Abstract
Young children transition in and out of synchronous states with their caregivers across physiology, behavior, and brain activity, but what do these synchronous periods mean? One body of two-brain studies using functional near-infrared spectroscopy (fNIRS) finds that individual, family, and moment-to-moment behavioral and contextual factors are associated with caregiver-child neural synchrony, while another 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 foundational developmental mechanism linking children's experiences to their healthy development, but many questions remain. In this review, we synthesize recent findings and open questions from caregiver-child studies using fNIRS, which is uniquely well suited for use with caregivers and children, but also laden with unique constraints. Throughout, we highlight open questions alongside best practices for optimizing two-brain fNIRS to examine hypothesized developmental mechanisms. 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.