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.