Alternating Fixation and Saccade Behavior in Nonhuman Primates with Alternating Occlusion–Induced Exotropia

Purpose: Nonhuman primates reared with daily alternating monocular occlusion (AMO) during their first few months of life develop large horizontal strabismus, A/V patterns and dissociated vertical deviation (DVD). In addition, these animals often alternate or switch the fixating eye during binocular viewing. The purpose of this study was to characterize the alternating fixation behavior of these animals during visually guided saccade tasks. Methods: Binocular eye movements were measured in two monkeys with AMO-induced exotropia as they performed a visually guided saccade task (random target presentation over a ±15° grid horizontally and vertically) during either monocular or binocular viewing. Results: During binocular viewing, large target steps into the temporal hemifield of the nonfixating eye (nasal retina of the nonfixating eye) produced fixation switches. Target steps into the nasal hemifield of the nonfixating eye (temporal retina of the nonfixating eye) tended not to produce a fixation switch. There were no significant differences in the amplitude–peak velocity or amplitude–duration main sequence relationships between alternating (binocular viewing) and nonalternating saccades (monocular or binocular viewing). Saccade latency tended to be greater during binocular viewing than during monocular viewing. Conclusions: This study shows that the AMO model for strabismus may be used for studying neural circuits involved in generating alternating fixation and alternating saccade behavior. Since patterns of alternating fixation are likely to be influenced by patterns of visual suppression, alternating saccade behavior may also be used as a probe to study mechanisms of visual suppression in strabismus.