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).