Towards a Novel Contactless Autonomic Biomarker: investigating the relationship between heart rate variability and facial temperature during resting state

Facial temperature has been proposed as a non-invasive marker of autonomic nervous system (ANS) activity. Previous research indicates that nose-tip temperature increases with parasympathetic activation, whereas periorbital temperature reflects sympathetic arousal. However, facial thermal fluctuations during resting state have not yet been systematically investigated, which represents the aim of the present study. Here we examined the relationship between regional facial temperature - measured from nose-tip and periorbital regions using functional infrared thermal imaging - and cardiac parasympathetic activity, indexed by high-frequency (HF) power and 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 associations, and (iii) between-subject correlations to examine inter-individual variability of the cardiac and thermal signals. Across 32 participants during a 13.5-min resting-state session, HF power showed a positive association with nose-tip and a negative association with periorbital temperature, whereas RMSSD showed weaker effects. A differential temperature index (nose-tip minus periorbital) was the most robust correlate of HF power. These findings suggest that facial thermal dynamics mirror vagal modulation at rest and that the differential index converging the information from different facial areas may provide a sensitive, contactless indicator of autonomic activity.