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

Affective Touch is encoded by pupil dilation as a comprehensive social phenomenon | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Affective Touch is encoded by pupil dilation as a comprehensive social phenomenon Greta Bonino, Alessandro Mazza, Francesca Capiotto, Annamaria Berti, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4696797/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 16 Oct, 2024 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Affective Touch is characterized by both emotional and arousing dimensions that rely on specific features of a gentle human caress. 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, their pupil dilation increased more when the touch was promoted by a human compared to an artificial hand. This discrimination was not present 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 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. Pupil dilation Affective touch Stroke velocity Human hand Artificial hand Skin-to-skin touch Figures Figure 1 Figure 2 INTRODUCTION Social interaction is a fundamental aspect of human life, and interpersonal touch plays a crucial role in shaping relationships and encouraging social connections 1 . 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 2 . 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 3 , 4 . This form of touch can foster sentiments of care, intimacy, closeness, and trust among individuals 5 – 7 . 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 8 – 10 . 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) 11 , 12 . 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 13 , coupling it with both somatosensory and reward processing regions 14 . The posterior insula plays a pivotal role in autonomic regulation and interoception by integrating sensory, affective, and rewarding aspects of tactile stimulation 4 . Its direct connection with CT-fibers stimulation 15 further suggests how CT-targeted touch might trigger psychophysiological responses characterizing Affective Touch as a fundamental mechanism for emotion regulation and social-affective processing 16 , even though recent advances suggest the possible involvement of Aβ mechanoreceptors contributing to the affective aspects of touch as well 17 . 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 10 : a response that can be influenced by salient contextual factors both in the person receiving the touch 18 , 19 and in the person promoting it 20 . However, in line with the notion that Affective Touch can serve as a potential buffer against stressful situations 3 , 21 , 22 it has also been linked to reductions in blood pressure 23 , 24 , stress hormone levels 25 , 26 and heart rate 27 , 28 along with an increase in heart rate variability 28 . 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 29 . Emotional stimuli indeed trigger the release of norepinephrine, a neurotransmitter involved in the regulation of pupil dilation 30 , and heightened pupil responses have been previously noted for both positive and negative arousing stimuli in both visual 31 – 33 and auditory 34 , 35 domains. Thus, 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 36 , 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 27 , 28 , 36 , 37 . This might have restricted the possibility of targeting the hedonic effects associated with an actual human touch. Interestingly, Ellingsen and colleagues (2014) 38 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. Expanding on this literature, we investigated whether, beyond the bottom-up affective component of touch (CT-fibers) the social aspect of the effector (a real human hand) might play significant role in determining the salience of Affective Touch. Indeed, dynamic stimuli inherently convey more information than static ones and when targeting CT-fibers, they are known to evoke autonomic and affective responses 8 . Moreover, a human hand is characterized by an ensemble of specific sensory characteristics (e.g., softness, warmness, texture) that signal to the receiver’s sensory system that they are being touched by another individual. Consequently, this type of sensory information, processed at a low level, becomes socially relevant 1 . Thus, we hypothesized that the combination of these characteristics—a human hand and a dynamic stimulus—would be more salient than their individual counterparts (i.e., an artificial hand and a static stimulation, respectively), thereby eliciting a stronger pupil response. In the present study, we explored whether and how the nature of the stroking effector (Human vs. Artificial) modulates pupillary responses in individuals receiving caress-like touches at CT-optimal velocity (Dynamic condition, 3 cm/s 12 ). Additionally, we collected explicit pleasantness ratings to examine the hedonic experience when a CT-optimal touch was promoted by a real human hand compared to the other conditions. 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 36 , 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. MATERIALS and METHODS Participants Thirty right-handed Italian volunteers (16 females and 14 males, mean age 23.9 ± 2.3 and 24.6 ± 2.8 respectively) took part to this study. Most of the participants were undergraduate students at the Department of Psychology (University of Turin) and were recruited from a participants’ database or through flyers posted on the University website. All experimental subjects gave written informed consent to participate, which was approved by the local ethics committee and performed in accordance with the Declaration of Helsinki. At the end of the experiment, all participants were informed about the aims and the scopes of the experiment and did not receive any compensation for participation in this research study. Experimental setting and design Participants were invited to sit in a comfortable position, place their left arm on a table with their palm facing down, and lean their chin and forehead on a headrest to ensure stability and reduce any unintentional movement (Fig. 1 a). Given that in this study we were interested in investigating how pupillary dilation vary as a function of different tactile stimulations, the experimental session started with a 9-point grid system calibration. Each touch was delivered by either a Human hand (i.e., the experimenter’s hand; Human condition) or an Artificial hand (i.e., a wooden hand; Artificial condition) (Fig. 1 b). The wooden hand closely resembled a real human hand aesthetically. This enabled us to manipulate tactile low-level sensory aspects that characterize a real human hand while simultaneously controlling for visual similarities. Additionally, participants received two types of touch: a dynamic [i.e., a dynamic stroking at 3 cm/s 12 ; Dynamic condition] and a static touch (Static condition). Each trial started with a 2-second fixation cross (baseline) followed by a 10-second grey square (stimulus) presented in the center of the screen, during which the participant received a tactile stimulation (Fig. 1 c). In each experimental session, one of two experimenters delivered the tactile stimulation. The female experimenter delivered touch only to male participants, while the male experimenter delivered touch only to female participants. This allowed to control for any possible bias due to the experimenter’s biological sex. Both experimenters were trained to deliver touch at a constant pressure and velocity. Across all experimental sessions, the two experimenters were always the same. Before the beginning of the experiment, participants’ left dorsal hand and forearm were marked with two 12-cm distant signs in order to guide the experimenter in the action of promoting the touch for the Dynamic Touch conditions. Moreover, a point in the middle of the subjects’ hand and forearm was measured to indicate the area for the Static Touch (Fig. 1 d). In each experimental session, one of two experimenters delivered the tactile stimulation. The female experimenter delivered touch only to male participants, while the male experimenter delivered touch only to female participants. This allowed to control for any possible. Both experimenters were trained to deliver touch at a constant pressure and velocity. Across all experimental sessions, the two experimenters were always the same. Given that pupil dilation recording is sensitive to eye movements and blinks, participants were instructed to keep their gaze fixed on the target stimulus and blink as little as possible. A 10-second period of tactile stimulation was followed by a 2-second ITI where subjects were allowed to rest. Before the beginning of the next trial participants were asked to rate the pleasantness of the touch received, on a scale from 0 to 10. Participants' subjective ratings were recorded by the experimenter as an indicator of the pleasantness associated with each touch. Each participant received 4 tactile stimuli per condition (i.e., Dynamic_Human, Dynamic_Artificial, Static_Human, and