Saccade Strategy in Alternating Exotropia

Abstract

Patient with exotropia frequently alternate fixation, looking at something with one eye and then switching their attention to acquire a new target with the other eye. Which eye informs the brain about the location of the new target? To address this issue, we presented targets dichoptically to 16 exotropes that were visible to the fixating eye, the deviated eye, or to both eyes. We then compared the subjects' choice of eye for target acquisition with the organization of their suppression scotomas. There was a correspondence between suppression scotoma maps and the eye used to acquire peripheral targets. In other words, a target perceived via an eye was also fixated by it. These studies reveal how patients with alternating strabismus, despite eye misalignment, manage to localize and fixate efficiently visual targets in their environment.

Keywords: diplopia; eye fixation preference; ocular dominance; scotoma; suppression.

Figure 1:

Schematic representation of portions of the visual field perceived via each eye (red = by the right eye, blue = by the left eye), shown here during left eye fixation. Gray shading indicates suppressed temporal retina in each eye. F = fovea.

Figure 2

(top): An exotrope views the cut end of the fish with her left eye(L = left fovea), while the right eye is deviated(R = right fovea). (bottom): She then switches her gaze to the saw blade, acquiring it with her right eye. This study asks which eye provided the brain with information about the location of the saw blade, so that an accurate saccade could be made?

Figure 3:. Target acquisition by a 36-year-old subject with alternating exotropia

a)Saccade data for blue cross/blue target trials. The targets appeared randomly at interstices of the grid. Red(right eye)or blue(left eye)dots indicate which eye was closest after a target acquisition, with the error indicated by a line to the target location. b)Saccade data for red cross/red target trials. R (right) and L (left)indicate mean positions of the eyes during central cross fixation.

Figure 4:. Crossover saccades elicited by a target presented nasally to a temporally deviated eye

a)Red cross/blue target trials. b)Blue cross/red target trials. The target color is the same as in Figure 3a and b respectively, but the eye to which they are presented is temporally displaced in the orbit by the amount of the ocular deviation(~18°). As a result, the subject often executes a”crossover”saccade by acquiring the target with the other eye. Crossover saccades are events represented by red dots in(a)and blue dots in(b).

Figure 5:. Saccades to targets presented to both eyes

a, b)Blue cross/purple target and red cross/purple target trials. A purple target elicits a different pattern of saccadic behavior than a blue(Figure 3a)or a red(Figure 3b)target. There are separate zones, where either the left eye or the right eye makes a saccade to the target. c, d)Perceptual data for blue cross/purple target and red cross/purple target trials acquired in a separate experiment, showing the patient’s verbal identification of target color. In regions shaded blue the right eye is suppressed, and vice-versa. Correlation between saccade data and perceptual data implies that saccades are made to each target with the eye that perceives the target.