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 abnorma lities and motion visually evoked potential (VEP) abnormalities observ ed in human infantile esotropes. Methods: Recordings were obtained of horizontal pursuit eye movements and fixation nystagmus under conditio ns of monocular viewing. Motion VEPs in response to horizontal motion were recorded with the animals sedated to reduce the possibility of ey e movement artifact. Results: The strabismic monkeys had a naso-tempor al asymmetry of horizontal pursuit favoring nasally directed motion wh en viewing with either eye. When fixating stationary targets, latent n ystagmus was apparent; the eyes drifted conjugately and the slow phase of the nystagmus was always nasally directed with respect to the fixa ting eye. Motion VEPs were characterized by a strong horizontal asymme try with the directional bias inverted approximately 180 deg in the tw o eyes. These eye movement and motion VEP asymmetries were not observe d in a normal macaque. Conclusions: Macaque monkeys who have infantile esotropia possess asymmetries of horizontal pursuit and motion VEPs l ike those documented in strabismic humans. Macaques with infantile eso tropia appear to be an appropriate animal model for study of neural me chanisms in strabismus.