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).