Synchronizing brains and hearts: A practical guide for caregiver-child fNIRS-ECG multimodal hyperscanning

The study of synchrony-concordant neural, physiological, and behavioral activity between individuals-has advanced our understanding of co-regulation and socio-emotional development, particularly within caregiver-child relationships. While previous research has often examined neural and physiological synchrony separately, recent multimodal approaches highlight the importance of integrating these systems. This paper presents a comprehensive, practical guide to multimodal hyperscanning using functional near-infrared spectroscopy (fNIRS) and electrocardiography (ECG) in caregiver-child dyads. We outline theoretical foundations, methodological considerations, and detailed protocols for synchronizing fNIRS (via NIRx), ECG (via Mindware), behavioral video (via Mangold VideoSyncPro), and task stimuli (via PsychoPy) in a fully time-aligned manner. We describe equipment configurations, software integration, trigger-based synchronization workflows, and strategies to overcome common challenges in developmental psychophysiological research, particularly with young children. Drawing from our experience with children aged 3-7 years, we provide empirical benchmarks for feasibility, compliance, and data quality. This guide provides a practical, adaptable framework designed to lower the barrier to entry for multimodal hyperscanning and encourage its widespread use in developmental research. Integrating neural and physiological synchrony allows researchers to capture the dynamic, multi-systemic nature of caregiver-child interaction and opens new avenues for investigating mechanisms of co-regulation, emotional development, and risk for psychopathology.