Speaker
Description
Previous studies have demonstrated that vocal tract length (VTL) perception is strongly limited in most cochlear-implant users, with post-lingually deaf adults showing greater deficits than those implanted in early childhood. Three potential factors may underlie this difficulty, as well as this difference between groups: (1) the implant’s coding strategy, which may distort VTL cues as it is not designed to directly encode them; (2) current spread between the stimulating electrode and the activated neural terminals, which reduces the effective spectral resolution and may disproportionately affect VTL perception, as it relies on fine spectral contrasts; and (3) the listener’s ability to relearn associations between acoustic cues and speaker size, influenced by their age at implantation.To test the first two hypotheses, we recorded the electrical outputs of a Cochlear Nucleus 7 processor using the MP3000 strategy with a standard map, and resonified them using a noise vocoder simulating different degrees of current spread. By comparing normal-hearing participants’ VTL discrimination performance with the original and vocoded stimuli, we isolated the contributions of the coding strategy alone and the electrode-neuron interface. The results indicate that the implant coding strategy alone can account for the VTL discrimination thresholds observed in paediatric implant users. However, realistic levels of current spread substantially degraded performances, bringing them to the level of adult cochlear-implant users.