Preparatory and selective attention during multi-talker listening in normal and impaired hearing

One of the great challenges of hearing research is to work out how listeners can perceive what one talker is saying when other talkers are speaking at the same time. Faced with this requirement for ‘multi-talker listening’, normally-hearing listeners achieve improved speech intelligibility when they know characteristics of an upcoming talker before he or she begins to speak. One aim was to investigate the time course of this improvement in intelligibility and the brain activity that accompanies it. A task was devised in which participants had to report key words spoken by a ‘target’ talker when one or two other talkers spoke simultaneously. Before the talkers began to speak, a visual cue indicated the location (left/right) or gender (male/female) of the target talker. The accuracy and latency of reporting key words progressively improved when participants had longer to prepare for the location or gender of the target talker. Preparatory brain activity, measured with electro-encephalography, began with a short latency (< 100 ms) after the reveal of the visual cue and was sustained until the talkers began to speak. Hearing-impaired listeners, both children and adults, typically show poorer speech intelligibility during multi-talker listening than normally-hearing listeners. One advantage of the experimental design was that brain activity during preparatory attention (before the onset of acoustical stimuli) could be compared between normally-hearing and hearing-impaired listeners and atypical attention identified, without confounding differences in transduction at the auditory periphery. This thesis demonstrates atypical preparatory EEG activity in children, aged 7-16 years, with bilateral moderate cochlear hearing loss, which provides evidence for atypical preparatory attention. Therefore, atypical preparatory attention might be one factor that contributes to poorer speech intelligibility in noisy environments. An implication is that acoustic hearing aids may not have the potential alone to restore normal processing of acoustical stimuli in hearing-impaired listeners.