Investigating the neural mechanisms underlying auditory and audio-visual perception in younger and older adults

This thesis aimed to address questions in two distinct areas of research in ageing and cognitive neuroscience. Firstly, given that the pre-stimulus state of cortical oscillations had been shown to predict behavioural and neural responses, we addressed the question of whether pre-stimulus oscillatory mechanisms change or remain consistent in the ageing brain. Secondly, previous research had shown that Audio-visual (AV) speech influences the amplitude and latency of evoked activity. Our research addressed the questions of whether/how AV enhancement and visual predictability of AV speech is represented in evoked activity in noisy listening conditions, and whether such Electroencephalographic (EEG) signatures remain stable with age. In Chapter 3 we investigated the consistency of how pre-stimulus activity influences auditory frequency discrimination performance in young and older participants. In both groups the power of pre-stimulus activity influenced the encoding of sensory evidence reflected by early evoked components, while the phase influenced choice formation in later-activated EEG components. Importantly, for the early EEG components we did not find evidence for a systematic difference in the time scales of the perceptually relevant pre-stimulus activity. In the later-activated EEG component we found a trend for perceptually relevant rhythmic activity to arise from slower frequencies in the ageing brain. At the same time our data replicate previous findings of a significant age-related slowing of Auditory Evoked Potential (AEP) latency, modulations of AEP amplitudes, and a flattening of the spectral profile of EEG activity. In Chapter 4, we investigated the consistency of behaviour and evoked activity underlying AV speech integration in a speech-in-noise discrimination task in younger and older adults. Behaviourally, younger and older adults performed comparably. Performance was greater for Audio-visually informative (AVinf) speech compared to Auditory-only informative (AOinf) speech across groups and noise levels, and was poorer at low noise levels. AV enhancement was greater in high noise levels, across all participants, and older adults derived greater AV enhancement compared to younger adults (an effect that was consistent across noise levels). In terms of visual predictability, we found that word discrimination performance was greater for target words with non-labial initial phonemes (assumed least visually predictive), compared to labial initial phonemes (assumed most visually predictive). Furthermore, we found that AV enhancement was greater for labial initial phonemes, compared to non-labial initial phonemes, and this was consistent across age groups.Neurally, we found that AV enhancement is represented by a centro-parietal P3-like activity in older adults and an N4-like fronto-central activity in younger adults, but found that this activity did not correlate with behavioural AV enhancement. Our results point to distinct patterns of late evoked activity underlying AV enhancement between younger and older adults, possibly representing distinct cognitive (memory) strategies in predicting upcoming target stimuli. At the same time our data replicate previous findings of a significant age-related slowing of AEP latency, modulations of AEP amplitudes, and a flattening of the spectral profile of EEG activity. In Chapter 5 we investigated the consistency of evoked activity underlying the visual predictability of AV speech. We found that visual predictability was reflected by late fronto-central negativity in older adults, but not in younger adults. However, we did not find evidence of an interaction between visual predictability and AV enhancement in terms of evoked activity, raising further questions about how visual predictability of speech is represented the brain’s electrophysiology. Our results point to distinct patterns of late evoked activity underlying visual predictability of visual speech, again possibly reflecting differential strategies in predictive coding. In summary, the results of this thesis demonstrate that pre-stimulus mechanisms in auditory pitch perception remain consistent in the younger and older adult brain, while spectral dynamics change with age. Our results also replicate previous work demonstrating age-related delays in peak latency, and changes in peak amplitude, of early auditory evoked activity. And lastly, we demonstrate that differences in the EEG signatures of AV enhancement between younger and older adults emerge in late evoked activity, and that visual predictability of speech is represented in late evoked activity only in older adults