Statistical analyses
The four stimulus intensities (i.e., none, low, medium, high) were
included as predictors of pupil size in a linear mixed effects model.
The trial number was also included to control for possible habituation
effects.
As in Experiment 1, the time to the maximum pupil size derivative was
compared between the four stimulus intensities.
To assess whether participants could discriminate the different stimulus
intensities within each pair, we tested whether the number of correct
responses was above chance level (i.e., 5 out of 10 correct responses)
using parametric one-sample t -tests or non-parametric Wilcoxon
signed rank tests. Next, we compared the performance (i.e., number of
correct responses) and participants’ confidence in their responses
between the three pairs of stimulus intensities using a non-parametric
Friedman’s ANOVA and post-hoc Wilcoxon signed-rank tests. One
participant had no data available on the discrimination task and was
excluded from these analyses.