Pitch ranking with nonsimultaneous dual-electrode electrical stimulation of the cochlea

It has already been established that simultaneous activation of two intracochlear electrodes can evoke a pitch percept which is intermediate to that of either electrode when activated by itself. In the present study, this result has been extended to nonsimultaneous activation of nearby electrodes. Pitch perception was investigated for electric stimuli presented on one or two intracochlear electrode pairs. All stimuli were pulse trains of period 4 ms. In the dual-electrode stimuli, each period contained two biphasic pulses, separated by 0.4 ms, with one pulse for each electrode pair. These stimuli were compared with loudness-balanced single-electrode stimuli, having one pulse per period, generated on the same electrode pairs. Their pitches were ranked in a two-alternative forced-choice procedure by five experienced users of the 22-electrode implant manufactured by Cochlear Pty Limited. The studies showed that the pitch of a dual-electrode stimulus was intermediate to, and moved monotonically between, those of the component electrode pairs as the relative currents were altered in an orderly fashion. Intermediate pitches were achieved in all subjects at a range of cochlear positions, for electrode separations generally up to 3 mm. In half the cases a significantly different intermediate pitch could be created between adjacent electrodes. Further studies are necessary to establish whether intermediate pitches can be obtained at larger separations, as for simultaneous stimulation, and how they are affected by other factors such as the time between pulses or the spatial extent of the stimulation.