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F. Froussart, C. Roumes, M. Maille; Assessing Binocular Imbalance: A New Test . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5637.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Heterophoria results from binocular imbalance along the horizontal direction. In such oculomotor conditions, binocularity is strictly constrained by fusion. Psychophysical experiments have pointed out that a double visual processing underlying fusion can be exhibited when stimuli supporting a horizontal binocular disparity are displayed dichoptically (Roumes et al, 1997, Human factors, 39, 359–373). The neural component is instantaneously efficient; the vergent component occurs secondarily. Methods: A clinical test was developed using contrast stimuli supporting a binocular disparity ranging from 0 to 220 min of arc, both in convergent or divergent condition. The stimuli are displayed either for 200 or 1000 ms. Rate of binocular fusion is used to determine fusion limits in each case. The difference between the two limits reveals the oculomotor contribution to fusion. Heterophoria should impair neural and vergent fusion in opposite way according to eso or exophoria. Results: Data were collected over a set of 17 subjects supporting various amount of ocular imbalance. Results confirm the hypothesis. In the case of exophoria, for instance, the observer exhibits a coherent increase of divergent amplitude of fusion relative to the convergent one and an associated reduction of divergent neural fusion. Nevertheless, subjects with a normal neural fusion were found despite an oculomotor imbalance. This latter result suggests that binocular vision can be perfectly adapted to a given ocular imbalance. Conclusions: A new test for binocular fusion could provide a functional approach and a complementary argument for diagnosis and potential reeducation.
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