Campos Espinoza, Ana; (2023) Cortical dynamics and speech perception during early childhood. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Early childhood is a period of important linguistic development; however, measuring the neural correlates of speech perception at this age is challenging. So far, there is no clarity about whether, between the ages of 3-6 years old, brain dynamics associated with speech perception (i) are different from those observed in adults, (ii) show atypical patterns in children with language difficulties, or (iii) are modulated by language skills. This thesis aimed to examine cortical dynamics related to speech perception in children from 4.7 to 5.7 years old with typical language development (TLD, n=12) and developmental language disorder (DLD, n=17) and compare them to that observed in neurotypical adults (n=20). I used electroencephalography (EEG) to investigate whether young children exhibit top-down modulations on cortical speech processing and whether TLD and DLD children differed in different measures of cortical oscillatory synchrony. Considering previous findings, we expected different EEG measures to reflect more efficient speech perception for participants with better language skills and stimuli with more informative linguistic content. We also expected an association between children's EEG responses and phonological and speech perception task performance. Experiment 1 validated a task-free, ERP multifeature paradigm in adults to determine whether contrasts between speech stimuli elicited consistent MMN and LDN responses, compared to a non-speech control condition. In Experiment 2, we used the multifeature experiment in the speech condition to determine whether the MMN and LDN were present in children and if they resembled the adult's responses. Here, we complemented ERPs analysis with time-frequency measures, evaluating the synchrony of the neural responses independently from their amplitude (power). We also assessed the children's phonological awareness skills and investigated their association with EEG measures. Experiment 3 examined the relationship between language skills (TLD/DLD children and adults) and the power and lateralisation of endogenous oscillations. We also related EEG resting-state measures to children's performance in speech-in-noise and filtered speech perception tests. Finally, in Experiment 4, we explored more ecological speech perception measures, recording EEG responses to unattended continuous speech. We then compared cortical entrainment to different speech features between children and adults. Our results demonstrated age-related differences in EEG responses between adults and children, confirming that increases in neural synchrony are relevant for cortical development. A central finding is that adults also exhibited a significant effect of the linguistic content, with greater cortical synchrony for lexical stimuli, but this effect was absent in children. This indicates that adult-like top-down language modulations on speech perception could not be already in place during early childhood. Notably, there was no evidence of differences between TLD and DLD children on most EEG measures, despite TLD children performing significantly better than DLD in phonological awareness and filtered speech perception tasks. These results indicate that cortical speech processing at the acoustic and lexical level may be preserved in young children with DLD. Although it is essential to consider methodological limitations when interpreting these findings, they were consistent across experiments. This thesis contributes to understanding the cortical dynamics of speech perception in young children. Our findings are relevant to inform oscillatory models of typical and atypical language development and future objective clinical measures of speech processing.

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