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 p = 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.