higher than, the modulation frequency. The results showed that the 'pitch' of pulsatile electrical stimulation resulting from periodicities in the time structure of the electrical stimulus has similarities to the 'pitch' observed for temporal patterns in acoustic stimulation such as amplitude-modulated noise. There were some differences, however, which may be attributable at least in part to the physiological response differences for electric and acoustic stimulation.