Disturbances of small-field horizontal and vertical optokinetic nystagmus in amblyopia.

Horizontal and vertical small-field optokinetic nystagmus (OKN) were examined in persons with strabismic or anisometropic amblyopia. Reduced velocity for the slow phase of OKN driven by temporalward and upward target motion presented monocularly was observed in both the amblyopic and nonamblyopic eyes of some subjects. Several experiments were conducted in search of a sensory disturbance of perceived motion sensitivity which could account for the abnormal OKN. Comparisons between the frequency response for OKN and the contrast sensitivity function for perceived motion revealed that amblyopes with asymmetric OKN had equal sensitivity to nasal and temporal target motion. Contrast thresholds for driving the temporal slow phase of OKN were elevated by over 1 log unit above contrast thresholds for perceived temporal target motion, whereas contrast thresholds for stimulating nasal movement and driving the nasalward slow phase of OKN were equal. Contrast sensitivity to nasal and temporal target motion was symmetrical at the fovea and parafovea of the amblyopic eye. These studies did not reveal a sensory a /mso observed in both eyes of persons with congenital strabismus without amblyopia and in the nondeprived eye in monocular congenital cataract. These observations suggest a relationship between directional asymmetries of OKN and the incomplete development of binocular vision.