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).