gms | German Medical Science

Fundamental frequency (F0), related to the glottal pulse rate, and vocal tract length (VTL), related to the size of the speaker, are important cues to differentiate between different speakers. While normal-hearing listeners are good at discriminating voices both in terms of F0 and VTL, cochlear implant (CI) users show much poorer discrimination of these two cues.

This study investigated the discrimination of F0 and VTL in simulated bimodal CI users, i.e., CI users with usable acoustic hearing in the opposite ear. Of special interest was the question how the combined access to electric and different amounts of acoustic hearing affected performance in F0 and VTL discrimination.

CI listening was simulated using a vocoder mimicking the details of signal processing with the advanced combinational encoder strategy. The electrical pulses were given to an auralization stage including physiological details of electric listening to produce audible output. Three different contralateral hearing impairments were simulated with audiometric thresholds representative for bimodal patients. Unprocessed speech was taken as control. F0 and VTL were manipulated independently using nonsense syllables.

The results confirmed poorer-than-normal F0 and VTL discrimination with simulated electric hearing, but as good F0 discrimination for the simulated hearing losses, as in normal hearing (just noticeable difference of 0.6 semitones). Combined access to electric and acoustic hearing resulted in performance that was as good as in acoustic hearing, except for the case when the electric side provided considerably better performance alone. This largely confirms a "better-ear-listening"-strategy for bimodal access of information for this psychoacoustic task.