Comparison of latent nystagmus and nasotemporal asymmetries of optokinetic nystagmus in adult humans and macaque monkeys who have infantile strabismus.

To determine whether macaque monkeys with infantile strabismus have latent nystagmus and directional asymmetries of horizontal optokinetic nystagmus (OKN) similar to those of humans with infantile strabismus, the authors recorded eye movements under conditions of monocular viewing. The presence of latent fixation nystagmus was tested by requiring the subjects to steadily fixate a stationary target subtending less than I deg of visual arc. OKN was tested using high-contrast, vertically-oriented moving stripes that filled 80 deg of the visual field. A macaque monkey who had infantile strabismus induced by alternating occlusion from birth showed latent nystagmus highly similar to that recorded in an adult human subject with infantile strabismus. The strabismic monkey also had asymmetric OKN similar to that of the strabismic human, favoring nasally-directed stimulus motion when viewing with either eye. Neither nystagmus nor an OKN asymmetry was observed in a normal macaque or in humans who had normal binocular vision. The findings of latent nystagmus and OKN asymmetries in the strabismic monkey support the notion that monkeys who have infantile-onset strabismus are an appropriate ocular motor model of human infantile strabismus.