Pupil size reveals the perceptual quality and effortless nature of synesthesia
- Experimental Psychology, Helmholtz Institute, Utrecht University, Netherlands
- Brain and Cognition, Department of Psychology, University of Amsterdam, Netherlands
Figures
Figure 1
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 synesthetes' 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 disk was presented, colored according to the synesthetes' average indicated color for that digit. At its center sat a gray patch matching the luminance and pixel area of the original digit from Block 1, together allowing assessment of externally triggered light responses.
Figure 2 with 1 supplement
Color reports by synesthetes and controls.
(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 reported none to moderate coupling and varied in consistency (gray circles). Note that higher consistency is reflected in lower color distance, hence lower values (Rothen et al., 2013). Larger dots indicate group means. (c, d) (HS) Lightness of color reports per synesthete (c) and control (d). Black dashed line represents lightness being 0.5. See Figure 2—figure supplement 1 for color reports on the hue and saturation axes.
Figure 2—figure supplement 1
Hue (angle) and saturation (eccentricity) for color reports for (a) synesthetes and (b) controls.
Figure 3 with 3 supplements
Pupil size change to graphemes, median-split 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 participants' average synesthetic color lightness per bin, dot size to the number of trials. **p<0.01, ***p<0.001 based on within samples and one sample t-tests. Significance relative to zero for lightness bins (left, right) and between bins (center). Participants with less than 25 trials per bin excluded for visualization (controls: n = 3, synesthetes: n = 4, see Figure 3—figure supplement 1 for pupil size per grapheme, Figure 3—figure supplement 2 for visualization without data exclusion, Figure 3—figure supplement 3 for visualization without demeaning).
Figure 3—figure supplement 1
Pupil size change to graphemes, irrespective of reported color.
Horizontal dashed line represents baseline pupil size. (a) Active controls, (b) synesthetes, and (c) passive controls.
Figure 3—figure supplement 2
Pupil size change to graphemes, split by 0.5 reported color lightness (dark gray = low lightness; light gray = high lightness) without removing participants with little trials per bin.
Top row: pupil responses to graphemes in controls. Bottom row: pupil responses to graphemes in synesthetes. (a, c) depict average, baseline-corrected, and within-participant demeaned pupil responses. Shaded error bands represent ± 1 SEM across participant means. (b, d) 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 participants' average photism lightness per bin, dot size to the number of trials in the respective bin. *p<0.05, **p<0.01. Asterisks denote significance relative to 0 for lightness bins (left, right) and for the difference between lightness bins (center).
Figure 3—figure supplement 3
Pupil size change to graphemes, split by 0.5 reported color lightness (dark gray = low lightness; light gray = high lightness) without demeaning (i.e., removing the average pupil response shape in the 4 s stimulus interval per individual irrespective of brightness perception).
Top row: pupil responses to graphemes in controls. Bottom row: pupil responses to graphemes in synesthetes. (a, c) depict average, baseline-corrected pupil responses. Shaded error bands represent ± 1 SEM across participant means. (b, d) 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 participants' average photism lightness per bin, dot size to the number of trials in the respective bin. ***p<0.001. Asterisks denote significance between lightness bins (center).
Figure 4 with 3 supplements
Results of per-time-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), with a trend for higher PA scores (purple). 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. See Figure 4—figure supplement 2 for visualization of this interaction. (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 Figure 4—figure supplement 3 for Block 2 featuring colored discs.
Figure 4—figure supplement 1
per time point LME (intercept and graphemes not shown) for the control group indicating colors.
Without PA score predictor, as PA score was not assessed in controls.
Figure 4—figure supplement 2
Demeaned pupil responses to digits eliciting dark (black) versus bright (gray) lightnesses (0.5 cutoff).
Solid lines depict responses for graphemes rated with high coupling strength (3,4, strong), dashed for graphemes with low coupling strength (1,2, weak). Participant counts per line: weak dark: n=11, weak light n=10, strong dark n=15, strong light n=15.
Figure 4—figure supplement 3
Results of per-time-point linear mixed effects model predicting pupil size in synesthetes in Block 2 (colored discs).
Covariates for the individual graphemes and intercept are not visualized here. All visualizations as in Figure 4 of the main manuscript.
Figure 5
Average pupil responses to graphemes from baseline, split by group: controls picking a color forced-choice (‘active’, gray), 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% CIs across participant means. Horizontal black line represents average pupil size during baseline. (b) Mean pupil size (0.8–4 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 0.7–2 s interval. **p<0.01: ***p<0.001 based on two-sided independent sample t-tests.
Appendix 1—figure 1
Questionnaire used to assess the coupling strength between grapheme and color in synesthetes and active controls.
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Appendix 1—figure 1—source data 1
Source table for Appendix 1—figure 1.
- https://cdn.elifesciences.org/articles/110390/elife-110390-app1-fig1-data1-v1.xlsx
Appendix 1—figure 2
Screening questionnaire to identify and recruit participants with grapheme-color synesthesia.
Additional files
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MDAR checklist
- https://cdn.elifesciences.org/articles/110390/elife-110390-mdarchecklist1-v1.docx
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Appendix 1—figure 1—source data 1
Source table for Appendix 1—figure 1.
- https://cdn.elifesciences.org/articles/110390/elife-110390-app1-fig1-data1-v1.xlsx
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Pupil size reveals the perceptual quality and effortless nature of synesthesia
eLife 15:RP110390.
https://doi.org/10.7554/eLife.110390.4
