DISCUSSION
In the present study, we investigated whether Affective Touch is encoded
as a comprehensive social phenomenon at the autonomic level. We explored
whether and how the nature of the stroking effector (Human vs.
Artificial) modulates pupillary responses and subjective experiences in
individuals receiving a caress-like touch. We employed a static touch as
a control to ensure that any observed differences were specific only for
touch delivered at CT-optimal speed (3 cm/s; Löken et al., 2009) and not
for other types of touch. Overall, we observed that when participants
received a dynamic touch, they displayed an increase in pupil dilation
for touch administered by a human compared to an artificial hand.
Interestingly, such a difference did not emerge for the control static
touch condition. Additionally, participants’ self-reports consistently
indicated that dynamic touch delivered by a human hand was perceived as
the most pleasant in comparison to all other touch conditions.
Previous studies (van Hooijdonk et al., 2019) investigated and compared
the impact of different stroking velocities on autonomic parameters,
including pupil dilation, and reported that pupil dilation increases as
a function of stimulation velocity. However, it is noteworthy that most
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 as we found that a
dynamic touch elicits higher pupil dilation responses but only when
touch is characterized by skin-to-skin contact. Thus, the autonomic
nervous system seems to encode also low-level characteristics of the
stroking effector. Indeed, as
haptic features convey information about the nature of an external
object (Kappers and Bergmann Tiest, 2013), both the temperature and the
softness of the touching hand likely inform the nervous system that the
dynamic touch is coming from another individual. As such, this
information becomes socially relevant (Cascio et al., 2019), yielding
autonomic reactions such as the strong modulation we observed in pupil
dilation. Taken together, these 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. 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. Consistently 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 received a gentle stroking promoted by a human
rather than an artificial hand. This suggests that C-tactile afferents,
the neural pathways responsible for the emotional and rewarding aspects
of touch (McGlone et al., 2014), may have a preference for slow,
caress-like touch (Löken et al., 2009) and are finely tuned to touch
that mimics human skin temperature (Ackerley, 2014b). However, recent
evidence has begun to challenge the complex but apparently not direct
relationship between Affective Touch and CT-system, given that numerous
unresolved questions have emerged about the mechanisms of CT-fibers and
their role in affect and emotion (Schirmer et al., 2023).
Nonetheless, our findings emphasize the pivotal role of human contact in
evoking positive emotional responses. Crucially, participants’
self-reports aligned with the pupil results, indicating that they
reported the highest levels of pleasantness when tactile stimulation was
delivered by a human hand at a speed resembling that of a caress. This
observation implies that the heightened arousal, as indexed by larger
pupil size, could be linked to a more positive hedonic experience
reported by the participants during that kind of touch. Indeed, pupil
dilation has been also 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 stronger pupil
responses may reflect the reward-related processing of a socially
relevant interaction occurring. These results 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), and underscore that real human-to-human interaction triggers a
wide range of positive physiological, emotional, and behavioral effects
that cannot be replicated by an artificial social touch (Willemse et
al., 2017).
It is important to acknowledge some 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, and possibly compare these effects with those of an
artificial hand from both a physiological and hedonic point of view.
Secondly, touch pressure and intensity 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). The experimenter was trained to maintain the pressure
consistently, as described in the literature (Pawling et al., 2017; van
Hooijdonk et al., 2019). Nevertheless, given the importance of the
applied force, future studies should also investigate this feature in a
more controlled manner as an essential aspect of Affective Touch. Next,
to avoid effects of habituation and tiredness on pupillary responses
(Morad et al., 2000), in our study we only exposed participants to four
trials per condition. However, even though most studies adopted less
than 10 trials, recent research showed that this might not be an
adequate number of repetitions (Schirmer et al., 2022). Therefore,
future research should consider adopting a larger number of repetitions
when investigating the hedonic aspects 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
(Devine et al., 2020; Sailer and Ackerley, 2019). 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 a touch resembling a caress
was delivered by a real human hand compared to an artificial hand. These
results shed light on the uniqueness of real human-to-human contact in
shaping Affective Touch as a means of support and affection (Bytomski et
al., 2020; Croy et al., 2016; Lo et al., 2021) having a strong adaptive
and evolutionary value central to our relational and social development.