Interim discussion
As hypothesised, tactile stimulation elicited pupil dilation responses
that could not be attributed to visual or auditory input. In particular,
we observed more pronounced pupil dilation following tactile stimulation
of body locations with a presumed higher tactile sensitivity.
Specifically, stronger responses were observed for the little finger
versus calf, the little finger versus forearm, and the forearm versus
calf. A slower pupil response to tactile stimulation of the calf
compared to the finger could partly contribute to the differences in
pupil response strength between these body locations. However, no
differences were found in the latency of the pupil response between the
forearm and the finger. The presumed difference in subjective tactile
sensitivity between the finger, the arm, and the calf was replicated
using a tactile detection paradigm with Von Frey filaments and showed
superior tactile detection abilities for the little finger versus calf,
the little finger versus forearm, and the forearm versus calf,
consistent with existing work (Weinstein, 1968). A tentative direct
relationship was observed between the differences in pupil responses
between pairs of stimulated body locations (although not for the finger
versus arm comparison), and the differences in tactile sensitivity
between the same pairs. However, these relationships must be interpreted
with caution due to the relatively small sample size for the correlative
analyses. In conclusion, our results show that tactile stimulation
applied to different parts of the body elicits pupil responses of
varying magnitude, probably due to differences in tactile sensitivity.
In Experiment 2, we investigated whether pupil responses following
tactile stimulation at a single body location would scale with stimulus
intensity, expecting stronger pupil responses with more intense
stimulation as shown in less controlled previous work (van Hooijdonk et
al., 2019).