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Avi Aizenman, Dennis Levi; Eye on the prize: Fixational stability as a metric for the recovery of visual acuity in amblyopia. Journal of Vision 2020;20(11):558. doi: https://doi.org/10.1167/jov.20.11.558.
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© ARVO (1962-2015); The Authors (2016-present)
Amblyopia is a developmental disorder which leads to impaired form vision and oculomotor abnormalities including eccentric and unsteady fixation. Increased fixational unsteadiness is associated with poor visual acuity and stereopsis. In this study, we investigated whether improvements in visual function with treatment are accompanied by changes in fixational stability, i.e., whether changes in fixation stability can serve as an objective proxy for changes in visual acuity in children undergoing treatment for amblyopia.
Five children with amblyopia currently undergoing patching treatment or vision therapy (3 strabismic, 2 anisometropic, mean age 8.2 years) and five normally sighted children (control group, mean age 9.4 years) participated in 3 sessions over 3 months. During each session, we measured visual acuity and stereopsis. To measure fixational stability, observers were asked to look at a 1-degree colorful smiley face on a black screen for 20-second intervals. To ensure attention, the smiley face changed color after 10 (+/-3) seconds and observers responded when the color changed. There were ten 20 second trials in total. Eye movements were recorded with an Eyelink II eyetracker.
We calculated the 68% isoline area (ISOA) values (similar to the more common bivariate contour ellipse metric, but without assumptions of normality) as a measure of fixational stability, and computed correlations between clinical measurements and ISOA. ISOA showed a correlation with visual acuity (r-squared=.41), and stereopsis (r-squared=.46) replicating previous work. Importantly, the change (pre/post ratio) in ISOA showed a correlation with changes in visual acuity (r-squared=.59), but not stereopsis (r-squared=.02). Although stereopsis is correlated with fixational stability by session, the changes in fixational stability during the treatment of amblyopia do not track changes in stereopsis. As improvements in visual acuity are accompanied by changes in fixational stability, novel interventions for amblyopia may consider tracking eye movements as a measure of recovery of visual function.
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