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A VEP Measure of the Binocular Fusion of Horizontal and Vertical Disparities

¹From the Department of Ophthalmology, Bristol Eye Hospital, Lower Maudlin Street, Bristol, United Kingdom; and ²The Smith-Kettlewell Eye Research Institute, San Francisco, California.

PURPOSE. Because of the lateral separation of the orbits, the retinal images differ in the two eyes. These differences are reconciled into a single image through sensory and motor fusional mechanisms. This study demonstrates electrophysiologically the effects that normal horizontal and vertical fusional processes have on the processing of monocular position signals.

METHODS. VEPs were recorded in 16 healthy adults in response to a vernier onset–offset target presented to one eye. The vernier offsets appeared and disappeared at 2 Hz and were introduced into bar targets that were oriented either vertically (horizontal offsets) or horizontally (vertical offsets). The magnitude of the offsets was varied over the range of 0.5 to 10 arc min. VEP amplitude was measured as a function of the size of the dynamic offset under monocular viewing conditions and in the presence of two different static targets presented to the other eye. One of the static targets matched the dynamic test, except that it had no vernier offsets. The other static target, the static pedestal, matched the dynamic test, but contained a set of static vernier offsets in locations corresponding to the locations of the dynamic offsets presented to the other eye.

RESULTS. VEP amplitude was a monotonically increasing function of vernier offset size under monocular viewing conditions. The addition of the static target without offsets in the other eye resulted in an increased amplitude VEP response. The addition of the static target with vernier offsets resulted in a decrease in VEP amplitude for both horizontal and vertical disparities.

CONCLUSIONS. The normal process of fusion results in a single visual direction. To obtain a single visual direction, the visual system must synthesize a binocular visual direction that differs from the monocular components. One of the conditions (the static pedestal with offsets) produces binocular visual direction shifts that degrade the appearance of vernier onset-offset, and reduce VEP amplitude for both horizontal and vertical disparities. This characteristic evoked response marker is a promising tool for measuring binocular fusion objectively in patients with strabismus.

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				A. M. Norcia, J. Hale, M. W. Pettet, S. P. McKee, and R. A. Harrad Disparity Tuning of Binocular Facilitation and Suppression after Normal versus Abnormal Visual Development Invest. Ophthalmol. Vis. Sci., March 1, 2009; 50(3): 1168 - 1175. [Abstract] [Full Text] [PDF]