Eye Choice for Acquisition of Targets in Alternating Strabismus

Abstract

In strabismus, potentially either eye can inform the brain about the location of a target so that an accurate saccade can be made. Sixteen human subjects with alternating exotropia were tested dichoptically while viewing stimuli on a tangent screen. Each trial began with a fixation cross visible to only one eye. After the subject fixated the cross, a peripheral target visible to only one eye flashed briefly. The subject's task was to look at it. As a rule, the eye to which the target was presented was the eye that acquired the target. However, when stimuli were presented in the far nasal visual field, subjects occasionally performed a "crossover" saccade by placing the other eye on the target. This strategy avoided the need to make a large adducting saccade. In such cases, information about target location was obtained by one eye and used to program a saccade for the other eye, with a corresponding latency increase. In 10/16 subjects, targets were presented on some trials to both eyes. Binocular sensory maps were also compiled to delineate the portions of the visual scene perceived with each eye. These maps were compared with subjects' pattern of eye choice for target acquisition. There was a correspondence between suppression scotoma maps and the eye used to acquire peripheral targets. In other words, targets were fixated by the eye used to perceive them. These studies reveal how patients with alternating strabismus, despite eye misalignment, manage to localize and capture visual targets in their environment.

Strabismus and the Oculomotor System: Insights from Macaque Models

“…Thus, strabismic patients with relatively little amblyopia can switch the eye of fixation depending on the location of the object they want to fixate, and this fixation-switch feature of strabismus is likely the result of visual suppression of portions of each eye’s retina. Analyses of the spatial patterns of fixation switch in strabismic humans and monkeys have provided insight into how visual information from the two eyes is processed and converted into action in strabismus (Agaoglu et al 2014b; Das 2009; Economides et al 2012, 2014; van Leeuwen et al 2001). Fundamentally, the results support the idea that fixation-switch behavior follows from suppression of a portion of the temporal retina in exotropes and a portion of the nasal retina in esotropes.…”

Capturing the Moment of Fusion Loss in Intermittent Exotropia

“…In exotropes, divergence tone is excessive relative to convergence tone, 35 explaining why adducting saccades are hypometric relative to abducting saccades. 18 The eyes are prevented from drifting apart by cortical binocular mechanisms, which maintain fusion. When 1 eye is occluded, cortical drive is interrupted, and the covered eye drifts outward.…”

“…Further details regarding the equipment, experimental design, and tracker resolution have been published. 18,20 An excerpt from a typical data recording session is available for review (Supplemental Video 1; available at www.aaojournal.org). …”

“…18 Such transitions were sometimes involuntary, or even occult, made while subjects were attempting to maintain alignment on a visual target. In other cases, subjects were able to exert some control over the process, and in fact were encouraged to release ocular fusion so that the resulting eye movement could be documented.…”

Author Response: Motion Responses in Human Strabismus: What Optokinesis in the Deviating Eye Is Telling Us

“…Our data and conclusions fit in nicely with the previous work in the literature. For instance, Economides, Adams, and Horton 11 , 12 used a visual psychophysical paradigm, and showed that the fovea of the deviated eye was not suppressed. In addition, other work from our lab in which we used a saccade paradigm to examine spatial patterns of fixation switch behavior (presumably driven by suppression) also revealed similar results.…”