Static_Artificial) for a total of 16 tactile stimulations presented in a random order. For each condition, the touch was delivered twice on the dorsal side of the hand and twice on the dorsal side of the forearm, two hairy CT-rich sites mostly involved in interpersonal touch 39 , 40 . We delivered tactile stimulation in two different locations to avoid habituation effects. Given that pupil dilation is sensitive to light we conducted the whole experimental session in a dark experimental room where the only source of illumination was the computer monitor. Specifically, stimuli were presented on a 17-inch LCD monitor at a screen resolution of 1280 × 1024 pixels (60‐Hz refresh rate), and the distance from the eyes to the monitor was set at 58 cm. The task was implemented on Psychtoolbox (MATLAB©, The Mathworks Inc.), and pupil size was recorded at a 1000 Hz sampling rate using an Eyelink®‐1000 monocular‐arm (SR Research, Osgoode, ON, Canada). a) Experimental setting: the participants sat facing a computer monitor with their chin and forehead on a headrest to ensure stability and reduce any unintentional movement during pupil recording. They were invited to place their left arm on the table with their palm facing down. The researcher standing behind on the left side of the experimental subject promoted different types of tactile stimulations on either the dorsal side of the hand or the dorsal side of the forearm of the participant. b) Experimental variables: participants received either a Dynamic touch (a dynamic stroking with a speed of 3 cm/s) or Static touch, both delivered for the full 10 seconds. The nature of the stroking effector promoting the touch was either a Human hand or an Artificial hand. c) Task progression: Each trial started with a 2-second fixation cross (baseline) followed by a 10-second grey square (stimulus) presented in the center of the screen, during which the participant received a tactile stimulation. The tactile stimulation ended with the beginning of a 2-second ITI. d) Tactile stimulation progression: during Dynamic touch condition participants received a dynamic stroking at 3cm/s 12 for the whole 10-second touch epoch whereas during Static touch condition participants received a static touch lasting 10-second as well. Data analysis Control analysis The spatial location control analysis allowed us to ensure that participants kept their gaze centered on the center of the screen while receiving tactile stimulations, and that pupillary measures were not biased by eye movements. Heatmaps in Fig. 2 a represent the spatial distribution of fixations during tactile stimulations. Axes represent pixels coordinates calculated according to standard Eyelink®-1000 1024x768 screen resolution. Pupillometry analysis Pupillary changes were first baseline corrected on a trial-by-trial basis by subtracting the mean change in pupil diameter 1000ms before the beginning of tactile stimulations. Next, to control for inter-individual variability, pupil data were Z-scored for each subject across all conditions 31 , 41 . In each trial, missing samples due to blinks or loss of the eye-tracking signal during the tactile stimulation period were interpolated via spline interpolation using the nearest valid adjacent samples. Pupil responses were then averaged across trials for each condition. Based on visual inspection of the average response profile, the mean change in pupil diameter was extracted for the time window ranging from 0–4 seconds after stimulus onset (Fig. 2 b). Data were analyzed via a 2-way repeated-measures ANOVA with Hand type (Human vs. Artificial) and Touch type (Dynamic vs. Static) as within subject factors. Post-hoc analyses following significant main effects and interactions were performed by running two-tailed pairwise t-tests, and multiple comparisons were corrected using False Discovery Rate (FDR 42 ). All p values < 0.05 were considered significant. Crucially, to investigate any possible effect of gender and age on pupil responses we first ran a 2x2x2 repeated measures ANOVA with Gender, Hand type and Touch Type as factors. Then, we conducted four Pearson’s correlations (one for each experimental condition) to examine if pupil size varied with age. To test the hypothesis that Dynamic_Human touch alone induced a larger pupil size than Dynamic_Artificial plus Static_Human touch, supralinearity was quantified by contrasting, for each participant, the average pupil size in the Dynamic_Human condition against the sum of the average pupil size in the Dynamic_Artificial plus Static_Human conditions. The effect of Dynamic_Human condition was then compared with the added Dynamic_Artificial and Static_Human condition with a paired-sample t test to determine significance. Finally, we investigated whether the blink rate changed across the four conditions by using nonparametric Wilcoxon Signed-Rank tests, as Kolmogrov-Smirnov tests showed that blink rate distributions were highly skewed in all conditions (all ps < 0.001). Subjective Rating To test whether different kinds of tactile stimulations impacted the perceived pleasantness, subjective ratings were analyzed by running a 2-way repeated-measures ANOVA with Hand type (Human vs. Artificial) and Touch type (Dynamic vs. Static) as within subject factors. Post-hoc analyses following significant main effects and interactions were performed by running two-tailed pairwise t-tests, and multiple comparisons were corrected using FDR. All p values < 0.05 were considered significant. RESULTS Pupil size We found a main effect of Hand type [F (1,119) = 10.196, p = 0.002, η 2 = 0.079], indicating a stronger pupil dilation when participants received a touch from a Human hand compared to an Artificial hand [t (119) = 3.193, p = 0.002]. Crucially, we also found a significant Hand type by Touch type interaction [F (1,119) = 7.402, p = 0.007, η 2 = 0.059], indicating that the magnitude of increase in pupil dilation during the touch promoted by a Human hand differed depending on the type of touch. Specifically, post-hoc t-tests showed that only during Dynamic touch participants exhibited a stronger pupil dilation when receiving a touch from a Human hand compared to an Artificial hand [t (119) = 4.023; p < 0.001], indicating that pupil dilation specifically encodes skin-stroking caress only when promoted by a Human hand. Furthermore, we observed that a touch promoted by a Human hand elicited a significant increase in pupil dilation for Dynamic compared to Static touch [t (119) = 2.966; p = 0.007]. During Static touch participants did not show any difference between a Human and Artificial hand [t (119) = 0.213; p = 0.832]. More importantly, we did not observe any difference in pupil dilation between Dynamic and Static conditions when the touch was promoted by an Artificial hand [t (119) = 1.079; p = 0.379] (Fig. 2 b and 2 c). Crucially these effects were not influenced by participant’s gender (all ps > 0.05) nor by the age (all correlations across all conditions showed a p > 0.05 between age and pupil size). For supralinearity analyses, we summed, for each participant, the pupil size of “Human Static” and “Artificial Dynamic” conditions and scattered this sum against the participant’s pupil size in the only “Human Dynamic” condition. As such, a participant whose pupil size is larger in the “Human Dynamic” than in the sum of “Human Static” and “Artificial Dynamic” conditions, would fall above the unity line indicating equality between the two measures plotted on the X and Y axes. These analyses showed that 70% (n = 21) of participants fell above the unity line, thus displaying a supralinear effect revealing a larger pupil size in the Dynamic_Human condition alone than in the Dynamic_Artificial plus Static_Human conditions summed together [t (29) = 1.781, p = 0.043] (Fig. 2 d). Our results show a stronger pupil dilation when touch was delivered simultaneously at CT-optimal speed and by a human hand. This kind of touch invoked a supralinear enhancement of pupil dilation indicating that the combination of these two features induced a significantly stronger physiological activation than the summed effects of each delivered separately. Moreover, we did not find any differences in blink rates across conditions (all ps > 0.160), suggesting that participants did not show differences in blinking activity depending on the Hand type nor on Touch type. Subjective ratings In line with pupil dilation findings, we observed a main effect of Hand type [F (1,119) = 32.062, p < 0.001, η 2 = 0.212], indicating that participants preferred to receive a touch from a Human hand than from an Artificial hand [t (119) = 5.662, p < 0.001]. Also, we found a main effect of Touch type [F (1,119) = 15.087, p < 0.001, η 2 = 0.113], indicating that participants preferred to receive a Dynamic than a Static touch [t (119) = 3.884, p < 0.001]. Finally, we also found a significant Hand type by Touch type interaction [F (1,119) = 4.125, p = 0.045, η 2 = 0.034], which showed that participants preferred to receive a Dynamic touch from a Human hand. Indeed, post-hoc pairwise comparisons showed that Dynamic touch from a Human hand condition received the highest ratings compared to all other conditions [Dynamic_Human vs. Dynamic_Artificial: t (119) = 3.657, p < 0.001; Dynamic_Human vs. Static_Human: t (119) = 2.343, p = 0.021; Dynamic_Human vs. Static_Artificial: t (119) = 7.070, p < 0.001] (Fig. 2 e). These results, in line with physiological findings, indicate that participants rated as the most pleasant a dynamic touch delivered by a human hand. 