Mechanism and paradigm.

a phenomenology results from external (solid arrow) and internal contributions (dashed arrow). The integrated brightness should affect pupil size: Light (dark) synesthetic colors should cause constrictions (dilations) at equal physical luminance in synesthetes, but not in controls where externally and internally generated brightnesses align. b We expected synethetes’ pupils to be larger for reported lower brightness and smaller for reported higher brightness. c Paradigm. Block 1: a digit was presented. Participants (except passive controls) subsequently indicated the color that most closely corresponded to the digit in their opinion. This was followed by an interstimulus interval (ISI). Block 2 (synesthetes only): a colored disk and gray central patch, matching the average indicated color per digit and the number and luminance of pixels of said digit were presented to assess externally triggered light responses. d), setting the basis for a possible inference of color phenomenology via the pupil light responses in both synesthetes and controls. Importantly, reported lightness of colors almost exactly matched between controls (M = 0.478, SD = 0.065, lightness scaled between 0 = black and 1 = white) and synesthetes (M = 0.479, SD = 0.070; t(30) = 0.031, p = 0.975, d = 0.011, 95% CI = [-0.704, 0.682]; BF01 = 2.973). Together, our synesthete participants were grapheme-color synesthetes as per the gold standard of the field (Eagleman et al., 2007), reporting specific, strong and consistent colors in response to graphemes.

a Reported colors per grapheme on all trials for synesthetes (left) and controls (right). b Synesthetes showed (near) perfect grapheme-color consistency and moderate to very strong grapheme-color couplings (rainbow circles), while controls report none to moderate coupling and vary in their consistency (grey circles); smaller values indicate higher consistency following (Rothen et al., 2013). Larger dots indicate group means. c,d (HS)Lightness of color reports per synesthete (c) and control (d). Blacked dashed line represents lightness being 0.5. between 800 ms and 4000 ms, split by (reported) color lightness, showed different pupil responses for synesthetes both for synesthetic colors (Figure 3d, t(11) = 4.399, p = 0.001) and externally triggered light responses in synesthetes (Block 2, see Figure 1c; Figure 3f, t(10) = 4.009, p = 0.003), but not in controls (Figure 3b, t(12) = 1.018, p = 0.329).

Pupil size change to graphemes, mediansplit by reported color lightness (dark gray = low lightness; light gray = high lightness).

Top row: pupil responses to graphemes in controls. Mid row: pupil responses to graphemes in synesthetes. Bottom row: pupil responses to colored discs in synesthetes (Block 2). a, c, e Depict average, baseline-corrected and within-participant demeaned pupil responses. Shaded error bands: ±1 SE across participant means. b, d, f depict mean pupil size (800–4000 ms) for dark vs. bright colors. Dots show individual participants; squares denote grand means with 95% CIs as whiskers. Dot luminance corresponds to the participant’s average synesthetic color lightness per bin, dot size to the number of trials in the respective bin. **: p < .01. Asterisks denote significance relative to 0 for lightness bins (left, right) and for the difference between lightness bins (center). Participants with less than 25 trials per bin excluded for visualization (controls: n = 3, synesthetes: n = 4, see Supplementary Figure 1 for similar visualization without data exclusion).

Results of pertime-point linear mixed effects model (LME) predicting pupil size in synesthetes while presented with graphemes.

Covariates for the individual graphemes and intercept are not visualized here. a depicts t-values of the LME over time. Horizontal lines denote significance threshold (p = 0.05 dashed, p = 0.01 dot-dashed, p = 0.001 dotted). Higher lightness was associated with smaller pupil size (red), this effect was stronger for stronger reported grapheme-color couplings (orange), as well as for higher PA scores (purple, indicating more projecting). Furthermore, higher lightness constricted the pupil more for stronger grapheme-color couplings in synesthetes with higher PA scores (gray, three-way interaction). b-d visualize interactions for the LME run on the average pupil size between 800 ms and 4000 ms. Dotted denotes low, dashed high of median splits. b Interaction of grapheme-color coupling strength with lightness: lightness affected the pupil more when grapheme color couplings were reported higher. c Interaction of PA scores with lightness: lightness affected the pupil more for synesthetes with higher PA scores, but note that this effect only reached borderline significance for a short interval. d three-way interaction of lightness, coupling strength, and PA score. see Supplementary Figure 2 for the same model in controls.

Average pupil responses to graphemes, split by group (controls picking a color forced-choice (’active’, gray) or controls passively viewing the graphemes (’passive’, black) and synesthetes (purple).

a Pupils dilated more for active controls than both synesthetes and passive controls. Shaded error bands represent 95% confidence intervals across participant means. Horizontal black line represents average pupil size during baseline. b Mean pupil size (0.8 s–4 s interval) per group and participant. Dots show individual participants; squares denote grand means with 95% CIs. c as a, but for the velocity of pupil size changes (first derivative, filtered). d as b, but for the velocity of pupil size changes and the 0.7 s-2 s interval. p < .01: **, p < .001: ***, based on two-sided independent sample t-tests.