Figure 4. A) Baseline-corrected pupil response over time after
tactile stimulation, expressed in arbitrary units (a.u.), plotted per
stimulation intensity. 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 intensity. Positive values
indicate the change in the amount of pupil size increase, negative
values 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 stimulation intensities over
time in seconds. Each line represents the t -values of the
comparisons between stimulation intensities on pupil response after
tactile stimulation over time, with an additive effect of trial number
and random intercepts for each participant. The dotted line representst = |1.96|, corresponding to p = 0.05.D) Time to maximum pupil response in seconds, averaged per
participant and split between stimulation intensities.
Participants performed above chance level in the forced-choice task
discriminating between the three stimulus intensities (low vs. medium:W = 0, p < 0.001, r = 0.51; low vs. high:W = 1, p < 0.001, r = 0.92; medium vs.
high: t (18) = 5.22, p < 0.001, d = 3.92;
see Supplementary Figure 8A for performance per stimulus intensity
pair). Performance differed between the three stimulus intensity pairs,χ2 (38) = 26.63, p < 0.001.
Participants performed worse in discriminating the medium vs. high
intensities as compared to the low vs. high intensities (W = 0,p = 0.002, r = -0.82), and as compared to the low vs.
medium intensities (W = 0, p = 0.001, r = -0.85),
both with large effect sizes. No differences were found between the low
vs. medium intensities as compared to the low vs. high intensities
(W = 6, p = 0.655, r = -0.10). Thus, participants
were better able to discriminate the low intensity from the medium and
high intensities; whereas the medium and high intensities were perceived
as being more similar.
Participants differed in the certainty of their responses in the
stimulus intensity discrimination task,
χ2(38) = 25.62, p < 0.001 (see
Supplementary Figure 8B for the certainty scores per stimulus intensity
pair). Participants were less confident discriminating the medium vs.
high intensities as compared to the low vs. high intensities
(W = 5, p < 0.001, r = 0.83), and as
compared to the low vs. medium intensities (W = 1, p< 0.001, r = 0.87), both with large effect sizes. There
were no differences between the low and medium intensities as compared
to the low and high intensities (W = 44, p = 0.124,r = 0.38). Thus, the certainty scores were consistent with the
accuracy in discriminating the different stimulus intensities.
As an exploratory analysis, we examined whether the ability to
discriminate between the medium and high stimulus intensities, and the
certainty of this judgement, were correlated with the difference in
pupil size response to tactile stimulation at medium and high stimulus
intensities. Only the effect for the medium vs. high stimulus intensity
was evaluated, as participants showed the most variation in their
discriminative performance for this stimulus pair.
We calculated difference scores for the discriminability and certainty
scores for tactile stimulation of the medium vs. high stimulus intensity
and for pupil responses over time. These difference scores were
correlated using Spearman correlations. No consistent correlations
between differences were observed over time (see Supplementary Figure 9
for details).