Speech recognition and temporal amplitude modulation processing by Mandarin-speaking cochlear implant users

Fundamental frequency (F0) information is important to Chinese tone and speech recognition. Cochlear implant (CI) speech processors typically provide limited F0 information via temporal envelopes delivered to stimulating electrodes. Previous studies have shown that English-speaking CI users' speech performance is correlated with amplitude modulation detection thresholds (AMDTs). The present study investigated whether Chinese-speaking CI users' speech performance (especially tone recognition) is correlated with temporal processing capabilities.

Chinese tone, vowel, consonant, and sentence recognition were measured in 10 native Mandarin-speaking CI users via clinically assigned speech processors. AMDTs were measured in the same subjects for 20- and 100-Hz amplitude modulated (AM) stimuli presented to a middle electrode at five stimulation levels that spanned the dynamic range. To further investigate the CI users' sensitivity to temporal envelope cues, AM frequency discrimination thresholds (AMFDTs) were measured for two standard AM frequencies (50 and 100 Hz), presented to the same middle electrode at 30% and 70% dynamic range with a fixed modulation depth (50%).

Results showed that AMDTs significantly improved with increasing stimulation level and that individual subjects exhibited markedly different AMDT functions. AMFDTs also improved with increasing stimulation level and were better with the 100-Hz standard AM frequency than with the 50-Hz standard AM frequency. Statistical analyses revealed that both mean AMDTs (averaged for 20- or 100-Hz AM across all stimulation levels) and mean AMFDTs (averaged for the 50-Hz standard AM frequency across both stimulation levels) were significantly correlated with tone, consonant, and sentence recognition scores, but not with vowel recognition scores. Mean AMDTs were also significantly correlated with mean AMFDTs.

These preliminary results, obtained from a limited number of subjects, demonstrate the importance of temporal processing to CI speech recognition. The results further suggest that CI users' Chinese tone and speech recognition may be improved by enhancing temporal envelope cues delivered by speech processing algorithms.