EFFECTS OF EARLY-ONSET ARTIFICIAL STRABISMUS ON PURSUIT EYE-MOVEMENTSAND ON NEURONAL RESPONSES IN AREA MT OF MACAQUE MONKEYS

In humans, esotropia of early onset is associated with a profound asym metry in smooth pursuit eye movements. When viewing is monocular, targ ets are tracked well only when they are moving nasally with respect to the viewing eye. To determine whether this pursuit abnormality reflec ts an anomaly in cortical visual motion processing, we recorded eye mo vements and cortical neural responses in nonamblyopic monkeys made str abismic by surgery at the age of 10-60 d. Eye movement recordings reve aled the same asymmetry in the monkeys' pursuit eye movements as in hu mans with early-onset esotropia. With monocular viewing, pursuit was m uch stronger for nasal-ward motion than for temporalward motion, espec ially for targets presented in the nasal visual field. However, for ta rgets presented during ongoing pursuit, temporalward and nasalward ima ge motion was equally effective in modulating eye movement. Single-uni t recordings made from the same monkeys, under anesthesia, revealed th at MT neurons were rarely driven binocularly, but otherwise had normal response properties. Most were directionally selective, and their dir ection preferences were uniformly distributed. Our neurophysiological and oculomotor measurements both suggest that the pursuit defect in th ese monkeys is not due to altered cortical visual motion processing. R ather, the asymmetry in pursuit may be a consequence of imbalances in the two eyes' inputs to the ''downstream'' areas responsible for the i nitiation of pursuit.