Discrimination of Voice Pitch and Vocal-Tract Length in Cochlear Implant Users

When listening to two competing speakers, normal-hearing (NH) listeners can take advantage of voice differences between the speakers. Users of cochlear implants (CIs) have difficulty in perceiving speech on speech. Previous literature has indicated sensitivity to voice pitch (related to fundamental frequency, F0) to be poor among implant users, while sensitivity to vocal-tract length (VTL; related to the height of the speaker and formant frequencies), the other principal voice characteristic, has not been directly investigated in CIs. A few recent studies evaluated F0 and VTL perception indirectly, through voice gender categorization, which relies on perception of both voice cues. These studies revealed that, contrary to prior literature, CI users seem to rely exclusively on F0 while not utilizing VTL to perform this task. The objective of the present study was to directly and systematically assess raw sensitivity to F0 and VTL differences in CI users to define the extent of the deficit in voice perception.

The just-noticeable differences (JNDs) for F0 and VTL were measured in 11 CI listeners using triplets of consonant-vowel syllables in an adaptive three-alternative forced choice method.

The results showed that while NH listeners had average JNDs of 1.95 and 1.73 semitones (st) for F0 and VTL, respectively, CI listeners showed JNDs of 9.19 and 7.19 st. These JNDs correspond to differences of 70% in F0 and 52% in VTL. For comparison to the natural range of voices in the population, the F0 JND in CIs remains smaller than the typical male-female F0 difference. However, the average VTL JND in CIs is about twice as large as the typical male-female VTL difference.

These findings, thus, directly confirm that CI listeners do not seem to have sufficient access to VTL cues, likely as a result of limited spectral resolution, and, hence, that CI listeners' voice perception deficit goes beyond poor perception of F0. These results provide a potential common explanation not only for a number of deficits observed in CI listeners, such as voice identification and gender categorization, but also for competing speech perception.