Concurrent sound segregation in electric and acoustic hearing

We investigated potential cues to sound segregation by cochlear implant (CI) and normal-hearing (NH) listeners. In each presentation interval of experiment 1a, CI listeners heard a mixture of four pulse trains applied concurrently to separate electrodes, preceded by a "probe" applied to a single electrode. In one of these two intervals, which the subject had to identify, the probe electrode was the same as a "target" electrode in the mixture. The pulse train on the target electrode had a higher level than the others in the mixture. Additionally, it could be presented either with a 200-ms onset delay, at a lower rate, or with an asynchrony produced by delaying each pulse by about 5 ms re those on the nontarget electrodes. Neither the rate difference nor the asynchrony aided performance over and above the level difference alone, but the onset delay produced a modest improvement. Experiment 1b showed that two subjects could perform the task using the onset delay alone, with no level difference. Experiment 2 used a method similar to that of experiment 1, but investigated the onset cue using NH listeners. In one condition, the mixture consisted of harmonics 5 to 40 of a 100-Hz fundamental, with the onset of either harmonics 13 to 17 or 26 to 30 delayed re the rest. Performance was modest in this condition, but could be improved markedly by using stimuli containing a spectral gap between the target and nontarget harmonics. The results suggest that (a) CI users are unlikely to use temporal pitch differences between adjacent channels to separate concurrent sounds, and that (b) they can use onset differences between channels, but the usefulness of this cue will be compromised by the spread of excitation along the nerve-fiber array. This deleterious effect of spread-of-excitation can also impair the use of onset cues by NH listeners.