Slow Vergence Research Articles

Changes in vergence mediated by saccades.

When binocular fixation is shifted between two targets which require change in vergence as well as an equivalent or greater alteration in the mean visual direction, the observed eye motions do not--as asserted by Yarbus (1957) and widely accepted today--consist of slow symmetrical change in vergence, upon which a conjugate (binocularly balanced) saccade is additively superimposed. In all tested target configurations, an unexpectedly large fraction of the total change in vergence occurred during the saccades; observed values ranged from about 40% in certain tasks, to essentially 100% when large version (4 degrees) was combined with small vergence change (less than 1 degree). In these latter situations, binocular congruence can be restored within about 50 ms by appropriately unbalanced saccades, rather than about 500 ms, as expected if slow fusional vergence movement were required. When larger vergence changes are demanded, additivity between vergence movement and conjugate saccade is also violated in that the rate of vergence change during the saccades is several-fold larger than the rate before the saccade or during subsequent completion of the required change in vergence. Furthermore, the residual fusional vergence movement observed in these tests was usually strongly asymmetrical, and often almost entirely monocular. Vertical saccades are nearly as effective as horizontal saccades in mediating a large fraction of an intended change in vergence. In saccades, which contributed strongly to (or fully mediated) an intended vergence change, target-specific binocular differences in saccadic excursion of as much as 40-50% were observed; hence, these eye movements are not fully yoked, as the term 'conjugate' implies. Instead, the eyes behave in such situations as though visual information from each eye is processed separately prior to the saccade, in order to generate the neural signals which control open-loop saccadic movement of the eye.

Fixation Disparity

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.

The relationship between fusional vergence eye movements and fixation disparity

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.

The influence of rapid prism adaptation upon fixation disparity

Small errors of fusional vergence (fixation disparity) were examined as a function of the magnitude of horizontal prism stimulating convergence or divergence for a short (30sec) duration. Marked differences were observed between the amplitude of fixation disparity resulting from convergent and divergent stimulus disparities. In another experiment, subjects wore a horizontal prism for 30 sec after which time one eye was occluded for 40 sec. Measurements of vergence eye movements revealed an incomplete relaxation of fusional vergence (prism adaptation) after 40 sec of occlusion. Marked differences were observed between the amplitude of prism adaptation resulting from convergent and divergent stimuli. Maximum prism adaptation and minimum fixation disparity occurred with the same direction of prism, suggesting that a slow fusional vergence mechanism minimizes errors of binocular vergence.