INTRODUCTION
Social interaction is a fundamental aspect of human life, and interpersonal touch plays a crucial role in shaping relationships and encouraging social connections (Cascio et al., 2019). Notably, social touch refers to the physical contact or tactile exchanges occurring between individuals during social engagements. It serves as a means of conveying greetings, affection, support, and comfort across diverse social scenarios (Hertenstein et al., 2006). A specific kind of social touch is Affective Touch, characterized by a gentle and enjoyable tactile stimulation capable of triggering profound emotional reactions and positive emotional states (Morrison et al., 2010; Morrison, 2016a). This form of touch can foster sentiments of care, intimacy, closeness, and trust among individuals (Field, 2010; Gulledge et al., 2007; Robinson et al., 2015).
Recent studies have shed light on the distinctive attributes of Affective Touch, revealing the existence of dedicated neural pathways and supporting its sui generis nature (Gallace and Spence, 2016; Morrison, 2016b; Olausson et al., 2008). A specialized somatosensory system, referred to as the CT-afferent system, stands out as it is selectively activated by soft and gentle strokes. Specifically, CT-fibers are sensitive to slow-moving caresses (1-10 cm/s) and exhibit heightened activation in response to touch stimuli with a temperature that closely aligns to human skin (i.e., 32°C) (Ackerley et al., 2014a; Löken et al., 2009). These two unique characteristics lend support to the notion that CT-fibers are finely tuned to warm and gentle human contact, effectively distinguishing Affective Touch from other kinds of touch exchange. Moreover, gentle stimulation of CT-innervated skin triggers the activation of the posterior insula (Gordon et al., 2013), coupling it with both somatosensory and reward processing regions (Sailer et al., 2016). The posterior insula plays a pivotal role in autonomic regulation and interoception by integrating sensory, affective, and rewarding aspects of tactile stimulation (Morrison et al., 2010). Its direct connection with CT-fiber stimulation (Kirsch et al., 2020) further underscores the distinctiveness of Affective Touch as a fundamental mechanism for emotion regulation and social-affective processing (Björnsdotter et al., 2009).
The complex interplay between Affective Touch, emotions, and the autonomic nervous system has been extensively investigated through psychophysiological responses. Notably, Affective Touch has been shown to induce an increase in skin conductance (Olausson et al., 2008): a response that can be influenced by salient contextual factors both in the person receiving the touch (Nava et al., 2021) and in the person promoting it (Mazza et al., 2023a). In line with the notion that Affective Touch can also serve as a potential buffer against stressful situations (Mazza et al., 2023b; Morrison, 2016a) it has been linked to reductions in blood pressure (Grewen et al., 2005; Lee and Cichy, 2020), stress hormone levels (Heinrichs et al., 2003; Henricson et al., 2008) and heart rate (Pawling et al., 2017; Triscoli et al., 2017) along with an increase in heart rate variability (Triscoli et al., 2017). Although skin conductance and heart rate have been extensively explored as markers of physiological modulation induced by Affective Touch, pupil dilation, a well-established indicator of physiological activation (Gusso et al., 2021), remains relatively unexplored in this context. Indeed, Affective Touch by engaging the sympathetic nervous system leads to the release of norepinephrine, a neurotransmitter involved in the regulation of pupil dilation (Aston-Jones and Cohen, 2005). Heightened pupil responses have been previously noted for both positive and negative arousing stimuli in visual (Basile et al., 2021; Dal Monte et al., 2015; Pagliaccio et al., 2019) and auditory (Partala and Surakka, 2003) domains. Understanding the relationship between Affective Touch and pupil dilation will provide important insights into the physiological responses evoked by this kind of tactile stimulation.
Earlier research has indicated that pupil dilation is influenced by the speed of touch rather than its pleasantness (van Hooijdonk et al., 2019), concluding that pupil responses primarily encode the sensory characteristics of tactile stimulation and do not distinctly respond to the emotional aspects of touch. However, the majority of the studies investigating Affective Touch employed brushes or mechanical tools to deliver tactile stimuli (Bertheaux et al., 2020; Pawling et al., 2017; Triscoli et al., 2017; van Hooijdonk et al., 2019). This might have restricted the possibility of targeting the hedonic effects associated with an actual human touch. Building on this premise, a few investigations have discovered that the pupil dilates more in response to human touch compared to artificial touch, particularly when Affective Touch is accompanied by the presentation of images displaying a positive facial expression (Ellingsen et al., 2014). This observation implies that pupil response can discern between distinct types of tactile interactions and potentially even capture the emotional experience accompanying touch. Thus, a touch promoted by a human hand, as opposed to artificial means, appears to be a pivotal factor in evoking distinct pupillary responses that are aligned with the emotional aspect of touch.
Although previous studies have made strides in understanding the significance of specific attributes of Affective Touch, such as the stroking velocity and the nature of the touching effector, they have largely focused on investigating these features individually, examining one characteristic at a time. Thus, this approach has made it challenging to draw comprehensive conclusions on the intricate interplay between these distinct characteristics and how those contribute to eliciting a physiological response. The current study aims to bridge this gap by exploring whether and how the nature of the stroking effector and the speed of touch interact to shape pupillary responses. Our investigation delves into how touch velocity (CT-optimal vs. CT-suboptimal) and the nature of the hand promoting the touch (Human vs. Artificial) influence both pupil dilation and the subjective experience in the person receiving tactile stimulations. We seek to ascertain whether pupil size can distinctly encode Affective Touch, by preferentially reacting to CT-optimal touch promoted by a Human hand. Our hypotheses encompass several scenarios. If pupil size indeed encodes the nature of the hand promoting the touch, we expect to observe greater pupil responses during Human-initiated touch compared to Artificial-initiated touch, irrespective of velocity. Conversely, if pupil size merely tracks stroking speed, as hinted by prior research (van Hooijdonk et al., 2019), we anticipated to find greater pupil responses during the CT-optimal velocity condition compared to the CT-suboptimal velocity condition, regardless of the nature of the hand promoting the touch (Human vs. Artificial). Finally, if pupil size has the capacity to encode Affective Touch as a whole, we hypothesize that pupil responses to CT-optimal touch will be further influenced by the nature of the hand promoting the touch. This would be reflected in larger pupil dilation when touch is promoted by a Human hand, but exclusively under CT-optimal velocity conditions.