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 12 ); 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 36 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 28 , 36 , 37 . 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 high-level characteristics of the stroking effector. Indeed, as haptic features convey information about the nature of an external object 43 , 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 1 , 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. 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 (velocity and stroking effector) 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 36 , 44 – 47 , 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 48 , may have a preference for slow, caress-like touch 12 and are finely tuned to touch that mimics human skin temperature 49 . 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 17 . Nonetheless, our findings emphasize the pivotal role of human contact in evoking positive emotional responses, as our participants reported the highest levels of pleasantness when tactile stimulation was delivered by a human hand at a speed resembling that of a caress. It's worth noting that these findings exhibited a similar pattern to those observed for pupil dilation. As pupil dilation has been associated with salient and rewarding stimuli 50 , 51 and to social interest in others 52 , a stronger pupil responses may reflect the reward-related processing of a socially relevant interaction occurring. However, although it has been reported that CT-optimal speed tactile stimulation carries a positive affective valence 27 , pupillary responses mostly track salience (not valence) of a stimulus and several top-down contextual factors might come at play in driving the association between pleasantness and autonomic activation 53 . Indeed, the way individuals experience social touch in general 54 and Affective Touch specifically 55 can be influenced by several contextual and top-down factors beyond the physical sensation of the touch itself. Hence, future studies might build upon the present results and explicitly address such an intriguing question. 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 36 , 56 , 57 has been identified as significant factor in other studies. Here the experimenter underwent a training before the beginning of the study to maintain consistent pressure, both during the dynamic and the static Touch, following procedures described in the literature 27 , 36 . Nevertheless, given the importance of applied force, future studies should also investigate this aspect in a more controlled manner as an essential aspect of Affective Touch. Next, to avoid effects of habituation and tiredness on pupillary responses 58 , 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 59 . Therefore, future research should consider adopting a larger number of repetitions when investigating the hedonic aspects of Affective Touch. Furthermore, in this study, we always employed an opposite-gender experimenter and this choice did not allow us to account for same-sex affective interactions. It would be valuable for future studies to consider these variables as well as participants' sexual preferences and cultural differences to mitigate potential interference effects and personal attitudes towards interpersonal touch. 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 60 , 61 . 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 1 , 62 . 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 subjective experience in the person receiving a tactile stimulation. We not only replicated previous observations regarding each feature alone, but also reported, for the first time, that their combination triggers a stronger physiological reaction than their isolated components along with a positive hedonic experience. These results shed light on the uniqueness of real human-to-human contact in shaping Affective Touch as a means of support and affection 63 – 65 having a strong adaptive and evolutionary value central to our relational and social development. Declarations AUTHOR CONTRIBUTION G.B. and O.D.M. designed the study, G.B. performed the experiment, G.B. and A.M. analyzed the data, and G.B., A.M., F.C., and O.D.M. wrote the paper. All authors reviewed the manuscript. ACKNOWLEDGMENTS We thank Giuseppe Pica and Luca Melchionda for assistance with data collection. We also thank Lucia De Francesco and Giulia Romano Cappi for their insightful comments on the manuscript. This work was supported by MIUR (DALO_RILO_19_01) and GFI (DALO_GFI_22_01_F) grant to ODM. DATA AVAILABILITY STATEMENT Data and code used for this paper’s analyses are made publicly available at https://github.com/SocialInteractionLabUnito/Pupil_AffectiveTouch CONFLICT OF INTEREST STATEMENT The authors declare no conflict of interest. ETHICAL STATEMENT The experimental procedure was approved by the Bioethical Committee of the University of Turin and conducted in accordance with the Declaration of Helsinki (World Medical Association, 2013). References Cascio, C. J., Moore, D. & McGlone, F. Social touch and human development. Dev. Cogn. Neurosci. 35 , 5–11 (2019). Hertenstein, M. J., Keltner, D., App, B., Bulleit, B. A. & Jaskolka, A. R. Touch communicates distinct emotions. Emotion 6 , 528–533 (2006). Morrison, I. Keep Calm and Cuddle on: Social Touch as a Stress Buffer. Adapt. Hum. Behav. Physiol. 2 , 344–362 (2016). Morrison, I., Löken, L. S. & Olausson, H. 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The outcome of tactile touch on oxytocin in intensive care patients: a randomised controlled trial. J. Clin. Nurs. 17 , 2624–2633 (2008). Pawling, R., Cannon, P. R., McGlone, F. P. & Walker, S. C. C-tactile afferent stimulating touch carries a positive affective value. PloS One 12 , e0173457 (2017). Triscoli, C., Croy, I., Steudte-Schmiedgen, S., Olausson, H. & Sailer, U. Heart rate variability is enhanced by long-lasting pleasant touch at CT-optimized velocity. Biol. Psychol. 128 , 71–81 (2017). Gusso, M. de M., Serur, G. & Nohama, P. Pupil Reactions to Tactile Stimulation: A Systematic Review. Front. Neurosci. 15 , (2021). Joshi, S. & Gold, J. I. Pupil size as a window on neural substrates of cognition. Trends Cogn. Sci. 24 , 466–480 (2020). Basile, B. M. et al. Autonomic arousal tracks outcome salience not valence in monkeys making social decisions. Behav. Neurosci. 135 , 443–452 (2021). Dal Monte, O., Costa, V. D., Noble, P. L., Murray, E. A. & Averbeck, B. B. Amygdala lesions in rhesus macaques decrease attention to threat. Nat. Commun. 6 , 10161 (2015). Pagliaccio, D. et al. Cross-species convergence in pupillary response: understanding human anxiety via non-human primate amygdala lesion. Soc. Cogn. Affect. Neurosci. 14 , 591–599 (2019). Oliva, M. & Anikin, A. Pupil dilation reflects the time course of emotion recognition in human vocalizations. Sci. Rep. 8 , 4871 (2018). Partala, T. & Surakka, V. Pupil size variation as an indication of affective processing. Int. J. Hum.-Comput. Stud. 59 , 185–198 (2003). van Hooijdonk, R. et al. Touch-induced pupil size reflects stimulus intensity, not subjective pleasantness. Exp. Brain Res. 237 , 201–210 (2019). Bertheaux, C. et al. Emotion Measurements Through the Touch of Materials Surfaces. Front. Hum. Neurosci. 13 , (2020). Ellingsen, D.-M. et al. In touch with your emotions: Oxytocin and touch change social impressions while others’ facial expressions can alter touch. Psychoneuroendocrinology 39 , 11–20 (2014). Cruciani, G. et al. Strengths and weaknesses of affective touch studies over the lifetime: A systematic review. Neurosci. Biobehav. Rev. 127 , 1–24 (2021). Pyasik, M. et al. Self-other distinction modulates the social softness illusion. Psychol. Res. 86 , 1165–1173 (2022). Rudebeck, P. H. et al. A role for primate subgenual cingulate cortex in sustaining autonomic arousal. Proc. Natl. Acad. Sci. U. S. A. 111 , 5391–5396 (2014). Benjamini, Y. & Hochberg, Y. Controlling the False Discovery Rate: A Practical and Powerful Approach to Multiple Testing. J. R. Stat. Soc. Ser. B Methodol. 