Fixation of disparity: a steady state error of disparity-induced vergence.

Fusional vergence eye movements are controlled by a fast-acting mechanism that aligns the eyes in response to retinal image disparity and a slow-acting mechanism that sustains binocular alignment. These two control mechanisms are distinguished by their decay time constants and stimuli. Fast fusional vergence responds to retinal image disparity, and slow fusional vergence responds to the effort or output of the fast control mechanism. Fixation disparity is modeled as a steady-state error of fast fusional vergence. Fixation disparity is shown to be inversely related to prism adaptation, a manifestation of slow fusional vergence, in normal but not abnormal binocular vision. Sensory factors such as interocular suppression influence the magnitude of fixation disparity in strabismus. Clinical implications concerning the relation between fixation disparity and heterophoria in normal and abnormal binocular vision are discussed.