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, suggesting 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, 2014a; Löken et al., 2009). These two key characteristics lend support to the notion that CT-fibers could distinguish Affective Touch from other kinds of touch exchange. Also, 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 suggests how CT-targeted touch might trigger psychophysiological responses characterizing Affective Touch as a fundamental mechanism for emotion regulation and social-affective processing (Björnsdotter et al., 2009), even though recent advances suggest the possible involvement of Aβ mechanoreceptors contributing to the affective aspects of touch as well (Schirmer et al., 2023).
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 transient increases 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; Novembre et al., 2021) and in the person promoting it (Mazza et al., 2023a). However, in line with the notion that Affective Touch can serve as a potential buffer against stressful situations (Mazza et al., 2023b; Morrison, 2016a; Walker et al., 2022) it has also 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, remains relatively unexplored in this context (Gusso et al., 2021). Emotional stimuli indeed trigger the release of norepinephrine, a neurotransmitter involved in the regulation of pupil dilation (Joshi and Gold, 2020), and heightened pupil responses have been previously noted for both positive and negative arousing stimuli in both visual (Basile et al., 2021; Dal Monte et al., 2015; Pagliaccio et al., 2019) and auditory (Oliva and Anikin, 2018; Partala and Surakka, 2003) domains. Thus, understanding the relationship between the pleasantness of 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. Interestingly, Ellingsen and colleagues (2014) have reported that 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. 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 explore whether and how the nature of the stroking effector (Human vs. Artificial) modulates pupillary responses and subjective experiences in individuals receiving caress-like touches at CT-optimal velocity (Dynamic condition, 3 cm/s; Löken et al., 2009). As a control, experimental subjects also received static touch (Static condition) from both hand types, as we aimed to ensure that any observed differences between human and artificial hands were specific for the dynamic touch.
Our hypotheses encompass several scenarios. If pupil size merely tracked stroking speed, as hinted by prior research (van Hooijdonk et al., 2019), we anticipated finding greater pupil responses during a dynamic touch condition compared to the static touch condition, regardless of the nature of the hand promoting the touch (Human vs. Artificial). Conversely, if pupil size only encoded the nature of the hand promoting the touch, we expected to observe greater pupil responses during human-initiated touch compared to artificial-initiated touch, irrespective of the type of touch (Dynamic vs. Static). Finally, if pupil size had the capacity to jointly encode distinct features characterizing Affective Touch, we hypothesized that pupil responses to dynamic touch would be specially 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 dynamic conditions.