57 , 289–300 (1995). Kappers, A. M. L. & Bergmann Tiest, W. M. Haptic perception. WIREs Cogn. Sci. 4 , 357–374 (2013). Ali, S. H. et al. Hold me or stroke me? Individual differences in static and dynamic affective touch. PLOS ONE 18 , e0281253 (2023). Pfabigan, D. M. et al. Ghrelin is related to lower brain reward activation during touch. Psychophysiology n/a , e14443 (2023). von Mohr, M., Kirsch, L. P. & Fotopoulou, A. The soothing function of touch: affective touch reduces feelings of social exclusion. Sci. Rep. 7 , 13516 (2017). Zheng, C. Y. et al. Comparing soft robotic affective touch to human and brush affective touch. in 2021 IEEE World Haptics Conference (WHC) 352–352 (2021). doi:10.1109/WHC49131.2021.9517156. McGlone, F., Wessberg, J. & Olausson, H. Discriminative and affective touch: sensing and feeling. Neuron 82 , 737–755 (2014). Ackerley, R. Human C-tactile afferents are tuned to the temperature of a skin-stroking caress. J. Neurosci. Off. J. Soc. Neurosci. 34 , 2879–2883 (2014). Beatty, J. Task-evoked pupillary responses, processing load, and the structure of processing resources. Psychol. Bull. 91 , 276–292 (1982). Laeng, B., Sirois, S. & Gredebäck, G. Pupillometry: A Window to the Preconscious? Perspect. Psychol. Sci. J. Assoc. Psychol. Sci. 7 , 18–27 (2012). Laeng, B. & Falkenberg, L. Women’s pupillary responses to sexually significant others during the hormonal cycle. Horm. Behav. 52 , 520–530 (2007). Sailer, U. & Leknes, S. Meaning makes touch affective. Curr. Opin. Behav. Sci. 44 , 101099 (2022). Saarinen, A., Harjunen, V., Jasinskaja-Lahti, I., Jääskeläinen, I. P. & Ravaja, N. Social touch experience in different contexts: A review. Neurosci. Biobehav. Rev. 131 , 360–372 (2021). Ellingsen, D.-M., Leknes, S., Løseth, G., Wessberg, J. & Olausson, H. The Neurobiology Shaping Affective Touch: Expectation, Motivation, and Meaning in the Multisensory Context. Front. Psychol. 6 , (2016). Case, L. K. et al. Pleasant Deep Pressure: Expanding the Social Touch Hypothesis. Neuroscience 464 , 3–11 (2021). Ten Brink, A. F., Heiner, I., Dijkerman, H. C. & Strauch, C. Pupil dilation reveals the intensity of touch. Psychophysiology e14538 (2024) doi:10.1111/psyp.14538. Morad, Y., Lemberg, H., Yofe, N. & Dagan, Y. Pupillography as an objective indicator of fatigue. Curr. Eye Res. 21 , 535–542 (2000). Schirmer, A. et al. Understanding sex differences in affective touch: Sensory pleasantness, social comfort, and precursive experiences. Physiol. Behav. 250 , 113797 (2022). Devine, S. L. et al. Childhood Adversity and Affective Touch Perception: A Comparison of United Kingdom Care Leavers and Non-care Leavers. Front. Psychol. 11 , (2020). Sailer, U. & Ackerley, R. Exposure shapes the perception of affective touch. Dev. Cogn. Neurosci. 35 , 109–114 (2019). Sehlstedt, I. et al. Gentle touch perception across the lifespan. Psychol. Aging 31 , 176–184 (2016). Bytomski, A. et al. Maternal stroking is a fine-tuned mechanism relating to C-tactile afferent activation: An exploratory study. Psychol. Neurosci. 13 , 149–157 (2020). Croy, I. et al. Interpersonal stroking touch is targeted to C tactile afferent activation. Behav. Brain Res. SreeTestContent1 297 , 37–40 (2016). Lo, C., Chu, S. T., Penney, T. B. & Schirmer, A. 3D Hand-Motion Tracking and Bottom-Up Classification Sheds Light on the Physical Properties of Gentle Stroking. Neuroscience 464 , 90–104 (2021). Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4696797","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":336069737,"identity":"b37406ec-f268-4e44-9e79-ee77fe60c3f4","order_by":0,"name":"Greta Bonino","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Greta","middleName":"","lastName":"Bonino","suffix":""},{"id":336069740,"identity":"9b25d707-d31b-4304-ac77-1978c1b6556a","order_by":1,"name":"Alessandro Mazza","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Alessandro","middleName":"","lastName":"Mazza","suffix":""},{"id":336069741,"identity":"07a93c0d-7b5d-425a-90f4-5100e3b1d488","order_by":2,"name":"Francesca Capiotto","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Francesca","middleName":"","lastName":"Capiotto","suffix":""},{"id":336069742,"identity":"1f65d356-1c57-42db-aa89-c19abf74abb0","order_by":3,"name":"Annamaria Berti","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Annamaria","middleName":"","lastName":"Berti","suffix":""},{"id":336069744,"identity":"d172c54f-b671-44c0-aa52-a7616a140e6a","order_by":4,"name":"Lorenzo Pia","email":"","orcid":"","institution":"University of Turin","correspondingAuthor":false,"prefix":"","firstName":"Lorenzo","middleName":"","lastName":"Pia","suffix":""},{"id":336069745,"identity":"5c2b9e71-96b5-48e0-9fb1-a0dd4f1b1e5a","order_by":5,"name":"Olga Dal Monte","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA6klEQVRIiWNgGAWjYDACdhDBxiAD4VWA2ECQgE8LM0QLD5BkbGA4A2SD9CTg04OihbGNgbA1/MzsFz8wlNnx6Lb3Hn/wcd7hPD755gMMD3/g1iLZzFMswXAumcfszLnExpnbDhezsbEl4HWYwWGeBAnGNmYesxs5hs282w4ntrHxGBDSkvyDsa0eouXvHKK0sB8D2nIYooWxgQgtQL+wWSScOw70yxnDmT3H0oFa0hIOJKTh1sLP3v74xoeyajmz4z0GH37UWCfObz588OEPG9xaGBhAzkAXO4BPAzDFPMAvPwpGwSgYBaMAAOV8Ta426FSfAAAAAElFTkSuQmCC","orcid":"","institution":"University of Turin","correspondingAuthor":true,"prefix":"","firstName":"Olga","middleName":"Dal","lastName":"Monte","suffix":""}],"badges":[],"createdAt":"2024-07-06 12:47:10","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4696797/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4696797/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41598-024-74566-3","type":"published","date":"2024-10-16T15:57:45+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":62657642,"identity":"fc8636e0-2703-4ad6-a1ed-97cfba3d2021","added_by":"auto","created_at":"2024-08-17 02:11:39","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":178319,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eExperimental setting and variables.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ea) Experimental setting: the participants sat facing a computer monitor with their chin and forehead on a headrest to ensure stability and reduce any unintentional movement during pupil recording. They were invited to place their left arm on the table with their palm facing down. The researcher standing behind on the left side of the experimental subject promoted different types of tactile stimulations on either the dorsal side of the hand or the dorsal side of the forearm of the participant. b) Experimental variables: participants received either a Dynamic touch (a dynamic stroking with a speed of 3 cm/s) or Static touch, both delivered for the full 10 seconds. The nature of the stroking effector promoting the touch was either a Human hand or an Artificial hand. c) Task progression: Each trial started with a 2-second fixation cross (baseline) followed by a 10-second grey square (stimulus) presented in the center of the screen, during which the participant received a tactile stimulation. The tactile stimulation ended with the beginning of a 2-second ITI. d) Tactile stimulation progression: during Dynamic touch condition participants received a dynamic stroking at 3cm/s\u003csup\u003e12\u003c/sup\u003e for the whole 10-second touch epoch whereas during Static touch condition participants received a static touch lasting 10-second as well.\u0026nbsp;\u003c/p\u003e","description":"","filename":"Figure1.png","url":"https://assets-eu.researchsquare.com/files/rs-4696797/v1/83717f12ecbfee94ed5dd6c1.png"},{"id":62657640,"identity":"1ec1cf61-5a5f-418e-b074-f7d9df258115","added_by":"auto","created_at":"2024-08-17 02:11:39","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":384153,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePupil Dilation responses and subjective rating\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ea) Spatial location control: heatmaps show the gaze position during a 10-second grey square (stimulus) in which the participant received a tactile stimulation. None of the four heat maps (depicting the 4 experimental conditions) showed any meaningful eye movements deviation from the stimulus presented on the center of the screen. b) On the top, pupil dilation traces aligned to the time of CT-optimal touch promoted by a Human hand (pink) and Artificial hand (yellow). The shaded traces represent ± s.e.m. centered around the mean. Vertical dotted grey line indicates the beginning CT-optimal touch (10-second duration). The grey-shaded area represents the analyzed epoch. On the bottom, pupil dilation traces aligned to the time of Static touch promoted by a Human hand (purple) and Artificial hand (light blue). c) Bar plot shows the Z-scored mean pupil size values normalized to baseline during Dynamic and Static touch promoted by a Human hand and Artificial hand. d) Scatter plot shows the supralinearity effect by contrasting participants’ pupil size in Dynamic_Human condition alone (y-axis) against Dynamic_Artificial plus Staticl_Human conditions summed together (x-axis). e)\u003cstrong\u003e \u003c/strong\u003eViolin plots show the mean subjective ratings reported by participants in the four conditions: Dynamic touch promoted by a Human hand (pink), Dynamic touch promoted by an Artificial hand (yellow), Static touch promoted by a Human hand (purple), and Static touch promoted by an Artificial hand (light blue). Data points overlaid on top show each subject. In black it is depicted the mean and in red the median. ***, \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.001; **, \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.01; *, \u003cem\u003ep\u003c/em\u003e \u0026lt; 0.05; n.s., not significant.