The relationship between fusional vergence eye movements and fixation disparity

Abstract The fusional vergence system is under the control of a fast neural integrator which aligns the eyes and a slow neural integrator which maintains binocular alignment. These controllers are distinguished by their decay time constants and their stimuli. Previous studies indicate that the fast fusional vergence controller responds to retinal image disparity and the slow fusional vergence controller responds to the output of the fast neural integrator. Slow fusional vergence as evidenced by adaptation of the phoria is unaffected by accommodative vergence when disparity vergence is open loop. Under closed loop conditions both accommodation and disparity induced vergence influence slow fusional vergence. These results indicate that the slow vergence controller is located before the site of interaction between convergence and accommodation. Fixation disparity is described as a steady-state error of the neural integrator controlling fast fusional vergence and its amplitude is shown to be inversely related to adaptation of the phoria to prism.

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