The specialised regional functionality of the mature human cortex partly emerges
through experience-dependent specialisation during early development. Our existing understanding
of functional specialisation in the infant brain is based on evidence from unitary imaging modalities
and has thus focused on isolated estimates of spatial or temporal selectivity of neural or haemodynamic activation, giving an incomplete picture. We speculate that functional specialisation will
be underpinned by better coordinated haemodynamic and metabolic changes in a broadly orchestrated physiological response. To enable researchers to track this process through development, we
develop new tools that allow the simultaneous measurement of coordinated neural activity (EEG),
metabolic rate, and oxygenated blood supply (broadband near-infrared spectroscopy) in the awake
infant. In 4- to 7-month-old infants, we use these new tools to show that social processing is accompanied by spatially and temporally specific increases in coupled activation in the temporal-parietal
junction, a core hub region of the adult social brain. During non-social processing, coupled activation decreased in the same region, indicating specificity to social processing. Coupling was strongest
with high-frequency brain activity (beta and gamma), consistent with the greater energetic requirements and more localised action of high-frequency brain activity. The development of simultaneous
multimodal neural measures will enable future researchers to open new vistas in understanding functional specialisation of the brain
