Hemispheric asymmetry and functional lateralisation in speech-related processes: Behavioural, psychophysiological, and structural factors

A leftward functional asymmetry of speech was first identified by Broca (1861) and Wernicke (1874), followed by observations of a related role of structural asymmetry by Geschwind and Levitsky (1968). Such insights have since been profoundly developed and continue to expand with contemporary advancements in technology and theory. The present thesis is aimed towards improved understanding of behavioural, psychophysiological, and structural factors of hemispheric asymmetry and lateralisation regarding speech-related processing. Findings are drawn from a single multi-faceted study (n = 63) and reported across three sections. In section A background theory and experimental investigation of behavioural and psychophysiological measures is provided focusing on behavioural gap detection threshold tasks and dichotic listening tasks in relation to working-memory span, processing speed, and general intelligence. Psychophysiological processing of rapid temporal cues in noise and silence is also explored using an auditory-evoked potential mismatch negativity paradigm. In section B background theory and experimental investigation of structural measures is reported, focusing on morphometry of the planum temporale (PT), Heschl’s gyrus, and inferior frontal gyrus pars opercularis, as well as connectivity of the transcallosal PT-to-PT pathway and arcuate fasciculus. Section C explores the integration of these behavioural, psychophysiological, and structural measures in relevance to behavioural rapid temporal processing and language lateralisation using hierarchical regression. Overall, I provide theoretical and experimental evidence that leftward lateralisation of speech can be attributed to acoustic and/or linguistic factors, which can be better understood through the exploration of behavioural, psychophysiological, and structural substrates. The primary implication is that context is part and parcel to the parts and parcels of speech-related processing, and careful methodology is crucial to clarifying existing complex yet meaningful patterns of structural asymmetry and functional lateralisation in such processing.