Siciliano, C.M.; (2009) Perceptual adaptation to binaurally mismatched frequency-to-place maps: implications for bilateral stimulation with cochlear implants. Doctoral thesis , UCL (University College London).

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Abstract

Simulations of monaural cochlear implants in normal-hearing listeners have shown that the deleterious effects of upward spectral shifting on speech perception can be overcome with training. This thesis examines whether the same is true when simulating bilateral stimulation. Can listeners adapt to upward-shifted speech information presented together with contralateral unshifted information? In two series of experiments, perceptual adaptation was investigated for both speech in quiet with a large interaural spectral mismatch, and speech in noise with a moderate interaural spectral mismatch. For speech in quiet, a six-channel dichotic sine-carrier vocoder simulated the binaurally mismatched frequency-to-place map. Odd channels were presented to one ear with an upward shift equivalent to a 6 mm basilar membrane distance, while even channels were presented to the contralateral ear unshifted. For speech in noise, the number of vocoded channels was increased to ten, and the upward spectral shift applied to the odd channels was decreased to 3.8 mm. Prior to vocoding, speech was combined with speech-shaped noise at a signal-to-noise ratio of 10 dB (or 0 dB for vowels). Listeners were trained with Connected Discourse Tracking for 5.3 hours or 10 hours, with the binaurally mismatched processor and/or just the shifted monaural bands. Speech perception was tested with sentence and vowel tests before, during and after training. Listeners showed adaptation to the upwardly shifted speech, but for nearly every speech test, intelligibility with the binaurally mismatched processor matched intelligibility with just the unshifted bands. Consistent with earlier findings with monaural spectral shifts, then, this research suggests that listeners are capable of adapting to a spectral shift, even in the presence of background noise. However, they appear to be resistant to integrating mismatches in frequency-place maps between the ears. A theory of “better ear” listening is proposed to account for this resistance. The findings are consistent with psychophysical studies of binaural hearing, which show maximal ITD and ILD sensitivity for similar interaural cochlear places. In optimizing bilateral cochlear implants for speech perception, it may thus be important to keep frequency-to-place maps similar in the two ears.

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