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E.H. Wong; First–Order and Second–Order Spatial Interactions in Human Amblyopia . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5363.
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© ARVO (1962-2015); The Authors (2016-present)
In normal subjects, first–order (luminance modulation) and second–order (contrast modulation) stimuli have been shown to be processed via separate pathways with little or no crosstalk. Lateral masking studies show that detection of a first– or second–order sinusoid flanked by respective first– or second–order, spatially separated, collinear or orthogonal sinusoids is facilitated in normal subjects but suppressed through amblyopic eyes. The present lateral masking study investigates whether amblyopic eyes also show abnormal processing of mixed first– and second–order stimuli relative to normal subjects.
A pilot group of four adults with normal (control) vision and four strabismic adults with amblyopic vision participated. We determined psychophysical (2–AFC) contrast detection thresholds for a 1 c/deg, horizontal, second–order sinusoid alone and when flanked by non–overlapping, 1 c/deg, first–order sinusoids in collinear, orthogonal or random–phase horizontal configuration. We tested at flank contrast 15% and 30%, separated target centroid and flank centroid by 4 lambda (where sigma = lambda), and used static, random noise as the carrier for all sinusoids.
In normal subjects (dominant eye), for both flank contrasts, detection was suppressed (average 18% +/– 11% (SE)) by orthogonal and random–phase flanks but slightly facilitated (average 11% +/– 7% (SE)) by collinear flanks. In amblyopic subjects, the preferred eye showed results similar to the control eyes. However, the amblyopic eyes showed suppression by all flanks – the collinear flanks did not facilitate detection. Additionally, all amblyopic eyes and most fellow preferred eyes showed second–order contrast sensitivity loss (target alone) relative to the control eyes.
The slight facilitation from collinear flanks shown by normal subjects and the preferred eyes of amblyopic subjects suggests the use of the first–order envelope (a second–order cue) and/or the first–order, in–phase orientation information. These mechanisms appear to be limited through the amblyopic eye. The moderate suppression from orthogonal and random–phase flanks shown by all eyes suggests a non–specific surround (inhibitory) effect on the center (target) gain. Results from each eye of amblyopic subjects appear to be independent of the loss of second–order contrast sensitivity.
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