Mismatch negativity (MMN) objectively reflects timbre discrimination thresholds in normal-hearing listeners and cochlear implant users

Abstract

Timbre perception is poor in cochlear implant (CI) users. Most behavioral studies assess timbre discrimination with real instrumental sounds differing in more than one timbre dimension. This study focuses on objective measures of individual discrimination of single timbre dimensions. A psychoacoustic and electrophysiological case-control study of timbre discrimination was conducted in 15 CI users and 15 normal-hearing (NH) listeners. Just noticeable differences (JNDs) for temporal envelope (TE) modulation differences and spectral distribution (S) differences of complex tones were measured in an adaptive three-alternatives forced-choice procedure. From individual JNDs, individual four tone pairs were computed: temporal envelope modulation/spectral distribution timbre discrimination, and above/below JND. With these tone pairs, four randomly arranged oddball paradigms were presented to derive mismatch negativity (MMN), which reflects the ability to automatically detect acoustic changes. JNDs could be derived for all NH listeners. Discrimination of spectral distribution differences and particularly temporal envelope modulation differences were more difficult for CI users. MMN was present in all NH listeners and CI users when the timbre difference was above the individual JND, but not when below JND. Thus, the timbre difference between ‘deviant’ and ‘standard’ tones reflected the individual threshold for automatic detection of timbre changes. We conclude that the timbre perception skills of NH listeners and CI users in particular can be assessed timbre dimension specifically with psychoacoustic measurements and MMN recordings. MMN occurrence reflects the individual JND for temporal envelope modulation and spectral distribution differences and can be used as a clinical tool to monitor auditory-verbal therapy after CI surgery.