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Sustained fixation induced changes in phoria and convergence peak velocity.

Kim EH, Vicci VR, Han SJ, Alvarez TL - PLoS ONE (2011)

Bottom Line: A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p<0.03).A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20).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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, New Jersey Institute of Technology, Newark, New Jersey, United States of America.

ABSTRACT

Purpose: This study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions.

Methods: Symmetrical 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).

Results: Phoria 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).

Conclusion: As 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.

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Related in: MedlinePlus

Haploscope experimental setup which stimulates retinal disparity while keeping accommodation constant.All visual stimuli were 40 cm away along midline for a constant 2.5D accommodative stimulus.
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pone-0020883-g001: Haploscope experimental setup which stimulates retinal disparity while keeping accommodation constant.All visual stimuli were 40 cm away along midline for a constant 2.5D accommodative stimulus.

Mentions: Visual stimuli were displayed using a haploscope. Two computer screens were used to generate a symmetrical disparity vergence stimulus, shown in figure 1. The stimulus was a green vertical line, 2 cm in height by 2 mm in width, and was presented on a black background. The two stimuli (green vertical lines) from each computer screen were projected onto the two partially reflecting mirrors and into the line of sight of the subject. Prior to the experiment, the stimuli from the computer screens were adjusted with the partially reflecting mirrors to calibrate the visual stimulus with real targets located at measured distances from the subject's midline. An inter-pupillary distance of 6 cm was assumed. During the experiment, only the visual stimulus displayed on the computer screens was seen by the subject. The subject's head was restrained using a custom chin rest to eliminate head movement and avoid any vestibular influences. No other visual stimuli were presented to the subject except for the visual stimuli presented on the computer screens. The stimuli screens were placed 40 cm away from the subject, hence the accommodation stimulus was held constant at 2.5D. A previous study showed that accommodative-vergence does not influence the initial peak velocity measurement [25]. Hence, vergence peak velocities observed in this present study will be assumed to be associated with the disparity-vergence system.


Sustained fixation induced changes in phoria and convergence peak velocity.

Kim EH, Vicci VR, Han SJ, Alvarez TL - PLoS ONE (2011)

Haploscope experimental setup which stimulates retinal disparity while keeping accommodation constant.All visual stimuli were 40 cm away along midline for a constant 2.5D accommodative stimulus.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3117850&req=5

pone-0020883-g001: Haploscope experimental setup which stimulates retinal disparity while keeping accommodation constant.All visual stimuli were 40 cm away along midline for a constant 2.5D accommodative stimulus.
Mentions: Visual stimuli were displayed using a haploscope. Two computer screens were used to generate a symmetrical disparity vergence stimulus, shown in figure 1. The stimulus was a green vertical line, 2 cm in height by 2 mm in width, and was presented on a black background. The two stimuli (green vertical lines) from each computer screen were projected onto the two partially reflecting mirrors and into the line of sight of the subject. Prior to the experiment, the stimuli from the computer screens were adjusted with the partially reflecting mirrors to calibrate the visual stimulus with real targets located at measured distances from the subject's midline. An inter-pupillary distance of 6 cm was assumed. During the experiment, only the visual stimulus displayed on the computer screens was seen by the subject. The subject's head was restrained using a custom chin rest to eliminate head movement and avoid any vestibular influences. No other visual stimuli were presented to the subject except for the visual stimuli presented on the computer screens. The stimuli screens were placed 40 cm away from the subject, hence the accommodation stimulus was held constant at 2.5D. A previous study showed that accommodative-vergence does not influence the initial peak velocity measurement [25]. Hence, vergence peak velocities observed in this present study will be assumed to be associated with the disparity-vergence system.

Bottom Line: A repeated measures ANOVA showed that convergence far steps were significantly faster than the near steps (p<0.03).A weaker correlation was observed for the near convergence steps (r = 0.59, p = 0.20).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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biomedical Engineering, New Jersey Institute of Technology, Newark, New Jersey, United States of America.

ABSTRACT

Purpose: This study sought to investigate the influence of phoria adaptation on convergence peak velocity from responses located at different initial vergence positions.

Methods: Symmetrical 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).

Results: Phoria 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).

Conclusion: As 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.

Show MeSH
Related in: MedlinePlus