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J.E. Hale, R.A. Harrad, S.P. McKee, A.M. Norcia; Binocular Fusional Suppression is Proportional to Stereoacuity . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4824.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:In normal observers, monocularly visible information is suppressed by a fusible target in the other eye, a phenomenon known as fusional suppression (McKee and Harrad, 1993). To study the relationship between this normal phenomenon and suppression in amblyopia, we measured the monocularly driven VEP response to vernier targets during binocular viewing in normal and stereoanomalous subjects. Methods:We recorded vernier thresholds by pairing a sweep VEP test target with a static fixed offset target in the other eye. Each vertical bar of the test target consisted of 5 segments; 3 static segments interspersed with 2 oscillating segments, repetitively aligning and misaligning at a rate of 2 HZ. When the vernier target was paired binocularly with the static fixed offset target, normal observers saw the moving segments oscillating in depth, towards and away from the static segments. Results: Monocularly-viewed vernier targets produce a low threshold in the normal. Binocular fusion with the fixed offset causes fusional suppression of the VEP response and a symmetrical elevation of threshold. Amblyopic observers have higher monocular vernier thresholds in their amblyopic eyes. The addition of the fixed offset to the amblyopic eye has no effect on the good eye’s response but presenting the fixed offset to the good eye elevates the threshold of the amblyopic eye somewhat. Stereoblind non-amblyopic observers did not show any significant VEP alteration in the disparity conditions. Non-amblyopic observers with reduced stereopsis exhibited some fusional suppression, but with less elevation of threshold and more asymmetry than was found in normal observers. Conclusions:In normal observers, the monocularly-driven VEP response to a vernier stimulus is suppressed by the presence of a disparate target in the other eye. The amblyope shows asymmetric fusional suppression, while the stereoblind observer shows no significant suppression. Stereoanomalous amblyopic observers have intermediate responses dependent on their degree of binocular function.
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