INTRODUCTION
Social interaction is a fundamental aspect of human life, and
interpersonal touch plays a crucial role in shaping relationships and
encouraging social connections (Cascio et al., 2019). Notably, social
touch refers to the physical contact or tactile exchanges occurring
between individuals during social engagements. It serves as a means of
conveying greetings, affection, support, and comfort across diverse
social scenarios (Hertenstein et al., 2006). A specific kind of social
touch is Affective Touch, characterized by a gentle and enjoyable
tactile stimulation capable of triggering profound emotional reactions
and positive emotional states (Morrison et al., 2010; Morrison, 2016a).
This form of touch can foster sentiments of care, intimacy, closeness,
and trust among individuals (Field, 2010; Gulledge et al., 2007;
Robinson et al., 2015).
Recent studies have shed light on the distinctive attributes of
Affective Touch, suggesting the existence of dedicated neural pathways
and supporting its sui generis nature (Gallace and Spence, 2016;
Morrison, 2016b; Olausson et al., 2008). A specialized somatosensory
system, referred to as the CT-afferent system, stands out as it is
selectively activated by soft and gentle strokes. Specifically,
CT-fibers are sensitive to slow-moving caresses (1-10 cm/s) and exhibit
heightened activation in response to touch stimuli with a temperature
that closely aligns to human skin (i.e., 32°C) (Ackerley, 2014a; Löken
et al., 2009). These two key characteristics lend support to the notion
that CT-fibers could distinguish Affective Touch from other kinds of
touch exchange. Also, gentle stimulation of CT-innervated skin triggers
the activation of the posterior insula (Gordon et al., 2013), coupling
it with both somatosensory and reward processing regions (Sailer et al.,
2016). The posterior insula plays a pivotal role in autonomic regulation
and interoception by integrating sensory, affective, and rewarding
aspects of tactile stimulation (Morrison et al., 2010). Its direct
connection with CT-fiber stimulation (Kirsch et al., 2020) further
suggests how CT-targeted touch might trigger psychophysiological
responses characterizing Affective Touch as a fundamental mechanism for
emotion regulation and social-affective processing (Björnsdotter et al.,
2009), even though recent advances suggest the possible involvement of
Aβ mechanoreceptors contributing to the affective aspects of touch as
well (Schirmer et al., 2023).
The complex interplay between Affective Touch, emotions, and the
autonomic nervous system has been extensively investigated through
psychophysiological responses. Notably, Affective Touch has been shown
to induce transient increases in skin conductance (Olausson et al.,
2008): a response that can be influenced by salient contextual factors
both in the person receiving the touch (Nava et al., 2021; Novembre et
al., 2021) and in the person promoting it (Mazza et al., 2023a).
However, in line with the notion that Affective Touch can serve as a
potential buffer against stressful situations (Mazza et al., 2023b;
Morrison, 2016a; Walker et al., 2022) it has also been linked to
reductions in blood pressure (Grewen et al., 2005; Lee and Cichy, 2020),
stress hormone levels (Heinrichs et al., 2003; Henricson et al., 2008)
and heart rate (Pawling et al., 2017; Triscoli et al., 2017) along with
an increase in heart rate variability (Triscoli et al., 2017). Although
skin conductance and heart rate have been extensively explored as
markers of physiological modulation induced by Affective Touch, pupil
dilation, a well-established indicator of physiological activation,
remains relatively unexplored in this context (Gusso et al., 2021).
Emotional stimuli indeed trigger the release of norepinephrine, a
neurotransmitter involved in the regulation of pupil dilation (Joshi and
Gold, 2020), and heightened pupil responses have been previously noted
for both positive and negative arousing stimuli in both visual (Basile
et al., 2021; Dal Monte et al., 2015; Pagliaccio et al., 2019) and
auditory (Oliva and Anikin, 2018; Partala and Surakka, 2003) domains.
Thus, understanding the relationship between the pleasantness of
Affective Touch and pupil dilation will provide important insights into
the physiological responses evoked by this kind of tactile stimulation.
Earlier research has indicated that pupil dilation is influenced by the
speed of touch rather than its pleasantness (van Hooijdonk et al.,
2019), concluding that pupil responses primarily encode the sensory
characteristics of tactile stimulation and do not distinctly respond to
the emotional aspects of touch. However, the majority of the studies
investigating Affective Touch employed brushes or mechanical tools to
deliver tactile stimuli (Bertheaux et al., 2020; Pawling et al., 2017;
Triscoli et al., 2017; van Hooijdonk et al., 2019). This might have
restricted the possibility of targeting the hedonic effects associated
with an actual human touch. Interestingly, Ellingsen and colleagues
(2014) have reported that pupil dilates more in response to human touch
compared to artificial touch, particularly when Affective Touch is
accompanied by the presentation of images displaying a positive facial
expression. This observation implies that pupil response can discern
between distinct types of tactile interactions and potentially even
capture the emotional experience accompanying touch. Thus, a touch
promoted by a human hand, as opposed to artificial means, appears to be
a pivotal factor in evoking distinct pupillary responses that are
aligned with the emotional aspect of touch.
Although previous studies have made strides in understanding the
significance of specific attributes of Affective Touch, such as the
stroking velocity and the nature of the touching effector, they have
largely focused on investigating these features individually, examining
one characteristic at a time. Thus, this approach has made it
challenging to draw comprehensive conclusions on the intricate interplay
between these distinct characteristics and how those contribute to
eliciting a physiological response. The current study aims to explore
whether and how the nature of the stroking effector (Human vs.
Artificial) modulates pupillary responses and subjective experiences in
individuals receiving caress-like touches at CT-optimal velocity
(Dynamic condition, 3 cm/s; Löken et al., 2009). As a control,
experimental subjects also received static touch (Static condition) from
both hand types, as we aimed to ensure that any observed differences
between human and artificial hands were specific for the dynamic touch.
Our hypotheses encompass several scenarios. If pupil size merely tracked
stroking speed, as hinted by prior research (van Hooijdonk et al.,
2019), we anticipated finding greater pupil responses during a dynamic
touch condition compared to the static touch condition, regardless of
the nature of the hand promoting the touch (Human vs. Artificial).
Conversely, if pupil size only encoded the nature of the hand promoting
the touch, we expected to observe greater pupil responses during
human-initiated touch compared to artificial-initiated touch,
irrespective of the type of touch (Dynamic vs. Static). Finally, if
pupil size had the capacity to jointly encode distinct features
characterizing Affective Touch, we hypothesized that pupil responses to
dynamic touch would be specially influenced by the nature of the hand
promoting the touch. This would be reflected in larger pupil dilation
when touch is promoted by a human hand, but exclusively under dynamic
conditions.