Factors limiting vocal-tract length perception in cochlear implants

Recent studies have highlighted the difficulties that cochlear-implant (CI) users have with perceiving vocal-tract length (VTL) variations across speakers. VTL perception is not only a necessary component of voice gender identification, it is also crucial for the segregation and understanding of concurrent talkers. In this study we aim to identify which aspects of CI stimulation are responsible for poor VTL discrimination performances. We measured just-noticeable-differences (JNDs) for VTL and fundamental frequency (F0) using an adaptive procedure, in normal-hearing participants listening to various vocoders simulating different aspects of electrical stimulation. The vocoders differed in terms of number of channels, carriers and spread of excitation. These measures are also to be compared to similar measures in actual CI users. The JND for F0 increases, i.e. worsens, when the spectral resolution is degraded, but quickly reaches a plateau. In contrast, the JND for VTL steadily increases when spectral resolution decreases. The type of carrier does not affect VTL discrimination. Finally, the electrical spread of excitation seems to be the strongest factor limiting VTL perception in CIs.