Present findings and theoretical contributions
In the current study, pupils responded more strongly to tactile
stimulation of more sensitive parts of the body. Specifically, stronger
pupil responses were observed after stimulation of the little finger
versus calf, the little finger versus forearm, and the forearm versus
calf. Secondly, pupils responded more strongly to more intense vibratory
stimulation on the finger as compared to less intense vibratory
stimulation. In both experiments, the enhanced response was reflected in
a greater increase in pupil size already within 0.5s of stimulation,
providing the basis for short measurement times and therefore minimal
effort on the part of the participant (to maintain fixation), as there
is sufficient time between trials for the pupil size to return to
baseline. Our results align with previous findings from less systematic
and less controlled studies, in which greater pupil dilations were
observed in response to more intense thermal stimulation (Drummond &
Clark, 2023; Eisenach et al., 2017) and faster stroke velocity (van
Hooijdonk et al., 2019).
The greater increase in pupil size in response to stimulation of more
sensitive body parts or more intense stimulation is likely due to more
intense cognitive processing associated with greater noradrenaline
release (Alnaes et al., 2014; Aston-Jones & Cohen, 2005; Joshi et al.,
2016; Murphy et al., 2014; Strauch et al., 2022), which has been
interpreted to subserve the communication between neural populations
(Dahl et al., 2022; Wainstein et al., 2022).
It is important to note that overt responses to tactile stimulation may
not solely reflect the objective intensity of the brain’s processing of
tactile stimulation, but also less tangible aspects such as
interoception and response biases. The here proposed method allows for a
more refined approach to disentangling these components and studying
where they align or diverge, respectively. The fact that our pupil-based
assessment of tactile processing intensity showed moderate correlations
with subjective indicators of tactile perception for Experiment 1, but
not Experiment 2 may be seen as a first pointer that the here indexed
processing intensity does not necessarily always reflect overt
responses.