Wavelet Transform Coherence Under Controlled Temporal Correspondence and Social Co-Presence: Implications for fNIRS Hyperscanning

Interpersonal neural synchrony research using functional near-infrared spectroscopy (fNIRS) frequently relies on wavelet transform coherence (WTC) to quantify temporal correspondence between cortical signals. Yet, whether WTC reliably recovers experimentally defined differences in signal correspondence remains insufficiently established. Here, we evaluated WTC using an auditory-guided finger-tapping paradigm in which matching and non-matching temporal structures were compared within participants and between dyad members during individual and joint performance. Task-evoked analyses confirmed reliable recruitment of contralateral motor and premotor regions across both contexts, whereas the ipsilateral motor cortex was significantly recruited only during joint performance and was the only region differing between sessions. WTC sensitivity was strongly modulated by social co-presence. Within participants, WTC failed to discriminate matching from non-matching temporal structures during individual performance, whereas robust matching-related increases emerged during joint performance, particularly in contralateral motor and premotor regions. Between participants, WTC showed limited and region-specific discrimination when participants were tested separately, but higher coherence for matching temporal structures emerged during joint performance, particularly in motor regions. These findings show that WTC can recover experimentally defined temporal correspondence between fNIRS signals, but that this sensitivity is not context-independent. The within-participant results further suggest that social co-presence may alter the temporal dynamics of the individual neural signals themselves. WTC should therefore be interpreted as a context-sensitive measure whose meaning depends on the functional and social processes generating the signals being compared.