The average proportion of horizontal fixations across age bins 1 s.e.m. (points and error bars, respectively). n = 6,720 participants were included in the analysis; for a more detailed distribution of participants across age groups, please refer to Fig. 2. Please note that each age bin spans two years. Due to space constraints, only every second tick mark is labelled. The black line shows the best-fitting polynomial. The horizontal lines at the top indicate the level of evidence for a given bin to be part of a broad linear change via shades of grey, as shown in the key (sliding window analysis; see main text and Methods). The red shading marks age bins with lower sensitivity for developmental changes, which were disregarded for curve fitting ( n < 90 per bin).
