Towards a novel contactless autonomic biomarker: Investigating the relationship between heart rate variability and...

Facial temperature is a promising non-invasive index of autonomic activity, yet its dynamics during resting state remain largely unexplored. In the present study, we investigated the relationship between facial temperature in the key regions of the nose tip and the periorbital area, and cardiac activity. Facial temperature was measured using functional infrared thermal imaging, while cardiac parasympathetic activity was indexed by high-frequency (HF) power and the root mean square of successive differences (RMSSD) of heart rate variability (HRV). Analyses included (i) grand-average correlations to identify overarching physiological patterns, (ii) within-subject correlations to explore the strength of individual-level relationship, and (iii) between-subject correlations to examine inter-individual variability in the association between cardiac and thermal signals. Across 32 participants, during a 13.5-min resting-state session, HF power showed a positive association with nose-tip temperature and a negative association with periorbital temperature, whereas RMSSD showed weaker effects. A differential temperature index (nose-tip minus periorbital) emerged as the most robust correlate of HF power. These findings suggest that facial thermal dynamics mirror vagal modulation at rest and that the differential index, which integrates information from distinct facial areas, may offer a sensitive, contactless indicator of autonomic activity.