Functional specialisation across the first five years of life: A longitudinal characterisation of social perception with fNIRS.

Research in typically developing infants has shown robust and consistent brain activation to social versus non-social visual and auditory stimuli in a network of brain regions, including the inferior frontal, anterior temporal, and posterior superior temporal cortex. However, large-scale, longitudinal neuroimaging studies across early childhood, particularly in low- and middle-income countries, are rare, yet important, given that they offer a powerful means of capturing within-person changes in neurodevelopment and identifying targets for intervention and support. Here, we investigated brain responses to social perception longitudinally across the first 5 years of life of participants in The Gambia with functional near-infrared spectroscopy (fNIRS) within the Brain Imaging for Global Health (BRIGHT) Project. We found that social visual stimuli elicited a selective right posterior temporal response across all six age points studied here (5, 8, 12, 18, 24 months and 3-5 years of age), with concurrent selective left responses at four of the six time points (5, 8, 18 months and 3-5 years of age). Inferior frontal regions showed brain activation at multiple ages, with the youngest (5 months) and oldest (24 months to 3-5 years) time points evidencing more widespread frontal social visual responses. Conversely, auditory social stimuli elicited a significantly stronger and more prolonged response than auditory non-social stimuli across all six age points from 5 months to 3-5 years across a range of frontal and temporal areas. Finally, from individual infant trajectories of brain activation, we identified different profiles of age-dependent specialization to auditory social stimuli, with some specializing earlier than others. Future work could take advantage of such profiles to investigate the effects of adversity and resilience factors on neurodevelopment.