Results

Figure 4A shows the changes in pupil size relative to baseline over time, Figure 4B shows the pupil size derivatives (see Supplementary Figure 5 for plots per participant). First, we found that tactile stimulation at each of the three stimulation intensities resulted in larger pupil size derivatives as compared to the no-stimulation condition (see Supplementary Figure 6 for the statistical comparisons), replicating the stimulation effect reported in Experiment 1.
We found faster increases in pupil size following high versus low intensity tactile stimulation, and medium versus low intensity tactile stimulation. No difference was found between high and medium intensity stimulation. In Figure 4C, the results of the linear mixed effects model are plotted, showing the t -values for comparisons between stimulus intensities over time. The results remained conceptually unchanged when simple functional t -tests were performed on the mean traces per condition and participant (Supplementary Figure 7). Thus, tactile stimulation at higher stimulus intensities resulted in larger pupil responses as compared to lower stimulus intensities.
The time to maximum pupil response (Figure 4D) did not differ between the three stimulus intensities, χ2(1998) = 0.58, = 0.748, indicating that differences in pupil response after tactile stimulation were not driven by differences in pupil response latency.