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Vallabh E. Das, Stephanie Kim, Anand C. Joshi, Ernest Baskin; Comparison Of Alternating Fixation Behavior In Esotropic And Exotropic Strabismic Non-human Primates. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4694.
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Nonhuman primates reared with daily alternating monocular occlusion (AMO) during the first four months of life develop large horizontal strabismus. We have also shown that these animals often alternate or switch the eye of fixation during binocular viewing conditions. The purpose of this study was to compare the spatial patterns of alternating fixation behavior in esotropic and exotropic monkeys.
Binocular eye movement data has been acquired in four monkeys, three with AMO-induced exotropia (two previously published) and the other with AMO-induced esotropia, as they performed a visually guided saccade task (random target presentation over a ±25° H x ±20°V grid) during either monocular or binocular viewing.
During binocular viewing, the exotropic monkeys showed fixation switch behavior for large target jumps into the temporal hemifield of the previously nonfixating eye (nasal retina of the previously nonfixating eye). Conversely, the esotropic monkey showed fixation switch behavior for large target jumps into the nasal hemifield of the previously nonfixating eye (temporal retina of the previously nonfixating eye). In the exotropes, target steps into the nasal hemifield of the nonfixating eye (temporal retina) tended not to produce a fixation switch, and in the esotrope target steps in the temporal hemifield of the nonfixating eye (nasal retina of the non-fixating eye) showed no fixation switch. Intermediate target steps that landed on the temporal retina of both eyes in the exotrope or the nasal retina of both eyes in the esotrope showed fixation switch on some trials and no fixation switch on others. Saccade latency tended to be greater during binocular viewing than during monocular viewing in both the exotropic and the esotropic monkeys.
Alternating saccade behavior in both exotropic and esotropic strabismic monkeys is consistent with expected patterns of retinal suppression (temporal retina is suppressed in exotropia and nasal retina is suppressed in esotropia) suggesting that visual suppression directly impacts alternating fixation and alternating saccade behavior. This study also raises some potentially interesting questions of how the brain chooses the eye of fixation for certain target jumps that land on the "suppressed" retina of both the eyes. Perhaps some of these questions are best answered using neural recording methods in areas of the brain related to target selection.
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