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, revealing 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 et al., 2014a;
Löken et al., 2009). These two unique characteristics lend support to
the notion that CT-fibers are finely tuned to warm and gentle human
contact, effectively distinguishing Affective Touch from other kinds of
touch exchange. Moreover, 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 underscores the distinctiveness of Affective Touch as
a fundamental mechanism for emotion regulation and social-affective
processing (Björnsdotter et al., 2009).
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 an increase 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) and in the person
promoting it (Mazza et al., 2023a). In line with the notion that
Affective Touch can also serve as a potential buffer against stressful
situations (Mazza et al., 2023b; Morrison, 2016a) it has 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
(Gusso et al., 2021), remains relatively unexplored in this context.
Indeed, Affective Touch by engaging the sympathetic nervous system leads
to the release of norepinephrine, a neurotransmitter involved in the
regulation of pupil dilation (Aston-Jones and Cohen, 2005). Heightened
pupil responses have been previously noted for both positive and
negative arousing stimuli in visual (Basile et al., 2021; Dal Monte et
al., 2015; Pagliaccio et al., 2019) and auditory (Partala and Surakka,
2003) domains. 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 (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. Building on this premise, a few
investigations have discovered that the 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 (Ellingsen et al., 2014). 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 bridge
this gap by exploring whether and how the nature of the stroking
effector and the speed of touch interact to shape pupillary responses.
Our investigation delves into how touch velocity (CT-optimal vs.
CT-suboptimal) and the nature of the hand promoting the touch (Human vs.
Artificial) influence both pupil dilation and the subjective experience
in the person receiving tactile stimulations. We seek to ascertain
whether pupil size can distinctly encode Affective Touch, by
preferentially reacting to CT-optimal touch promoted by a Human hand.
Our hypotheses encompass several scenarios. If pupil size indeed encodes
the nature of the hand promoting the touch, we expect to observe greater
pupil responses during Human-initiated touch compared to
Artificial-initiated touch, irrespective of velocity. Conversely, if
pupil size merely tracks stroking speed, as hinted by prior research
(van Hooijdonk et al., 2019), we anticipated to find greater pupil
responses during the CT-optimal velocity condition compared to the
CT-suboptimal velocity condition, regardless of the nature of the hand
promoting the touch (Human vs. Artificial). Finally, if pupil size has
the capacity to encode Affective Touch as a whole, we hypothesize that
pupil responses to CT-optimal touch will be further 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 CT-optimal velocity conditions.