Latent fixation nystagmus and nasotemporal asymmetries of motion visually evoked potentials in naturally strabismic primate

Purpose: To determine whether macaque monkeys who had onset of natural, alternating esotropia in early infancy have the eye movement abnormalities and motion visually evoked potential (VEP) abnormalities observed in human infantile esotropes. Methods: Recordings were obtained of horizontal pursuit eye movements and fixation nystagmus under conditions of monocular viewing. Motion VEPs in response to horizontal motion were recorded with the animals sedated to reduce the possibility of eye movement artifact. Results: The strabismic monkeys had a naso-temporal asymmetry of horizontal pursuit favoring nasally directed motion when viewing with either eye. When fixating stationery targets, latent nystagmus was apparent; the eyes drifted conjugately and the slow phase of the nystagmus was always nasally directed with respect to the fixating eye. Motion VEPs were characterized by a strong horizontal asymmetry with the directional bias inverted approximately 180 deg in the two eyes. These eye movement and motion VEP asymmetries were not observed in a normal macaque. Conclusions: Macaque monkeys who have infantile esotropia possess asymmetries of horizontal pursuit and motion VEPs like those documented in strabismic humans. Macaques with infantile esotropia appear to be an appropriate animal model for study of neural mechanisms in strabismus.