Neural and Behavioural Correlates of Auditory Discrimination and Language Processing in Infants and Children

Inconsistency of accounts on auditory and speech processing in early development tends to be attributed to maturational factors in children. However, variability in stimulus and design features used across studies is likely to contribute to this lack of consensus. The mismatch response (MMR), an electrophysiological measure elicited to auditory stimulation, can provide a neural index of speech processing and language development. The current study investigated the design and paradigm features which influence MMR in 5- to 11-month-old infants and 4-to 6- year-old children. Across experiments, trial duration; deviance type; stimulus type and number of simultaneous streams (phonemes and tone pairs) were systematically manipulated. Taking such an approach provided means to gain an understanding of those factors that influence auditory and speech development in infants and young children and address questions of interest to researchers working within the field of neurodevelopmental research. A secondary objective of this thesis was to delineate the relationship between neural and behavioural correlates of language development in the same participants at the time of testing and in a subset of the infants at 2 years.
The results indicated that the ‘oddball’ auditory deviance elicited the largest MMR, as did tone pairs compared to phonemes. The deviance modulation revealed larger MMR deflections to the oddball than roving or sequential change between deviant and standard stimuli. Furthermore, shorter trial duration produced no difference in MMR intensity, a finding that suggests a slightly shorter stimulus presentation time can be utilised at minimal processing cost. Stimulus modulation confirmed that tone pairs consistently elicited larger MMR than phonemes. Finally, the number of concurrent stimulation streams did not influence MMR to tone pairs but revealed dissociation in processing phonemes between infants and children. Infants discriminated phonetic deviance at the sensory level whereas children exhibited later MMR response, which was associated with attentional focus.
Results of the analyses related to the secondary question revealed a positive association that linked MMR to acoustic contrast and language proficiency in infants. A similar pattern was observed between MMR in infancy and language at 2 years of age, but an opposite trend was found in children. In this age group, the ability to ignore background sounds was linked to more advanced language. Nonetheless, a relationship between MMR to acoustic and language proficiency in children was identified with the potential for auditory processing to predict language outcomes. Recommendations deriving from this work may be of considerable interest to neuroscientists and neuropsychologists who specialise in enabling children achieving their linguistic aptitude. They may also inform the development of clinical interventions targeted at children with language difficulties.