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 across the first 2000 days of life (conception to five years of age) with functional near-infrared spectroscopy (fNIRS) longitudinally with participants in The Gambia within the Brain Imaging for Global Health (BRIGHT) Project. We found that social visual stimuli elicited a specialised 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 specialised left responses at four of the six time points (5, 8, 18 months and 3-5 years of age). Inferior frontal regions showed age-related changes in brain activation, with the youngest (5 months) and oldest (24 months to 3-5 years) time points evidencing more widespread frontal social visual responses. In contrast, social auditory stimuli elicited a significantly stronger and more prolonged response than non-social auditory stimuli across all six age points from 5 months to 3-5 years across a range of frontal and temporal areas. Furthermore, from individual infant trajectories of brain activation, we identified different profiles of age-dependent specialisation to social auditory stimuli, with some specialising earlier than others. Finally, in studying the hemodynamic responses, we showed that their different characteristics were influenced by age and stimulus type, highlighting the limitations of comparing fNIRS signal amplitudes across ages, and the importance of incorporating the timing of the fNIRS response to characterise brain activation more comprehensively.