Results

Figure 2A shows the pupil size as compared to baseline over time, and Figure 2B shows the pupil size derivatives (see Supplementary Figure 1 for plots per participant). First, we found that tactile stimulation at each of the three body locations resulted in larger pupil size derivatives as compared to those in the control condition (see Supplementary Figure 2 for statistical comparisons), demonstrating that the pupil responds to tactile stimulation.
In particular, the pupil showed a faster change in the amount of increase after stimulation of the little finger versus forearm, little finger versus calf, and forearm versus calf. In Figure 2C, the results of the linear mixed effects model are plotted, showing thet -values for comparisons between stimulus locations over time. The results remained conceptually unchanged when simple functionalt -tests were performed on the mean traces per condition and participant instead of LMEs (Supplementary Figure 3). Thus, tactile stimulation to the presumably more sensitive body locations resulted in stronger pupil responses as compared to the less sensitive body locations.
The time to the maximum pupil response (Figure 2D) differed between the three stimulation sites, χ2(1596) = 7.76, p = 0.021. The maximum pupil response derivative was later for the calf than for the little finger (W  = 139053, p  = 0.037, r = -0.10), which was a small effect. There were no differences in the time to maximum derivative between the calf and the forearm (W  = 145287, p  = 0.200, r = -0.06) and between the forearm and the little finger (W  = 151355, p = 0.564,r = -0.02).