(A) Subject views tangent screen through a hot mirror, with cameras (short arrows) mounted overhead to image each eye. Infrared illuminators (long arrows) are stationed laterally. Infrared filter shutters occlude each eye without interrupting tracking. A central video camera (*) records the experiment. (B) Left eye in primary gaze, with corneal light reflexes from each illuminator. The white crosshair marks the pupil center. (C) Left eye abducted 40°, showing that the temporal light reflex in (B) remains on the cornea, so accurate tracking continues. The nasal light reflex is lost on the conjunctiva.(A) Subject views tangent screen through a hot mirror, with cameras (short arrows) mounted overhead to image each eye. Infrared illuminators (long arrows) are stationed laterally. Infrared filter shutters occlude each eye without interrupting tracking. A central video camera (*) records the experiment. (B) Left eye in primary gaze, with corneal light reflexes from each illuminator. The white crosshair marks the pupil center. (C) Left eye abducted 40°, showing that the temporal light reflex in (B) remains on the cornea, so accurate tracking continues. The nasal light reflex is lost on the conjunctiva.

Recordings from an 11-year-old girl with a right eye fixation preference and a refraction of: -2.25 + 2.50 x 98 (right eye) and -4.25 + 3.75 x 80 (left eye). At t = 0 s, a shutter covered either eye, causing it to move outwards. Mean position is shown for the right (red) and left eye (blue), with shading denoting standard deviation. Positive values represent right gaze. n = number of trials. (A) Shutter occlusion of the left eye produced an exotropia of 25.5° (|-24.9° – 0.6°|). The fixating right eye’s mean position deviates from 0° by less than tracking error. (B) Shutter occlusion of the right eye produced a deviation of 30.1°.

Scatter plot showing a correlation between left exotropia and right exotropia (r = 0.94) for 37 subjects, measured with the fixating eye in primary gaze and the deviated eye occluded. The gray lines define the maximum deviation from the unity line attributable to tracking error (± 1° for each eye tracker). 16/37 subjects fall outside these error bounds, indicating the presence of incomitance. Horizontal bar = SD of left exotropia, vertical bar = SD of right exotropia, blue = left eye dominant, red = right eye dominant, gray = no eye dominance. * = patient illustrated in Fig. 2.

Eye preference is not correlated with stability of the fixating eye. Data were collected while one eye was fixating and the other was occluded. The standard deviation of the position of the fixating right eye versus the fixating left eye was correlated for individual strabismic subjects (r = 0.68). However, eye dominance (red = right eye; blue = left eye) did not confer better fixation stability. If it did, red points would lie above the unity line, and blue points would fall below. + = subjects with > 2° incomitance. * = patient illustrated in Fig. 2.

Eye preference is not correlated with variability of deviation angle. The standard deviation of exotropia is plotted for the right eye fixating and for the left eye fixating for each subject. The standard deviations are correlated (r = 0.74), but there is no clear relationship with eye dominance (red = right eye dominant, blue = left eye dominant). If fixation by the dominant eye rendered the exotropia more stable, blue points would be located above the line and red points below. + = subjects with > 2° incomitance. * = patient illustrated in Fig. 2.

Difference in accommodative effort (D = diopters exerted by each eye) required to focus at 57 cm versus incomitance, with the fixating eye in primary gaze. The same accommodative effort was required in each eye in the majority of subjects, accounting for the clustering of points along the horizontal meridian. Incomitance is present only in subjects with more than 2° difference in exotropia (vertical lines). Accommodative effort could contribute to incomitance only for subjects whose data fall in shaded quadrants. A difference in accommodative effort of > 0.25 diopters potentially contributed to incomitance > 2° in only 3/16 patients. * = patient illustrated in Fig. 2.