Figure 2. A) Baseline-corrected pupil response over time after tactile stimulation, expressed in arbitrary units (a.u.), plotted per stimulation site. Positive values indicate pupil dilation, negative values indicate pupil constriction. Error bands indicate one standard error above and below the mean. B) Pupil response derivative traces over time, averaged per stimulation site. Positive values indicate the change in the amount of pupil size increase, negative values indicate the change in the amount of pupil size decrease compared to the previous time point. Error bands indicate one standard error above and below the mean. C) Linear mixed effects model for pupil response comparing t -values between stimulus locations over time in seconds. Each line represents the t -values of the comparisons between stimulus locations on pupil response after tactile stimulation over time, with an additive effect of block number and trial number within a block and random intercepts for each participant. The dotted line represents t  = |1.96|, corresponding to = 0.05. D) Time to maximum pupil response in seconds, averaged per participant and split between stimulation sites.
Tactile sensitivity thresholds as assessed with the Von Frey filaments (see Supplementary Figure 4 for the thresholds per location) differed between the three body locations,χ2 (62) = 44.43, p  < 0.001. Lower subjective tactile sensitivity thresholds were observed for the little finger versus forearm (W = 1, p < 0.001,r = -0.86), little finger versus calf (W = 1, p< 0.001, r = -0.87), and forearm versus calf (W= 65, p < 0.001, r = -0.66), all with large effect sizes.
As an exploratory analysis, for each of the pairs of body locations, we examined whether the difference in tactile sensitivity thresholds (as assessed with the Von Frey filaments) was related to the difference in pupil response. In other words, did the decrease in subjective tactile sensitivity thresholds across body locations scale with the increase in the strength of pupil responses in response to tactile stimulation? For each pair of body locations (i.e., finger-calf, finger-forearm, forearm-calf) we calculated difference scores for tactile sensitivity thresholds and for pupil responses over time. These difference scores were correlated using Spearman correlations, shown in Figure 3A, together with functional p -values, shown in Figure 3B. Differences in Von Frey tactile sensitivity thresholds and pupil derivative were positively correlated for the finger versus calf, and for the arm versus calf, at approximately 0.5 to 0.7 s after stimulation, but not for the finger versus arm. Although correlations for n = 32 must be interpreted with caution, this suggests at least tentative support for the idea that the here proposed pupillometric estimate and Von-Frey filaments measure similar or related latent tactile processing.