DISCUSSION
In the present study, we investigated whether Affective Touch is encoded as a comprehensive social phenomenon by the autonomic nervous system. We measured participants’ pupil responses and pleasantness ratings while manipulating two key features characterizing Affective Touch, that are the stroking velocity and the sensory characteristics of the stroking hand. Overall, we observed a notable increase in pupil dilation when the touch received fell within the CT-optimal stroking range (3 cm/s; Löken et al., 2009) and was administered by a human hand. Additionally, participants’ self-reports consistently indicated that this type of touch was perceived as the most pleasant in comparison to all other touch conditions. This outcome aligns with existing research suggesting that C-tactile afferents, the neural pathways responsible for the emotional and rewarding aspects of touch (McGlone et al., 2014), exhibit a preference for slow, caress-like touch (Löken et al., 2009) and are finely attuned to touch that mimics human skin temperature(Ackerley et al., 2014b). Thus, these findings emphasize the pivotal role of human contact in evoking positive emotional responses.
In line with the well-documented positive effects of Affective Touch on human well-being, other studies investigating the autonomic responses to Affective Touch reported positive effects on both cardiovascular (Grewen et al., 2005; Lee and Cichy, 2020; Triscoli et al., 2017) and hormonal systems (Heinrichs et al., 2003; Henricson et al., 2008) of individuals receiving a touch. Also, physiological coupling has been observed among dyads during interpersonal touch (Chatel-Goldman et al., 2014), further highlighting its pivotal role in social bonding. However, it is noteworthy that most of these studies employed artificial tools to reproduce Affective Touch at a CT-optimal speed (Bertheaux et al., 2020; Triscoli et al., 2017; van Hooijdonk et al., 2019). While this approach is valuable for precisely controlling stroking velocity, it may lack ecological validity as it does not account for the nuances of human-to-human tactile interactions. Our results add knowledge to this body of work by revealing that pupil responses are generally more pronounced when the touch is administered by a human hand compared to an artificial hand. This observation aligns with a previous study by Ellingsen and colleagues (2014), which similarly found that pupil dilation is more responsive to human touch than machine touch. Pupil dilation is associated with salient and rewarding stimuli (Beatty, 1982; Laeng et al., 2012) and reflects social interest in others (Laeng and Falkenberg, 2007): this suggests that touch promoted by a real human hand may be perceived as more rewarding, thereby eliciting a stronger pupil response.
Moreover, we found that the stroking speed is also another crucial element for eliciting a high pupil dilation, but only when the touch is promoted by a human hand. Van Hooijdonk and colleagues (2019) investigated and compared the impact of different stroking velocities on autonomic parameters, including pupil dilation. They reported, in line with our findings, that pupil dilation increased as a function of stimulation velocity. Thus, our findings not only corroborate previous research but also expand on the concept that pupil response depends on the interaction between stroking velocity and low-level characteristics of the stroking effector. Both the temperature and the softness of the touching hand might be responsible for the strong modulation observed in pupil dilation. Indeed, the greatest pupil dilation occurred when the touch resembled a soft stroke that activated CT-system fibers at both optimal temperature and velocity. CT-fibers are known to convey feelings of pleasure and comfort (von Mohr et al., 2017), and the soothing and beneficial effects of this pleasant touch provide evidence supporting a direct connection between CT-fiber stimulation and autonomic modulation (Fotopoulou et al., 2022; Triscoli et al., 2017). Taken together, our results consistently support the idea that Affective Touch is linked to autonomic regulation and that pupil size encodes Affective Touch not only for the speed or the effector features, but as a holistic experience. Indeed, we observed a higher pupil dilation when touch was delivered simultaneously at CT-optimal speed and by a human hand. Also, the observation of supralinear enhancement of pupil dilation in this kind of touch further supports the idea that the combination of these two features can induce a significantly stronger autonomic activation than the summed effects of each delivered separately.
In our study, we also invited participants to rate the pleasantness of the touch they received. Consistent with prior research (Ali et al., 2023; Pfabigan et al., 2023; van Hooijdonk et al., 2019; von Mohr et al., 2017; Zheng et al., 2021), our participants reported higher levels of pleasantness when the touch was administered at the CT-optimal speed and when delivered by a human rather than an artificial hand. Crucially, self-reports mirrored pupil results by revealing that participants reported the highest ratings of pleasantness when tactile stimulation featured both CT-optimal speed and human contact simultaneously. These finding reinforce the notion that touch is perceived as more pleasant when it exhibits characteristics associated with human touch (Schirmer et al., 2023; Wijaya et al., 2020). It also underscores that real human-to-human interaction triggers a wide range of positive physiological, emotional, and behavioural effects that cannot be replicated by an artificial social touch (Willemse et al., 2017).
This is the first study investigating the combined effects of two key low-level features characterizing tactile interaction on both physiological activity and perceived pleasantness. Our results emphasize the uniqueness of Affective Touch as a standalone form of social interaction characterized by specific sensory attributes. Indeed, this type of social touch not only conveys positive emotions but also elicits feelings of support and affection (Bytomski et al., 2020; Croy et al., 2016; Lo et al., 2021). Collectively, our findings suggest that the connection between Affective Touch and pleasantness hinges on the sensory characteristics inherent to human-to-human tactile interactions. These features might be crucial for experiencing Affective Touch as a rewarding experience having a strong adaptive and evolutionary value central to our relational and social development.
It is important to acknowledge the possible limitations in our study and consider potential avenues for future research. Firstly, in our study, we only examined two different stroke speeds. Future investigations should explore a broader range of stroke-speed conditions while still using a human hand, as a touch promoted by a human hand is central for analysing the affective and social aspects of Affective Touch. Secondly, touch pressure and intensity, which have been identified as significant factors in other studies (Case et al., 2021; Iriki et al., 1996; van Hooijdonk et al., 2019; Wolfgang Ellermeier and Westphal, 1995), should also be investigated as essential features of Affective Touch. Furthermore, it would be valuable for future studies to consider participants’ personal attitudes towards interpersonal touch by using targeted questionnaires. For instance, research has indicated that individuals who lacked tactile, enjoyable experiences with close family members during early development may perceive Affective Touch as less pleasant (Sailer et al., 2016). Lastly, our study focused exclusively on young subjects. Future research should expand upon these findings and explore the effects of age. A more diverse and heterogeneous sample could provide further insights into the hedonic and physiological responses related to Affective Touch throughout the lifespan (Cascio et al., 2019; Sehlstedt et al., 2016).
Summarizing, the present study investigated how two key features characterizing Affective Touch such as touch velocity and the nature of the hand promoting the touch influence both pupil dilation and the subjective experience in the person receiving a tactile stimulation. We not only replicated previous observations regarding each feature alone, but also unveiled, for the first time, that their combination is crucial in promoting a stronger physiological activation and an optimal hedonic experience. In fact, we found that both pupil dilation and degree of pleasantness were significantly higher when tactile stimulation was delivered by a human hand and with a speed resembling the one used during a caress. These combined sensory elements appear to be fundamental for experiencing Affective Touch as a comprehensive and rewarding phenomenon, one that holds central importance in our social lives.