(A) Recordings from a 26-year-old woman (plano OU) during fixation on a 0.5° target at 57 cm. The mean position (solid line) and standard deviation (shading) are shown for the right eye (red) and left eye (blue). At t = 0 s, a shutter occluded either the right eye or the left eye, inducing an exodeviation. n = number of events. Positive and negative values for horizontal deviation denote right and left gaze respectively. (B) Spontaneous exotropia occurring intermittently between cover-induced episodes has an amplitude nearly equal to shutter-induced exotropia, with a difference of 0.1° for right exotropia and 0.4° for left exotropia. The shapes of the mean position traces and the variability in individual position traces are also similar. Traces are aligned at t = 0 s, with movement onset. (C) Recordings from a 36-year-old woman (-1.00 sphere OU), showing a larger exotropia. (D) Interspersed episodes of spontaneous fusion loss are similar to those from shutter occlusion, with an amplitude difference of 0.7° for right exotropia and 1.2° for left exotropia.

Recordings from a 35-year-old man (-1.25 + 0.75 OD; -1.25 OS), showing that (A) right exotropia induced by occlusion was smaller and more variable than (B) spontaneous right exotropia. (C) Covering the dominant left eye produced a highly variable outwards eye movement; spontaneous left exotropia never occurred. See Supplementary Video.

Deviation amplitude is correlated with peak velocity of the deviating eye’s movement during onset of spontaneous exotropia.

Peak velocity of the deviating eye’s movement is similar for spontaneous exotropia compared with shutter-induced exotropia.

A) A 35-year-old man, with left eye dominance. Strategy 1: after the non-dominant right eye was uncovered at t = 0 sec, it made a vergence-like adducting movement. Strategy 2: after the dominant left eye was uncovered, an alternating saccade was made, followed at variable intervals by a vergence-like movement of the right eye. Blinks have been removed (see Supplementary Video). B) A 31-year-old man, with left eye dominance. After the right eye was uncovered, Strategy 1 was employed. Convergence (small arrow) was faster than divergence (large arrow), which occurred with subsequent episodes of fusion loss. Strategy 3: after the left eye was uncovered, a disconjugate saccade (much larger in the adducting, left eye) was made, followed by vergence movements (arrows). The vergence movement was larger in the non-dominant right eye. (C) An 11-year-old girl, with right eye dominance. Strategy 4: After the dominant eye was uncovered, an alternating saccade was made to bring it onto the target (first stage of Strategy 2). This was followed by a disconjugate saccade and asymmetrical vergence movement (Strategy 3). After the left eye was uncovered, this child used Strategy 3 to recover fusion.

The eye trackers were calibrated at distance; 0° refers to fixation at 305 cm. A) Task 1: spontaneous episodes of right exotropia (amplitude 15.0°, 23 peak velocity 20.1 °/sec) in a 46-year-old man during fixation at 305 cm. B) Task 2: exotropia (amplitude 14.2°, peak velocity 24.5 °/sec) following shutter occlusion of the right eye, while fixating on a crosshair at ~ 21 cm from the left eye along its line of sight with a distant laser spot. The intent was to mimic the eye movement in (A), which moved the right eye from -15.0° to 0°. The crosshair may have been placed a few cm closer than intended, resulting in a movement of the right eye from -18.1° to -3.9°. C) Task 3: gaze shift along left eye’s line of sight from a crosshair nominally at 21 cm to a laser spot at 305 cm resulted in an outwards movement of the right eye (amplitude 18.1°, peak velocity 142.2 °/sec) that combined divergence with adisconjugate saccade. Replication of the right eye’s movement in (B) was intended, but foiled by the occurrence of a saccade. D) Task 4: Same task as (C), except that when the distant laser spot appeared, a shutter simultaneously covered the right eye. The right eye moved outwards (amplitude 33.0°, peak velocity 43.4 °/sec) from -17.4° to 15.3°, to assume the same final position as in (A).

Data are shown for 10/13 patients; the other 3 are illustrated in Figs. 1 & 2. Only subjects E and G, and the subject in Fig. 2 showed a measurable difference between shutter and spontaneous exotropia. Mean traces in bold; red = right eye; blue = left eye; n = number of episodes.

Example traces showing peak velocity of eye movement in exotropia, derived by differentiation of eye position versus time plots. A) Data from subject illustrated in Fig. 1A, for exotropia induced by shutter. B) Data illustrated in Fig. 1B, for 24 spontaneous exotropia. C) Data from subject illustrated in Fig. 1C, for exotropia induced by shutter. D) Data illustrated in Fig. 1D, for spontaneous exotropia. Peak velocities for each subject are plotted in Figs. 3 & 4.