Young children transition in and out of more and less synchronous states with their caregivers across physiology, behavior, and brain activity, but what do these synchronous periods mean? One body of caregiver-child two-brain studies using fNIRS finds that individual, familial, and moment-to-moment behavioral and contextual factors are correlated with caregiver-child neural synchrony, while a separate body of literature finds that neural synchrony is associated with positive child outcomes. Taken together, it is tempting to conclude that caregiver-child neural synchrony may act as a key developmental mechanism linking children’s experiences to their healthy development, but many questions remain. In this brief review, we synthesize key findings and open questions from two-brain caregiver-child studies using functional near-infrared spectroscopy (fNIRS), which is uniquely well-suited for use with caregivers and children, but also laden with unique constraints. Throughout, we highlight key questions and best practice recommendations to optimize two-brain fNIRS to examine these critical developmental mechanisms from assumptions guiding study design to final analysis. We particularly emphasize the need to consider immediate and global stressors as context for interpretation of neural synchrony findings, and the need for full inclusion of socioeconomically and racially diverse families in future studies.
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