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SP McKee, Y Bonneh, AM Norcia; Binocular Disparity Suppresses the Monocular VEP Response to Vernier Targets . Invest. Ophthalmol. Vis. Sci. 2002;43(13):4678.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Normal fusion of disparate targets involves a process that actively suppresses monocularly visible information (McKee & Harrad, 1993). In psychophysical measurements, if a standard vernier target, presented to one eye, is paired with a similar target in the other eye that has a large fixed offset, vernier acuity for the fused target is degraded, compared to the monocular vernier acuity. Is the monocular VEP response to vernier offsets also suppressed when paired with a static disparate target in the other eye? Methods:We recorded steady state visual evoked potentials to large offsets (6 arcmin) in vertical bars presented to one eye. Each bar contained multiple segments that repetitively aligned and misaligned at a rate of 1 Hz. We paired the oscillating vernier bars in one eye with static vertical bars in the other eye. The static bars were either straight or had fixed offsets corresponding to the oscillating segments in the other eye. When fused, the observer saw rows of bar segments moving in depth interspersed with fixed static segments. Results:The making and breaking of alignment of the monocularly-viewed vernier target produced a strong VEP response. When paired with static bars with no offset (0 disparity), the amplitude of the monocular VEP response was unaffected or enhanced. However, when a large offset was introduced into the static bars, thereby creating a large standing disparity, the amplitude of the monocular VEP response was strongly reduced. Similar measurements made in one amblyopic subject who lacked stereopsis showed a strong asymmetry. The VEP of the amblyopic eye was suppressed by the static bars at all disparities, but the VEP of the non-amblyopic was unaffected by the static bars in the amblyopic eye. Conclusion:The VEP response reflects the normal operations of binocular fusion. The binocular system must suppress the monocular signals that specify two discrepant visual directions, in favor of a unique binocularly-defined visual direction. This fusional operation may underlie the suppression found in non-binocular amblyopes, where it functions asymmetrically, suppressing only the amblyopic eye.
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