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E. H. Wong, J. Seshadri, D. Wiles, J. Liu; Dichoptic Study of Second-Order Lateral Masking in Normal and Amblyopic Vision. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5886.
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In past studies, monocular testing revealed that each eye of amblyopic observers show deficits in processing of second-order contrast. In the present study we directly investigated binocular integration and lateral interactions of second-order contrast using a novel dichoptic method.
A pilot group of six normal (control) and four amblyopic adult observers participated. We used a mirror stereoscope apparatus and psychophysically determined monocular contrast detection thresholds for an amplitude-modulated sinusoid target (1 c/deg, 2.75 deg patch, random noise carrier) presented alone and with two, vertically displaced flanks (0.125 deg separation with target). Flanks were identical to the target (but set at 30, 60 and 90% contrast) and in collinear or orthogonal orientation, or contained only the noise carrier. All ocular combinations of target and flank were presented to each observer. Every observer aligned four nonius lines before each trial (single 200 msec presentation) of a 25-trial sequence (method of limits). Thresholds were calculated following learning effects.
The control eyes and the preferred eye of amblyopes showed strong binocular integration of target and flanks. For binocular as well as only monocular presentation of stimuli, these eyes showed moderate to substantial suppression of target detection by all flank conditions. The noise-only flanks produced the greatest amount of suppression. In contrast, some amblyopic eyes showed no binocular integration, i.e. could not see the target when flanks were presented to the preferred eye. In either case, amblyopic eyes showed essentially no effect (facilitation or suppression) on target detection by all flank conditions.
Our preliminary results suggest that the mechanisms of binocular integration and lateral masking of second-order contrast are independent. The normal visual system shows strong binocular integration and orientation-independent suppression. The latter result suggests an inhibitory surround mechanism. The amblyopic visual system shows substantially limited binocular integration and lateral masking (suppression) only when viewing through the amblyopic eye. The latter is a novel second-order amblyopic deficit.
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