Disentangling the influence of attention in the auditory efferent system during speech processing

The physiological mechanisms allowing humans to selectively attend to a single conversation in acoustically adverse situations, such as overlapping conversations or background noise, are poorly understood. In particular, the extent to which goal-directed, top-down processes of auditory attention can modulate the inner ear activity via the auditory efferent system remains unclear. This thesis investigates the relationship between degraded speech and the auditory efferent control of the cochlea. Young, normal-hearing, participants were assessed in a series of three experiments where speech intelligibility was manipulated during Active and Passive listening to: 1) noise vocoded speech; 2) speech in babble noise and 3) speech in speech-shaped noise. A lexical decision task was used in the “Active” listening condition where subjects were instructed to press a button each time they heard a non-word. In the “Passive” listening condition they were instructed to ignore all auditory stimuli and watch a movie. Click-evoked OAEs (CEOAEs) were obtained from the ear contralateral to the speech stimuli, allowing the measurement of cochlear-gain changes. A 64-channel EEG was synchronized with the CEOAE recording system, enabling the simultaneous measurement of cortical speech-onset event-related potentials (ERPs), click-evoked auditory brainstem responses (ABRs) and behavioural responses. Behavioural results showed that accuracy declined as the speech signals were degraded, while ERPs components were enhanced during the Active condition compared to the Passive condition. A decrease in cochlear gain (reduction in CEOAE amplitudes) with increasing task difficulty was observed for noise vocoded speech, but not for speech in babble or speech-shaped noise. Brainstem components showed decreased activity linked to CEOAE suppression. These findings contribute to an integrative view of auditory attention as an adaptive mechanism that recruits cochlear gain control via the auditory efferent system in a manner dependent upon the auditory scene encountered.