The speed acuity test (SA)

The SA-test was administered binocularly at 5 m. Each trial consisted of a high-contrast black Landolt-C presented at the center of the computer screen against a white background. Children had to indicate, as quickly and accurately as possible, on which side, right or left, the opening of the C was located by pressing the corresponding mouse button (Fig. 1A). For the set-up at the Royal Dutch Visio, two big buttons (Supplementary Fig. ¹A, ø 125 mm each, one blue, one yellow) were used that were equivalent to mouse buttons (Eelke Verschuur, Smoothie groot). The stimulus was presented until the child responded after which a blank screen appeared for 0.5–1.5 s. Task difficulty was manipulated by presenting nine optotype sizes (− 0.43, − 0.25, − 0.13, − 0.03, 0.05, 0.27, 0.5, 0.68 and 1.09 LogMAR for children with a visual acuity of 0.1 LogMAR or better), each presented 10 times in pseudo random order. Optotype sizes were adjusted to the child’s visual acuity before the start of the SA-test (acuity between 0.1 and 0.5 LogMAR; optotype sizes − 0.13, − 0.03, 0.05, 0.27, 0.5, 0.68, 0.8, 1.09 and 1.3 LogMAR, acuity of 0.5 LogMAR or worse; optotype sizes − 0.03, 0.05, 0.27, 0.5, 0.68, 0.8, 1.09, 1.3 and 1.5 LogMAR).