Clear visual images require an ability to coordinate eye functions. Despite the central importance of binocular vision in everyday functioning, no standard exists for distinguishing good vs poor binocular vision, in contrast with monocular vision's eye chart and visual acuity measures. This study begins to relate the signs and symptoms of binocular vision with children's self-reported visual symptoms to substantiate a model of Functional Binocular Vision (FBV). Following Rasch's probabilistic application of Maxwell's method of analogy, we tested whether observations of visual symptoms and skills were sufficiently consistent to define interval units of measurement. Visual symptom data were obtained from over 1,000 students in grades 3-6 in four California schools using the Convergence Insufficiency Symptom Survey (CISS). An additional 21 optometric acuity and FBV measures were made for more than 459 of those students, as their CISS scores indicated problems in binocular vision. Of these 459, 78 completed an intervention training the eyes in binocular function and provided at least two sets of CISS and optometric measures. The CISS, acuity, and FBV measures all define interval measures spanning at least three statistically distinct groups (Cronbach's alpha 0.75). The final CISS measure was predicted in a regression model by the initial and final FBV measures (F = 3.5, 2 df1, 74 df2, p = .03). Further research will be needed to refine the measures and establish practical guidelines.
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