Affective Touch is encoded by pupil dilation as a comprehensive social phenomenon

Affective Touch plays a pivotal role in regulating emotions, fostering social bonds, and nurturing affiliations with others. Its emotional and arousing dimensions rely on specific features of tactile stimulations characterizing gentle human caresses, such as touch velocity and the nature of the stroking source. While previous research has examined the physiological responses to these individual features of Affective Touch, no study has explored how they interact to shape autonomic activity. In this study, we investigated whether and how both the nature of the touching effector (Human hand vs. Artificial hand) and touch type (Dynamic vs. Static) influenced the participants’ pupil dilation and their subjective experience during tactile stimulation. We observed that when participants received a dynamic touch, they displayed an increase in pupil dilation when the touch was promoted by a human compared to an artificial hand, and that such discrimination was absent for static touch. Also, dynamic touch promoted by a human hand invoked a supralinear enhancement of pupil dilation indicating that the combination of these two features induced a significantly stronger autonomic activation than the summed effects of each separately. Moreover, this specific type of touch was perceived as the most pleasant compared to all other tactile stimulations. Overall, our results suggest that pupil dilation could map the pleasant experience of human-to-human tactile interactions, supporting the notion that the autonomic nervous system encodes the emotional and hedonic aspects associated with Affective Touch as a complex and holistic social experience, rather than solely responding to its low-level sensory properties.