The effect of acute superior oblique palsy on vertical pursuit in monkeys

PURPOSE. To investigate vertical smooth pursuit eye movements in monkeys with acute acquired superior oblique palsy (SOP). METHODS. The trochlear nerve was severed intracranially in two rhesus monkeys. After surgery, the paretic eye was patched for 6 or 9 days, and then binocular viewing was allowed. Eye movements were measured with binocular, dual search coils, before and after surgery, under monocular viewing conditions. Vertical pursuit movements along the midline were elicited by using triangular-wave (20 deg/s, ±20°) or step-ramp (20 deg/s) stimuli at a distance of 66 cm. RESULTS. During the early post-lesion period, before binocular viewing was allowed, pursuit velocity of the paretic eye during triangular-wave tracking was lower than that of the normal eye. When the viewing eye crossed straight ahead, the changes in pursuit velocity conjugacy were similar for upward and downward tracking. After habitual binocular viewing was allowed, differences between upward and downward pursuit emerged. When measured ∼30 days after lesioning, this directional asymmetry was less during the open-loop period of step-ramp tracking than during triangular-wave tracking. CONCLUSIONS. Rhesus monkeys with acute acquired SOP show characteristic changes in vertical pursuit, with deficits for both upward and downward tracking, and differences between the initiation of step-ramp pursuit and the sustained response during triangular-wave tracking. The habitual viewing condition (monocular versus binocular) also affected the pattern of deficit.