Abstract
PurposeThis study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions.MethodsSymmetrical 4° convergence step responses and near dissociated phoria (measured at 40 cm from the subject's midline) were recorded from six subjects with normal binocular vision using an infrared limbus tracking system with a haploscope. Two different sustained fixations (1° and 16° convergent rotation along the subject's midline) were used to study whether phoria had an influence on the peak velocity of convergence responses located at two initial vergence positions (1° or ‘far’ steps and 12° or ‘near’ steps).ResultsPhoria was significantly adapted after a sustained fixation task at near (16°) and far (1°) (p<0.002). A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p<0.03). When comparing convergence steps with the same initial vergence position, steps measured after near phoria adaptation were faster than responses after far adaptation (p<0.02). A regression analysis demonstrated that the change in phoria and the change in convergence peak velocity were significantly correlated for the far convergence steps (r = 0.97, p = 0.001). A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20).ConclusionAs a result of sustained fixation, phoria was adapted and the peak velocity of the near and far convergence steps was modified. This study has clinical considerations since prisms, which evoke phoria adaptation, can be prescribed to help alleviate visual discomfort. Future investigations should include a systematic study of how prisms may influence convergence and divergence eye movements for those prescribed with prisms within their spectacles.
Highlights
Vergence, a binocular eye movement, is responsible for attaining visual information located at different distances
Retinal blur is the defocusing of images which stimulates the accommodative vergence system, whereas disparity is the difference between where a new target is projected onto the retina and the fovea
The right eye position response decaying to the phoria level was measured following the 30 convergence steps to determine whether phoria was still adapted
Summary
A binocular eye movement, is responsible for attaining visual information located at different distances. The two primary inputs to the vergence system are retinal blur and disparity[1]. Retinal blur is the defocusing of images which stimulates the accommodative vergence system, whereas disparity is the difference between where a new target is projected onto the retina and the fovea. Disparity stimulates the disparityvergence system and allows a person to perceive and maintain a single binocular vision. Through the use of different instruments, one can study accommodative-vergence or disparity-vergence independently. A haploscope can systematically change where an object is projected onto the retina while maintaining a fixed focal length. When using a haploscope, any changes within eye movements can be assumed to be associated with the disparity-vergence system since the retinal blur stimulus is constant
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