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. We used two types of touch velocity: a CT-optimal
[i.e., a dynamic stroking at 3 cm/s; (Löken et al., 2009); CT-optimal
condition] and a CT-suboptimal (i.e., a static touch; CT-suboptimal
condition). Both types of touch were 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 ), over
either the dorsal side of the hand or the dorsal side of the forearm,
two CT-rich sites mostly involved in interpersonal touch (Pyasik et al.,
2022; Suvilehto et al., 2015). 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., CT-optimal_Human,
CT-optimal_Artificial, CT-suboptimal_Human, and CT-suboptimal_
Artificial) for a total of 16 tactile stimulations. For each condition,
the touch was delivered twice on the dorsal side of the hand and twice
on the dorsal side of the forearm; the order of conditions presentation
was randomized. 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).