DISCUSSION
In the present study, we investigated whether Affective Touch is encoded
as a comprehensive social phenomenon by the autonomic nervous system. We
measured participants’ pupil responses and pleasantness ratings while
manipulating two key features characterizing Affective Touch, that are
the stroking velocity and the sensory characteristics of the stroking
hand. Overall, we observed a notable increase in pupil dilation when the
touch received fell within the CT-optimal stroking range (3 cm/s; Löken
et al., 2009) and was administered by a human hand. Additionally,
participants’ self-reports consistently indicated that this type of
touch was perceived as the most pleasant in comparison to all other
touch conditions. This outcome aligns with existing research suggesting
that C-tactile afferents, the neural pathways responsible for the
emotional and rewarding aspects of touch (McGlone et al., 2014), exhibit
a preference for slow, caress-like touch (Löken et al., 2009) and are
finely attuned to touch that mimics human skin temperature(Ackerley et
al., 2014b). Thus, these findings emphasize the pivotal role of human
contact in evoking positive emotional responses.
In line with the well-documented positive effects of Affective Touch on
human well-being, other studies investigating the autonomic responses to
Affective Touch reported positive effects on both cardiovascular (Grewen
et al., 2005; Lee and Cichy, 2020; Triscoli et al., 2017) and hormonal
systems (Heinrichs et al., 2003; Henricson et al., 2008) of individuals
receiving a touch. Also, physiological coupling has been observed among
dyads during interpersonal touch (Chatel-Goldman et al., 2014), further
highlighting its pivotal role in social bonding. However, it is
noteworthy that most of these studies employed artificial tools to
reproduce Affective Touch at a CT-optimal speed (Bertheaux et al., 2020;
Triscoli et al., 2017; van Hooijdonk et al., 2019). While this approach
is valuable for precisely controlling stroking velocity, it may lack
ecological validity as it does not account for the nuances of
human-to-human tactile interactions. Our results add knowledge to this
body of work by revealing that pupil responses are generally more
pronounced when the touch is administered by a human hand compared to an
artificial hand. This observation aligns with a previous study by
Ellingsen and colleagues (2014), which similarly found that pupil
dilation is more responsive to human touch than machine touch. Pupil
dilation is associated with salient and rewarding stimuli (Beatty, 1982;
Laeng et al., 2012) and reflects social interest in others (Laeng and
Falkenberg, 2007): this suggests that touch promoted by a real human
hand may be perceived as more rewarding, thereby eliciting a stronger
pupil response.
Moreover, we found that the stroking speed is also another crucial
element for eliciting a high pupil dilation, but only when the touch is
promoted by a human hand. Van Hooijdonk and colleagues (2019)
investigated and compared the impact of different stroking velocities on
autonomic parameters, including pupil dilation. They reported, in line
with our findings, that pupil dilation increased as a function of
stimulation velocity. Thus, our findings not only corroborate previous
research but also expand on the concept that pupil response depends on
the interaction between stroking velocity and low-level characteristics
of the stroking effector. Both the temperature and the softness of the
touching hand might be responsible for the strong modulation observed in
pupil dilation. Indeed, the greatest pupil dilation occurred when the
touch resembled a soft stroke that activated CT-system fibers at both
optimal temperature and velocity. CT-fibers are known to convey feelings
of pleasure and comfort (von Mohr et al., 2017), and the soothing and
beneficial effects of this pleasant touch provide evidence supporting a
direct connection between CT-fiber stimulation and autonomic modulation
(Fotopoulou et al., 2022; Triscoli et al., 2017). Taken together, our
results consistently support the idea that Affective Touch is linked to
autonomic regulation and that pupil size encodes Affective Touch not
only for the speed or the effector features, but as a holistic
experience. Indeed, we observed a higher pupil dilation when touch was
delivered simultaneously at CT-optimal speed and by a human hand. Also,
the observation of supralinear enhancement of pupil dilation in this
kind of touch further supports the idea that the combination of these
two features can induce a significantly stronger autonomic activation
than the summed effects of each delivered separately.
In our study, we also invited participants to rate the pleasantness of
the touch they received. Consistent with prior research (Ali et al.,
2023; Pfabigan et al., 2023; van Hooijdonk et al., 2019; von Mohr et
al., 2017; Zheng et al., 2021), our participants reported higher levels
of pleasantness when the touch was administered at the CT-optimal speed
and when delivered by a human rather than an artificial hand. Crucially,
self-reports mirrored pupil results by revealing that participants
reported the highest ratings of pleasantness when tactile stimulation
featured both CT-optimal speed and human contact simultaneously. These
finding reinforce the notion that touch is perceived as more pleasant
when it exhibits characteristics associated with human touch (Schirmer
et al., 2023; Wijaya et al., 2020). It also underscores that real
human-to-human interaction triggers a wide range of positive
physiological, emotional, and behavioural effects that cannot be
replicated by an artificial social touch (Willemse et al., 2017).
This is the first study investigating the combined effects of two key
low-level features characterizing tactile interaction on both
physiological activity and perceived pleasantness. Our results emphasize
the uniqueness of Affective Touch as a standalone form of social
interaction characterized by specific sensory attributes. Indeed, this
type of social touch not only conveys positive emotions but also elicits
feelings of support and affection (Bytomski et al., 2020; Croy et al.,
2016; Lo et al., 2021). Collectively, our findings suggest that the
connection between Affective Touch and pleasantness hinges on the
sensory characteristics inherent to human-to-human tactile interactions.
These features might be crucial for experiencing Affective Touch as a
rewarding experience having a strong adaptive and evolutionary value
central to our relational and social development.
It is important to acknowledge the possible limitations in our study and
consider potential avenues for future research. Firstly, in our study,
we only examined two different stroke speeds. Future investigations
should explore a broader range of stroke-speed conditions while still
using a human hand, as a touch promoted by a human hand is central for
analysing the affective and social aspects of Affective Touch. Secondly,
touch pressure and intensity, which have been identified as significant
factors in other studies (Case et al., 2021; Iriki et al., 1996; van
Hooijdonk et al., 2019; Wolfgang Ellermeier and Westphal, 1995), should
also be investigated as essential features of Affective Touch.
Furthermore, it would be valuable for future studies to consider
participants’ personal attitudes towards interpersonal touch by using
targeted questionnaires. For instance, research has indicated that
individuals who lacked tactile, enjoyable experiences with close family
members during early development may perceive Affective Touch as less
pleasant (Sailer et al., 2016). Lastly, our study focused exclusively on
young subjects. Future research should expand upon these findings and
explore the effects of age. A more diverse and heterogeneous sample
could provide further insights into the hedonic and physiological
responses related to Affective Touch throughout the lifespan (Cascio et
al., 2019; Sehlstedt et al., 2016).
Summarizing, the present study investigated how two key features
characterizing Affective Touch such as touch velocity and the nature of
the hand promoting the touch influence both pupil dilation and the
subjective experience in the person receiving a tactile stimulation. We
not only replicated previous observations regarding each feature alone,
but also unveiled, for the first time, that their combination is crucial
in promoting a stronger physiological activation and an optimal hedonic
experience. In fact, we found that both pupil dilation and degree of
pleasantness were significantly higher when tactile stimulation was
delivered by a human hand and with a speed resembling the one used
during a caress. These combined sensory elements appear to be
fundamental for experiencing Affective Touch as a comprehensive and
rewarding phenomenon, one that holds central importance in our social
lives.