\u003c/p\u003e","description":"","filename":"Figure2.png","url":"https://assets-eu.researchsquare.com/files/rs-4696797/v1/c8a3cacaa5bc4c5cf96724d5.png"},{"id":67149262,"identity":"ac58690e-f5d9-4795-a848-4fe5d49a63c6","added_by":"auto","created_at":"2024-10-21 16:12:55","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1003125,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4696797/v1/aaaece01-7dcc-4914-91d9-891197893ad2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Affective Touch is encoded by pupil dilation as a comprehensive social phenomenon","fulltext":[{"header":"INTRODUCTION","content":"\u003cp\u003eSocial interaction is a fundamental aspect of human life, and interpersonal touch plays a crucial role in shaping relationships and encouraging social connections \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. 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 \u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. 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 \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. This form of touch can foster sentiments of care, intimacy, closeness, and trust among individuals \u003csup\u003e\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eRecent studies have shed light on the distinctive attributes of Affective Touch, suggesting the existence of dedicated neural pathways and supporting its \u003cem\u003esui generis\u003c/em\u003e nature \u003csup\u003e\u003cspan additionalcitationids=\"CR9\" citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e. 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\u0026ndash;10 cm/s) and exhibit heightened activation in response to touch stimuli with a temperature that closely aligns to human skin (i.e., 32\u0026deg;C) \u003csup\u003e\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e,\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e. 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 \u003csup\u003e\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e\u003c/sup\u003e, coupling it with both somatosensory and reward processing regions \u003csup\u003e\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. The posterior insula plays a pivotal role in autonomic regulation and interoception by integrating sensory, affective, and rewarding aspects of tactile stimulation \u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e. Its direct connection with CT-fibers stimulation \u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e further suggests how CT-targeted touch might trigger psychophysiological responses characterizing Affective Touch as a fundamental mechanism for emotion regulation and social-affective processing \u003csup\u003e\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e, even though recent advances suggest the possible involvement of Aβ mechanoreceptors contributing to the affective aspects of touch as well \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe 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 \u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e: a response that can be influenced by salient contextual factors both in the person receiving the touch \u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e and in the person promoting it \u003csup\u003e\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. However, in line with the notion that Affective Touch can serve as a potential buffer against stressful situations \u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e,\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e it has also been linked to reductions in blood pressure \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e,\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e, stress hormone levels \u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e and heart rate \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e along with an increase in heart rate variability \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. 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 \u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. Emotional stimuli indeed trigger the release of norepinephrine, a neurotransmitter involved in the regulation of pupil dilation \u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e, and heightened pupil responses have been previously noted for both positive and negative arousing stimuli in both visual \u003csup\u003e\u003cspan additionalcitationids=\"CR32\" citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e and auditory \u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e domains. Thus, understanding the relationship between Affective Touch and pupil dilation will provide important insights into the physiological responses evoked by this kind of tactile stimulation.\u003c/p\u003e \u003cp\u003eEarlier research has indicated that pupil dilation is influenced by the speed of touch rather than its pleasantness \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e, 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 \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. This might have restricted the possibility of targeting the hedonic effects associated with an actual human touch. Interestingly, Ellingsen and colleagues (2014)\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u003c/sup\u003e 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.\u003c/p\u003e \u003cp\u003eAlthough 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. Expanding on this literature, we investigated whether, beyond the bottom-up affective component of touch (CT-fibers) the social aspect of the effector (a real human hand) might play significant role in determining the salience of Affective Touch. Indeed, dynamic stimuli inherently convey more information than static ones and when targeting CT-fibers, they are known to evoke autonomic and affective responses \u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Moreover, a human hand is characterized by an ensemble of specific sensory characteristics (e.g., softness, warmness, texture) that signal to the receiver\u0026rsquo;s sensory system that they are being touched by another individual. Consequently, this type of sensory information, processed at a low level, becomes socially relevant \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e. Thus, we hypothesized that the combination of these characteristics\u0026mdash;a human hand and a dynamic stimulus\u0026mdash;would be more salient than their individual counterparts (i.e., an artificial hand and a static stimulation, respectively), thereby eliciting a stronger pupil response. In the present study, we explored whether and how the nature of the stroking effector (Human vs. Artificial) modulates pupillary responses in individuals receiving caress-like touches at CT-optimal velocity (Dynamic condition, 3 cm/s \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e). Additionally, we collected explicit pleasantness ratings to examine the hedonic experience when a CT-optimal touch was promoted by a real human hand compared to the other conditions. 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.\u003c/p\u003e \u003cp\u003eOur hypotheses encompass several scenarios. If pupil size merely tracked stroking speed, as hinted by prior research \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e, 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.\u003c/p\u003e"},{"header":"MATERIALS and METHODS","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eParticipants\u003c/h2\u003e \u003cp\u003eThirty right-handed Italian volunteers (16 females and 14 males, mean age 23.9\u0026thinsp;\u0026plusmn;\u0026thinsp;2.3 and 24.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8 respectively) took part to this study. Most of the participants were undergraduate students at the Department of Psychology (University of Turin) and were recruited from a participants\u0026rsquo; database or through flyers posted on the University website. All experimental subjects gave written informed consent to participate, which was approved by the local ethics committee and performed in accordance with the Declaration of Helsinki. At the end of the experiment, all participants were informed about the aims and the scopes of the experiment and did not receive any compensation for participation in this research study.