Extended Data Figure 7. Physical image properties that constitute eyes vary significantly from video stimulus to video stimulus in lightness, color, contrast, orientation gradients, and motion.

a, Still images sampled from videos depicting dyadic mutual gaze stimuli (an entreating caregiver, engaging the child in mutual gaze and play routines). Still images from 5 of 15 videos are shown (all 15 dyadic mutual gaze videos included in actual analyses). b, Eye region demarcated from each still image in (a). Across all demarcated eye regions, across all frames of videos presented, physical image property profiles were analyzed. In the row to the right of each representative still image and corresponding eye region, physical image property profiles, analyzed across all video frames, are given as histograms. c, Lightness. d, Red-green color opponency. e, Yellow-blue color opponency. f, Contrast. g, Orientation gradients. h, Motion. i, For each physical image property analyzed in columns (a-h), row (i) gives corresponding comparison plots across the 5 histograms located in the column directly above. j, Statistical comparisons of the measured image property distributions by 2-sample Kolmogorov- Smirnov test. P values are corrected for multiple comparisons by the Bonferroni method. For each of the physical image properties analyzed in columns (a-h), row (j) presents the corresponding matrix of statistical comparisons (i.e., the 1^st row of colored circles presents comparisons for video 1 vs. 2, video 1 vs. 3, etc.; while the 2^nd row presents comparisons for video 2 vs. 3, 2 vs. 4, etc.). k, Still images sampled from videos depicting triadic peer interaction stimuli (scenes of children interacting in a daycare setting). Still images from 5 of 12 videos are shown (all 12 triadic peer interaction videos included in actual analyses). l, Eye regions demarcated from each still image in (k). m-t, All parts of (m-t) are as in (c-j).