The effects of fine structure strategies on pitch and speech perception by cochlear implant users

Cochlear implant (CI) users show close to normal speech understanding performances in quiet listening conditions. However, skills requiring the better use of Temporal Fine Structure (TFS) cues and low frequency (LF) resolution such as speech understanding in noise, pitch perception and music perception are more challenging. Some CI listeners show better performances in the Disharmonic Intonation (DI) test, which is used for pitch perception evaluation. CI users use two different mechanisms to discriminate pitch: Rate Pitch (RP) which is the ability to discriminate pitch changes within the same intracochlear electrode and Place Pitch (PP) which is to discriminate pitch changes by the change of coding place to an adjacent electrode. Most CI speech processing strategies don't convey TFS information, but Med-El's FS coding strategies provide the listener with the TFS information. Previous studies reported that most CI listeners perform poorly in the DI test, whilst there are also performers with normal/close to normal results. This can be related to speech processing strategies of the CIs. Participants were 15 unilateral and 15 bilateral postlingual adult CI users fitted with FS coding strategies. Pitch and speech perception are evaluated with the DI test, words and sentences recognition in quiet and in noise for +10 and +5 signal noise ratio (SNR) and the Matrix test. Overall group showed an average median just noticeable difference (JND) of 33 Hz in the DI test. RP performers had significantly better DI scores than PP performers, 9 Hz vs 148 Hz, respectively. DI scores within the clinical normal zone are achieved by 33.3% and only by RP performers. Group comparisons for RP/PP performers showed statistically significant differences for word recognition scores in noise and for the Matrix test. With the FS coding strategies, RP ability's positive effect on both pitch and speech perception is observed in this study.