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003eExperimental setting and design\u003c/h2\u003e \u003cp\u003eParticipants were invited to sit in a comfortable position, place their left arm on a table with their palm facing down, and lean their chin and forehead on a headrest to ensure stability and reduce any unintentional movement (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea). Given that in this study we were interested in investigating how pupillary dilation vary as a function of different tactile stimulations, the experimental session started with a 9-point grid system calibration. Each touch was delivered by either a Human hand (i.e., the experimenter\u0026rsquo;s hand; Human condition) or an Artificial hand (i.e., a wooden hand; Artificial condition) (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). The wooden hand closely resembled a real human hand aesthetically. This enabled us to manipulate tactile low-level sensory aspects that characterize a real human hand while simultaneously controlling for visual similarities.\u003c/p\u003e \u003cp\u003eAdditionally, participants received two types of touch: a dynamic [i.e., a dynamic stroking at 3 cm/s \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e; Dynamic condition] and a static touch (Static condition). Each trial started with a 2-second fixation cross (baseline) followed by a 10-second grey square (stimulus) presented in the center of the screen, during which the participant received a tactile stimulation (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec). In each experimental session, one of two experimenters delivered the tactile stimulation. The female experimenter delivered touch only to male participants, while the male experimenter delivered touch only to female participants. This allowed to control for any possible bias due to the experimenter\u0026rsquo;s biological sex. Both experimenters were trained to deliver touch at a constant pressure and velocity. Across all experimental sessions, the two experimenters were always the same.\u003c/p\u003e \u003cp\u003eBefore the beginning of the experiment, participants\u0026rsquo; left dorsal hand and forearm were marked with two 12-cm distant signs in order to guide the experimenter in the action of promoting the touch for the Dynamic Touch conditions. Moreover, a point in the middle of the subjects\u0026rsquo; hand and forearm was measured to indicate the area for the Static Touch (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed). In each experimental session, one of two experimenters delivered the tactile stimulation. The female experimenter delivered touch only to male participants, while the male experimenter delivered touch only to female participants. This allowed to control for any possible. Both experimenters were trained to deliver touch at a constant pressure and velocity. Across all experimental sessions, the two experimenters were always the same.\u003c/p\u003e \u003cp\u003eGiven that pupil dilation recording is sensitive to eye movements and blinks, participants were instructed to keep their gaze fixed on the target stimulus and blink as little as possible. A 10-second period of tactile stimulation was followed by a 2-second ITI where subjects were allowed to rest. Before the beginning of the next trial participants were asked to rate the pleasantness of the touch received, on a scale from 0 to 10. Participants' subjective ratings were recorded by the experimenter as an indicator of the pleasantness associated with each touch. Each participant received 4 tactile stimuli per condition (i.e., Dynamic_Human, Dynamic_Artificial, Static_Human, and Static_Artificial) for a total of 16 tactile stimulations presented in a random order. For each condition, the touch was delivered twice on the dorsal side of the hand and twice on the dorsal side of the forearm, two hairy CT-rich sites mostly involved in interpersonal touch \u003csup\u003e\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e,\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e. We delivered tactile stimulation in two different locations to avoid habituation effects.\u003c/p\u003e \u003cp\u003eGiven that pupil dilation is sensitive to light we conducted the whole experimental session in a dark experimental room where the only source of illumination was the computer monitor. Specifically, stimuli were presented on a 17-inch LCD monitor at a screen resolution of 1280 \u0026times; 1024 pixels (60‐Hz refresh rate), and the distance from the eyes to the monitor was set at 58 cm. The task was implemented on Psychtoolbox (MATLAB\u0026copy;, The Mathworks Inc.), and pupil size was recorded at a 1000 Hz sampling rate using an Eyelink\u0026reg;‐1000 monocular‐arm (SR Research, Osgoode, ON, Canada).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003ea) Experimental setting: the participants sat facing a computer monitor with their chin and forehead on a headrest to ensure stability and reduce any unintentional movement during pupil recording. They were invited to place their left arm on the table with their palm facing down. The researcher standing behind on the left side of the experimental subject promoted different types of tactile stimulations on either the dorsal side of the hand or the dorsal side of the forearm of the participant. b) Experimental variables: participants received either a Dynamic touch (a dynamic stroking with a speed of 3 cm/s) or Static touch, both delivered for the full 10 seconds. The nature of the stroking effector promoting the touch was either a Human hand or an Artificial hand. c) Task progression: Each trial started with a 2-second fixation cross (baseline) followed by a 10-second grey square (stimulus) presented in the center of the screen, during which the participant received a tactile stimulation. The tactile stimulation ended with the beginning of a 2-second ITI. d) Tactile stimulation progression: during Dynamic touch condition participants received a dynamic stroking at 3cm/s\u003csup\u003e12\u003c/sup\u003e for the whole 10-second touch epoch whereas during Static touch condition participants received a static touch lasting 10-second as well.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003eData analysis\u003c/h2\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003eControl analysis\u003c/h2\u003e \u003cp\u003eThe spatial location control analysis allowed us to ensure that participants kept their gaze centered on the center of the screen while receiving tactile stimulations, and that pupillary measures were not biased by eye movements. Heatmaps in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea represent the spatial distribution of fixations during tactile stimulations. Axes represent pixels coordinates calculated according to standard Eyelink\u0026reg;-1000 1024x768 screen resolution.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003ePupillometry analysis\u003c/h2\u003e \u003cp\u003ePupillary changes were first baseline corrected on a trial-by-trial basis by subtracting the mean change in pupil diameter 1000ms before the beginning of tactile stimulations. Next, to control for inter-individual variability, pupil data were Z-scored for each subject across all conditions \u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e,\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e. In each trial, missing samples due to blinks or loss of the eye-tracking signal during the tactile stimulation period were interpolated via spline interpolation using the nearest valid adjacent samples. Pupil responses were then averaged across trials for each condition. Based on visual inspection of the average response profile, the mean change in pupil diameter was extracted for the time window ranging from 0\u0026ndash;4 seconds after stimulus onset (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). Data were analyzed via a 2-way repeated-measures ANOVA with Hand type (Human vs. Artificial) and Touch type (Dynamic vs. Static) as within subject factors. Post-hoc analyses following significant main effects and interactions were performed by running two-tailed pairwise t-tests, and multiple comparisons were corrected using False Discovery Rate (FDR \u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e). All p values\u0026thinsp;\u0026lt;\u0026thinsp;0.05 were considered significant. Crucially, to investigate any possible effect of gender and age on pupil responses we first ran a 2x2x2 repeated measures ANOVA with Gender, Hand type and Touch Type as factors. Then, we conducted four Pearson\u0026rsquo;s correlations (one for each experimental condition) to examine if pupil size varied with age.\u003c/p\u003e \u003cp\u003eTo test the hypothesis that Dynamic_Human touch alone induced a larger pupil size than Dynamic_Artificial plus Static_Human touch, supralinearity was quantified by contrasting, for each participant, the average pupil size in the Dynamic_Human condition against the sum of the average pupil size in the Dynamic_Artificial plus Static_Human conditions. The effect of Dynamic_Human condition was then compared with the added Dynamic_Artificial and Static_Human condition with a paired-sample t test to determine significance.\u003c/p\u003e \u003cp\u003eFinally, we investigated whether the blink rate changed across the four conditions by using nonparametric Wilcoxon Signed-Rank tests, as Kolmogrov-Smirnov tests showed that blink rate distributions were highly skewed in all conditions (all ps\u0026thinsp;\u0026lt;\u0026thinsp;0.001).