Authors: Somashekara HS, Nisha Krishnani, Jayashree S Bhat, Arivudai P Nambi
Cochlear implant is electronic device which is surgically implanted into the cochlea to provide the sense of hearing for hearing impaired individual who may not benefit from hearing aids. The current day’s cochlear implant codes only the temporal envelope cues. Speech perception simulation studies have shown that, adding frequency modulation cues to the amplitude modulation improves the speech recognition under adverse listening condition. A similar processing cannot be incorporated in the cochlear implant as it would result in spectral mismatch. The current study evaluated the effect of such mismatch on speech recognition scores. Method: The study involved the subjects with normal hearing who listened to the spectrally shifted HINT sentences having only amplitude modulation (AM) cues and amplitude modulation with frequency modulations (FM). Sine wave vocoders were used to synthesize the signals with only AM and AM with FM cues. For simulating the spectral mismatch, carrier frequencies were decided based on Greenwood’s map. Sine wave vocoded speech was presented with and without background noise. Results: Paired‘t’ test showed significant main effect of FM on spectrally shifted speech in quiet as well as noise. Mean scores significantly improved when the speech was processed with AM+FM spectral shift than AM spectral shift alone in both the conditions. Conclusion: The results of the current study indicated the importance of frequency modulation cues even in the spectrally mismatched conditions.
Keywords: Cochlear implant, Spectral shift, Amplitude modulations (AM), Frequency modulations (FM)