Effective cortico-cortical connectivity in auditory and speech processing

• Main Author: Chiu, Faith
• Published: University College London (University of London) 2018
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.756372

The thesis aimed to update the traditional understanding of the speech chain with recent proposals on communicative behaviour from theoretical and computational neuroscience. For a generative brain that engages in active (Bayesian) inference, speech perception itself is considered as a form of predictive processing. Experiments presented in this thesis were designed to answer the questions of ‘what’, ‘how’, and ‘where’: 1. What constitutes an auditory prediction error, and is it selective to a specific sound type? 2. How is surprise minimisation implemented in hierarchical cortical networks for auditory and speech perception? 3. Where – at what level in cognition and linguistic knowledge – could predictions for speech perception come from? Study 1 answered the first question – of ‘what’ – by collecting passive Mismatch Negativity responses to speech and non-speech sounds. This was recorded as subjects ignored the auditory stimuli. It was hypothesised that if speech-specific auditory prediction errors existed, certain aspects of MMN – a change detection response – would be selective to sound type and to speech category. Source-analyses of the same EEG data in Study 1 provided clues to the question of ‘how’. To address ‘where’, Study 2 considered attentional modulations to the effect of categorical processing recorded in passive brain responses, and tested for a prelexical locus of speech-specific prediction and prior knowledge. Auditory-evoked brain responses were recorded in both Ignore and Attend conditions to acoustically identical stimuli from three subject groups of varying levels of familiarity to a certain speech category. Stimuli manipulation in an accompanying behavioural discrimination task allowed for the monitoring of top-down lexical influences. Findings from this project will provide a way to reframe existing theoretical debates in speech processing on topics such as speech-specific auditory processing (against non-speech sounds), categorical perception, and pre-lexical abstraction within spoken word recognition.