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSubjective Rating\u003c/h2\u003e \u003cp\u003eTo test whether different kinds of tactile stimulations impacted the perceived pleasantness, subjective ratings were analyzed by running a 2-way repeated-measures ANOVA with Hand type (Human vs. Artificial) and Touch type (Dynamic vs. Static) as within subject factors. Post-hoc analyses following significant main effects and interactions were performed by running two-tailed pairwise t-tests, and multiple comparisons were corrected using FDR. All p values\u0026thinsp;\u0026lt;\u0026thinsp;0.05 were considered significant.\u003c/p\u003e \u003c/div\u003e"},{"header":"RESULTS","content":"\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003ePupil size\u003c/h2\u003e\n \u003cp\u003eWe found a main effect of Hand type [F\u003csub\u003e(1,119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;10.196, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.002, \u0026eta;\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.079], indicating a stronger pupil dilation when participants received a touch from a Human hand compared to an Artificial hand [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;3.193, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.002]. Crucially, we also found a significant Hand type by Touch type interaction [F\u003csub\u003e(1,119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;7.402, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.007, \u0026eta;\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.059], indicating that the magnitude of increase in pupil dilation during the touch promoted by a Human hand differed depending on the type of touch. Specifically, post-hoc t-tests showed that only during Dynamic touch participants exhibited a stronger pupil dilation when receiving a touch from a Human hand compared to an Artificial hand [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;4.023; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001], indicating that pupil dilation specifically encodes skin-stroking caress only when promoted by a Human hand. Furthermore, we observed that a touch promoted by a Human hand elicited a significant increase in pupil dilation for Dynamic compared to Static touch [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2.966; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.007]. During Static touch participants did not show any difference between a Human and Artificial hand [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.213; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.832]. More importantly, we did not observe any difference in pupil dilation between Dynamic and Static conditions when the touch was promoted by an Artificial hand [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.079; \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.379] (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb and \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec). Crucially these effects were not influenced by participant\u0026rsquo;s gender (all ps\u0026thinsp;\u0026gt;\u0026thinsp;0.05) nor by the age (all correlations across all conditions showed a p\u0026thinsp;\u0026gt;\u0026thinsp;0.05 between age and pupil size).\u003c/p\u003e\n \u003cp\u003eFor supralinearity analyses, we summed, for each participant, the pupil size of \u0026ldquo;Human Static\u0026rdquo; and \u0026ldquo;Artificial Dynamic\u0026rdquo; conditions and scattered this sum against the participant\u0026rsquo;s pupil size in the only \u0026ldquo;Human Dynamic\u0026rdquo; condition. As such, a participant whose pupil size is larger in the \u0026ldquo;Human Dynamic\u0026rdquo; than in the sum of \u0026ldquo;Human Static\u0026rdquo; and \u0026ldquo;Artificial Dynamic\u0026rdquo; conditions, would fall above the unity line indicating equality between the two measures plotted on the X and Y axes. These analyses showed that 70% (n\u0026thinsp;=\u0026thinsp;21) of participants fell above the unity line, thus displaying a supralinear effect revealing a larger pupil size in the Dynamic_Human condition alone than in the Dynamic_Artificial plus Static_Human conditions summed together [t\u003csub\u003e(29)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;1.781, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.043] (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed). Our results show a stronger pupil dilation when touch was delivered simultaneously at CT-optimal speed and by a human hand. This kind of touch invoked a supralinear enhancement of pupil dilation indicating that the combination of these two features induced a significantly stronger physiological activation than the summed effects of each delivered separately.\u003c/p\u003e\n \u003cp\u003eMoreover, we did not find any differences in blink rates across conditions (all ps\u0026thinsp;\u0026gt;\u0026thinsp;0.160), suggesting that participants did not show differences in blinking activity depending on the Hand type nor on Touch type.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003eSubjective ratings\u003c/h2\u003e\n \u003cp\u003eIn line with pupil dilation findings, we observed a main effect of Hand type [F\u003csub\u003e(1,119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;32.062, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u0026eta;\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.212], indicating that participants preferred to receive a touch from a Human hand than from an Artificial hand [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;5.662, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001]. Also, we found a main effect of Touch type [F\u003csub\u003e(1,119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;15.087, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001, \u0026eta;\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.113], indicating that participants preferred to receive a Dynamic than a Static touch [t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;3.884, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001]. Finally, we also found a significant Hand type by Touch type interaction [F\u003csub\u003e(1,119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;4.125, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.045, \u0026eta;\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.034], which showed that participants preferred to receive a Dynamic touch from a Human hand. Indeed, post-hoc pairwise comparisons showed that Dynamic touch from a Human hand condition received the highest ratings compared to all other conditions [Dynamic_Human vs. Dynamic_Artificial: t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;3.657, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001; Dynamic_Human vs. Static_Human: t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;2.343, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;=\u0026thinsp;0.021; Dynamic_Human vs. Static_Artificial: t\u003csub\u003e(119)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;7.070, \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.001] (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ee). These results, in line with physiological findings, indicate that participants rated as the most pleasant a dynamic touch delivered by a human hand.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"DISCUSSION","content":"\u003cp\u003eIn 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 \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e); 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.\u003c/p\u003e \u003cp\u003ePrevious studies \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e 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 \u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. 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 high-level characteristics of the stroking effector. Indeed, as haptic features convey information about the nature of an external object \u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, 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 \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, 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. 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 (velocity and stroking effector) can induce a significantly stronger autonomic activation than the summed effects of each delivered separately.\u003c/p\u003e \u003cp\u003e In our study, we also invited participants to rate the pleasantness of the touch they received. Consistently with prior research \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan additionalcitationids=\"CR45 CR46\" citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e, 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 \u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e, may have a preference for slow, caress-like touch \u003csup\u003e\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u003c/sup\u003e and are finely tuned to touch that mimics human skin temperature \u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. 