Studies have provided evidence that infants exhibit distinct neural responses to social stimuli (e.g., faces) compared to non-social stimuli (e.g., toys) shortly after birth. This specialization for processing social input further evolves over months and years, leading to a sophisticated network of brain regions that emerges to handle social information. However, it is essential to consider the context in which brain development unfolds to unravel the underlying mechanisms and interpret the significance of these brain functions early in life. As a cornerstone of early infant social experiences, caregiver-infant interactions may influence the cortical specialisation to social stimuli while also reflecting developing brains' behavioural and cognitive capacities. This thesis analysed data from three longitudinal projects to investigate the interplay between cortical specialization to social stimuli and its relationships with behaviours sampled from infant social interactions.

Chapter 1 summarised the field regarding the development of social brain functions in infants and various factors that relate to their development, focusing on infant and caregiver social behaviours. Chapter 2 overviewed functional near-infrared spectroscopy (fNIRS) used to study cortical specialization in infants and discussed the method of behavioural coding.

In Chapter 3, the behavioural coding scheme to operationalise caregiver-infant behaviours was developed and infant social behaviours were studied between 1 and 5 months of age. Results revealed a significant concurrent and predictive relationship between infant and caregiver behaviours. However, mutual gaze at 1 month emerged as the strongest predictor of infant social behaviours at 5 months leading to the interim conclusion that behaviours measured dyadically in early interactions were better at predicting later infant behaviours.

Using fNIRS, Chapter 4 investigated the concurrent associations between 5-month-old infants' neural responses to social visual and auditory stimuli and caregiver-infant behaviours in interactions. Findings revealed that infant and dyadic behaviours, such as mutual gaze and multimodal engagement, predicted social auditory responses while caregiver behaviours predicted visual responses. However, the direction of associations was the opposite of what was predicted suggesting that enriched interactions were associated with decreased specialisation to social stimuli at this age.

Chapter 5 examined cortical specialization in 4-to-7-month-old infants at elevated likelihood (EL) for developing autism spectrum disorder (ASD) and/or attention-deficit hyperactivity disorder (ADHD), aiming to understand the neurobiological factors involved in this interplay. Results indicated that infants at EL displayed reduced specialization to vocal and visual stimuli compared to the typically developing (TL) group. Furthermore, neural activity in these areas did not show significant associations with caregiver- infant behaviours in the EL group, unlike the significant associations observed in the TL group. Based on these findings, it was concluded that EL infants exhibited differential susceptibility to the effects of social interactions on social brain functions.

The study in Chapter 6 employed High-Density Diffuse Optical Tomography (HD-DOT) utilised in the home setting to investigate cortical specialization to social stimuli within the first month of life. Preliminary findings suggested changes in how infants process social auditory stimuli within the first month of life, which appeared to be linked to their social responsiveness supporting the presence of brain-behaviour associations early in life. The study also highlighted the feasibility and challenges of employing visual paradigms with newborns. Although selective visual responses were observed in a small group of one- month-old infants, they were not analysed statistically.

Chapter 7 provided a comprehensive discussion of all the findings, including an overview of the limitations and suggestions for future directions. Taken together, the results of this thesis shed light on cortical specialization to social stimuli, caregiver-infant interactions, and the emergence of social capacities in infants in the first five months of life.