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 (Figure 1a ). Given that in this study we recorded pupillary dilation, the experimental session started with a 9-point grid system calibration. 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. 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) (Figure 1b ). Additionally, participants received two types of touch: a dynamic [i.e., a dynamic stroking at 3 cm/s; (Löken et al., 2009); Dynamic condition] and a static touch (Static condition). 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.
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 (Cruciani et al., 2021; Pyasik et al., 2022). 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 Tobii TX300).