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 \u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u003c/sup\u003e. Nonetheless, our findings emphasize the pivotal role of human contact in evoking positive emotional responses, as our participants reported the highest levels of pleasantness when tactile stimulation was delivered by a human hand at a speed resembling that of a caress. It's worth noting that these findings exhibited a similar pattern to those observed for pupil dilation. As pupil dilation has been associated with salient and rewarding stimuli \u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e,\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e and to social interest in others \u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e, a stronger pupil responses may reflect the reward-related processing of a socially relevant interaction occurring. However, although it has been reported that CT-optimal speed tactile stimulation carries a positive affective valence \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u003c/sup\u003e, pupillary responses mostly track salience (not valence) of a stimulus and several top-down contextual factors might come at play in driving the association between pleasantness and autonomic activation \u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e. Indeed, the way individuals experience social touch in general \u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e and Affective Touch specifically \u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e can be influenced by several contextual and top-down factors beyond the physical sensation of the touch itself. Hence, future studies might build upon the present results and explicitly address such an intriguing question.\u003c/p\u003e \u003cp\u003eIt 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 \u003csup\u003e\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e,\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e,\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u003c/sup\u003e has been identified as significant factor in other studies. Here the experimenter underwent a training before the beginning of the study to maintain consistent pressure, both during the dynamic and the static Touch, following procedures described in the literature \u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. Nevertheless, given the importance of applied force, future studies should also investigate this aspect in a more controlled manner as an essential aspect of Affective Touch. Next, to avoid effects of habituation and tiredness on pupillary responses \u003csup\u003e\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e, 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 \u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u003c/sup\u003e. Therefore, future research should consider adopting a larger number of repetitions when investigating the hedonic aspects of Affective Touch. Furthermore, in this study, we always employed an opposite-gender experimenter and this choice did not allow us to account for same-sex affective interactions. It would be valuable for future studies to consider these variables as well as participants' sexual preferences and cultural differences to mitigate potential interference effects and personal attitudes towards interpersonal touch. 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 \u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e,\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e. 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 \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSummarizing, 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 subjective experience in the person receiving a tactile stimulation. We not only replicated previous observations regarding each feature alone, but also reported, for the first time, that their combination triggers a stronger physiological reaction than their isolated components along with a positive hedonic experience. These results shed light on the uniqueness of real human-to-human contact in shaping Affective Touch as a means of support and affection \u003csup\u003e\u003cspan additionalcitationids=\"CR64\" citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e\u003c/sup\u003e having a strong adaptive and evolutionary value central to our relational and social development.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAUTHOR CONTRIBUTION\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eG.B. and O.D.M. designed the study, G.B. performed the experiment, G.B. and A.M. analyzed the data, and G.B., A.M., F.C., and O.D.M. wrote the paper. All authors reviewed the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eACKNOWLEDGMENTS\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eWe thank Giuseppe Pica and Luca Melchionda for assistance with data collection. We also thank Lucia De Francesco and Giulia Romano Cappi for their insightful comments on the manuscript. This work was supported by MIUR (DALO_RILO_19_01) and GFI (DALO_GFI_22_01_F) grant to ODM.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eDATA AVAILABILITY STATEMENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eData and code used for this paper\u0026rsquo;s analyses are made publicly available at https://github.com/SocialInteractionLabUnito/Pupil_AffectiveTouch\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eCONFLICT OF INTEREST STATEMENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eETHICAL STATEMENT\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe experimental procedure was approved by the Bioethical Committee of the University of Turin and conducted in accordance with the Declaration of Helsinki (World Medical Association, 2013).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eCascio, C. 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B. \u0026amp; Schirmer, A. 3D Hand-Motion Tracking and Bottom-Up Classification Sheds Light on the Physical Properties of Gentle Stroking. \u003cem\u003eNeuroscience\u003c/em\u003e \u003cstrong\u003e464\u003c/strong\u003e, 90\u0026ndash;104 (2021).\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Pupil dilation, Affective touch, Stroke velocity, Human hand, Artificial hand, Skin-to-skin touch","lastPublishedDoi":"10.21203/rs.3.rs-4696797/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4696797/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e Affective Touch is characterized by both emotional and arousing dimensions that rely on specific features of a gentle human caress. 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\u0026rsquo; pupil dilation and their subjective experience during tactile stimulation.\u003c/p\u003e \u003cp\u003eWe observed that when participants received a dynamic touch, their pupil dilation increased more when the touch was promoted by a human compared to an artificial hand. This discrimination was not present 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 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.\u003c/p\u003e","manuscriptTitle":"Affective Touch is encoded by pupil dilation as a comprehensive social phenomenon","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-08-17 02:11:34","doi":"10.21203/rs.3.rs-4696797/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-08-05T09:51:04+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-08-02T12:51:36+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-07-26T15:33:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"153571979290322202660253793048957077352","date":"2024-07-25T05:15:14+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"185105326887732177039979943808141439800","date":"2024-07-23T10:09:20+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-07-23T08:17:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-07-23T08:13:26+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2024-07-23T08:10:43+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-07-18T09:37:40+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2024-07-06T12:45:48+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"da9574d7-a376-4e01-89e7-437a4512f2d3","owner":[],"postedDate":"August 17th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-10-21T16:07:50+00:00","versionOfRecord":{"articleIdentity":"rs-4696797","link":"https://doi.org/10.1038/s41598-024-74566-3","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2024-10-16 15:57:45","publishedOnDateReadable":"October 16th, 2024"},"versionCreatedAt":"2024-08-17 02:11:34","video":"","vorDoi":"10.1038/s41598-024-74566-3","vorDoiUrl":"https://doi.org/10.1038/s41598-024-74566-3","workflowStages":[]},"version":"v1","identity":"rs-4696797","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4696797","identity":"rs-4696797","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}