Directionally selective short-term nonconjugate adaptation of vertical pursuits.

We investigated the contribution of fixation phoria and dynamic processes to short term (1 hr) nonconjugate adaptation of vertical pursuits. Unequal aftereffects were observed in vertical phoria measured during stationary gaze and during pursuits (static fixation and pursuit phorias) demonstrating direction-specific aftereffects of pursuit phoria that were not evident in measures of fixation phoria. A linear model describes the combination of fixation phoria and three dynamic direction-specific components which include a gain component that determines nonconjugate velocity, a phase component that determines the relative position of binocular pursuits, and a small position specific pursuit phoria adjustment process. Larger position-specific variations of fixation phoria appear to compensate for inappropriate or incomplete adaptive changes produced by two of the dynamic mechanisms which are not position specific.