Pupillometry analysis
Pupillary changes were first baseline corrected on a trial-by-trial
basis by subtracting the mean change in pupil diameter 1000ms before the
beginning of tactile stimulations. Next, to control for inter-individual
variability, pupil data were Z-scored for each subject across all
conditions (Basile et al., 2021; Rudebeck et al., 2014). In each trial,
missing samples due to blinks or loss of the eye-tracking signal during
the tactile stimulation period were interpolated via spline
interpolation using the nearest valid adjacent samples. Pupil responses
were then averaged across trials for each condition. Based on visual
inspection of the average response profile, the mean change in pupil
diameter was extracted for the time window ranging from 0-4 seconds
after stimulus onset (Figure 2b ). Data were analyzed via a
2-way repeated-measures ANOVA with Hand type (Human vs. Artificial) and
Touch type (Dynamic vs. Static) as within subject factors. Post-hoc
analyses following significant main effects and interactions were
performed by running two-tailed pairwise t-tests, and multiple
comparisons were corrected using false discovery rate (FDR; Benjamini &
Hochberg, 1995). All p values < 0.05 were considered
significant.
To test the hypothesis that Dynamic_Human touch alone induced a larger
pupil size than Dynamic_Artificial plus Static_Human touch,
supralinearity was quantified by contrasting, for each participant, the
average pupil size in the Dynamic_Human condition against the sum of
the average pupil size in the Dynamic_Artificial plus Static_Human
conditions. The effect of Dynamic_Human condition was then compared
with the added Dynamic_Artificial and Static_Human condition with a
paired-sample t test to determine significance.
Last, we investigated whether the blink rate changed across the four
conditions by using nonparametric Wilcoxon Signed-Rank tests.