Ali, Fatima; (2021) The role of sound offsets in auditory temporal processing and perception. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Sound-offset responses are distinct to sound onsets in their underlying neural mechanisms, temporal processing pathways and roles in auditory perception following recent neurobiological studies. In this work, I investigate the role of sound offsets and the effect of reduced sensitivity to offsets on auditory perception in humans. The implications of a 'sound-offset deficit' for speech-in-noise perception are investigated, based on a mathematical model with biological significance and independent channels for onset and offset detection. Sound offsets are important in recognising, distinguishing and grouping sounds. They are also likely to play a role in perceiving consonants that lie in the troughs of amplitude fluctuations in speech. The offset influence on the discriminability of model outputs for 48 non-sense vowel-consonant-vowel (VCV) speech stimuli in varying levels of multi-talker babble noise (-12, -6, 0, 6, 12 dB SNR) was assessed, and led to predictions that correspond to known phonetic categories. This work therefore suggests that variability in the offset salience alone can explain the rank order of consonants most affected in noisy situations. A novel psychophysical test battery for offset sensitivity was devised and assessed, followed by a study to find an electrophysiological correlate. The findings suggest that individual differences in sound-offset sensitivity may be a factor contributing to inter-subject variation in speech-in-noise discrimination ability. The promising measures from these results can be used to test between-population differences in offset sensitivity, with more support for objective than psychophysical measures. In the electrophysiological study, offset responses in a duration discrimination paradigm were found to be modulated by attention compared to onset responses. Overall, this thesis shows for the first time that the onset-offset dichotomy in the auditory system, previously explored in physiological studies, is also evident in human studies for both simple and complex speech sounds.

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