Rate modulation and speech perception with cochlear implants
Document TypePhD thesis
Access StatusOpen Access
© 2018 Dr. Tim Brochier
While cochlear implants (CI) have been successful in restoring a sense of hearing to people with severe to profound sensorineural hearing loss, there is still a wide variance in speech outcomes for CI users. Psychophysical experiments have shown that some of the variance can be explained by sensitivity to temporal modulations. If poor outcomes are partly caused by limited access to temporal speech cues, then improved transmission of those cues may provide perceptual benefits to CI users. The broad aim of the research presented in this thesis was to improve speech outcomes for CI users through better transmission of temporal speech information. The first study investigated the effect of stimulation rate and presentation level on speech perception and temporal modulation detection for CI users, in order to identify stimulation rates that provide a perceptual advantage in certain listening conditions. Speech perception (in quiet and in noise) and amplitude modulation detection thresholds (AMDTs) were measured at different presentation levels and stimulation rates. The reduction in speech perception due to added noise was significantly greater at higher rates. Speech perception was also significantly affected by presentation level, with scores increasing as the level increased. In contrast, AMDTs, as measured via acoustic input to the speech processor, exhibited no effect of rate or level. Correlations were found between AMDTs and speech perception for both the low-rate and high-rate processors. Therefore, while AMDTs explained inter-subject variability in speech perception, they did not explain within subject variability across stimulation rates and presentation levels. The second study evaluated the perception of amplitude modulation (AM) and rate modulation (RM), providing fundamental information about the temporal processing abilities of CI users and the perceptual mechanism underlying those abilities. The study was the first psychophysical evaluation of AM and RM detection in the same CI users. AM and RM detection thresholds were correlated and exhibited similar effects of modulation frequency and presentation level, indicating that AM and RM may be perceived by a common perceptual mechanism that involves central temporal integration. In the final study, a novel speech processing strategy called ARTmod (Amplitude and Rate Temporal modulation) was developed and tested. The ARTmod strategy encoded speech with simultaneous AM and RM, in order to observe whether RM can be used to enhance the perception of temporal envelopes of speech signals. In the experiment, the amount of AM was fixed and the amount of RM was varied, and speech perception was measured for the different RM amounts. A significant effect of RM amount was found, with speech scores improving as the RM amount was increased. The results indicated that RM can constructively combine with AM to enhance the perception of temporal speech envelopes and improve speech perception for CI users.
Keywordscochlear implants; psychophysics; signal processing; rate modulation; amplitude modulation; stimulation; stimulation rate; speech perception; hearing; temporal; temporal information
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