DISCUSSION
In the present study, we investigated whether Affective Touch is encoded as a comprehensive social phenomenon at the autonomic level. We explored whether and how the nature of the stroking effector (Human vs. Artificial) modulates pupillary responses and subjective experiences in individuals receiving a caress-like touch. We employed a static touch as a control to ensure that any observed differences were specific only for touch delivered at CT-optimal speed (3 cm/s; Löken et al., 2009) and not for other types of touch. Overall, we observed that when participants received a dynamic touch, they displayed an increase in pupil dilation for touch administered by a human compared to an artificial hand. Interestingly, such a difference did not emerge for the control static touch condition. Additionally, participants’ self-reports consistently indicated that dynamic touch delivered by a human hand was perceived as the most pleasant in comparison to all other touch conditions.
Previous studies (van Hooijdonk et al., 2019) investigated and compared the impact of different stroking velocities on autonomic parameters, including pupil dilation, and reported that pupil dilation increases as a function of stimulation velocity. However, it is noteworthy that most 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 as we found that a dynamic touch elicits higher pupil dilation responses but only when touch is characterized by skin-to-skin contact. Thus, the autonomic nervous system seems to encode also low-level characteristics of the stroking effector. Indeed, as haptic features convey information about the nature of an external object (Kappers and Bergmann Tiest, 2013), both the temperature and the softness of the touching hand likely inform the nervous system that the dynamic touch is coming from another individual. As such, this information becomes socially relevant (Cascio et al., 2019), yielding autonomic reactions such as the strong modulation we observed in pupil dilation. Taken together, these 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. 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. Consistently 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 received a gentle stroking promoted by a human rather than an artificial hand. This suggests that C-tactile afferents, the neural pathways responsible for the emotional and rewarding aspects of touch (McGlone et al., 2014), may have a preference for slow, caress-like touch (Löken et al., 2009) and are finely tuned to touch that mimics human skin temperature (Ackerley, 2014b). However, recent evidence has begun to challenge the complex but apparently not direct relationship between Affective Touch and CT-system, given that numerous unresolved questions have emerged about the mechanisms of CT-fibers and their role in affect and emotion (Schirmer et al., 2023).
Nonetheless, our findings emphasize the pivotal role of human contact in evoking positive emotional responses. Crucially, participants’ self-reports aligned with the pupil results, indicating that they reported the highest levels of pleasantness when tactile stimulation was delivered by a human hand at a speed resembling that of a caress. This observation implies that the heightened arousal, as indexed by larger pupil size, could be linked to a more positive hedonic experience reported by the participants during that kind of touch. Indeed, pupil dilation has been also 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 stronger pupil responses may reflect the reward-related processing of a socially relevant interaction occurring. These results 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), and underscore that real human-to-human interaction triggers a wide range of positive physiological, emotional, and behavioral effects that cannot be replicated by an artificial social touch (Willemse et al., 2017).
It is important to acknowledge some 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, and possibly compare these effects with those of an artificial hand from both a physiological and hedonic point of view. Secondly, touch pressure and intensity 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). The experimenter was trained to maintain the pressure consistently, as described in the literature (Pawling et al., 2017; van Hooijdonk et al., 2019). Nevertheless, given the importance of the applied force, future studies should also investigate this feature in a more controlled manner as an essential aspect of Affective Touch. Next, to avoid effects of habituation and tiredness on pupillary responses (Morad et al., 2000), in our study we only exposed participants to four trials per condition. However, even though most studies adopted less than 10 trials, recent research showed that this might not be an adequate number of repetitions (Schirmer et al., 2022). Therefore, future research should consider adopting a larger number of repetitions when investigating the hedonic aspects 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 (Devine et al., 2020; Sailer and Ackerley, 2019). 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 a touch resembling a caress was delivered by a real human hand compared to an artificial hand. These results shed light on the uniqueness of real human-to-human contact in shaping Affective Touch as a means of support and affection (Bytomski et al., 2020; Croy et al., 2016; Lo et al., 2021) having a strong adaptive and evolutionary value central to our relational and social development.