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E.H. Wong, D.M. Levi; Second-Order Spatial Summation and Contrast Discrimination in Amblyopia . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4804.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Amblyopes show both reduced contrast sensitivity and abnormal spatial interactions for second-order (contrast defined) stimuli (Wong, Levi & McGraw, 2000, 2001). In the present studies we investigate whether amblyopes show abnormal spatial summation and contrast discrimination for second-order stimuli. Methods: 6 amblyopic, 2 non-binocular/non-amblyopic, and 5 normal (control) adults participated. Spatial summation: We psychophysically measured monocular detection contrast thresholds for 9 sizes of 1 c/deg, amplitude modulated noise. Stimuli differed in σ of the 2-D Gaussian envelope, resulting in 0.625 – 7.5 cycles (at 2.5σ). We repeated the experiment with luminance modulated noise (first-order control experiment). Contrast discrimination: We psychophysically measured monocular increment thresholds (pedestal + increment contrast) for 1 c/deg, amplitude modulated sinusoids (pedestal). We varied pedestal contrast (9 levels, 0.2 – 3.0 pedestal contrast threshold (PCT)) and plotted threshold versus pedestal contrast (TvC function). Results: Spatial summation: All eyes, except the amblyopic eyes of deep amblyopes, showed similar reduction in detection contrast thresholds with increasing stimulus size. A 1.5 cycle stimulus reduced detection contrast by about 50% from that for the smallest (0.625 cycle) stimulus. Deep amblyopes showed threshold reduction with larger stimuli, e.g. 5.5 cycles for 50% reduction. All eyes showed similar first-order spatial summation (control experiment). Contrast discrimination: Control eyes showed a dipper shaped TvC function. Relative to pedestal detection contrast, increment threshold was 30 – 70% lower at 0.2 – 1.4 PCT and 10 – 40% greater at 3.0 PCT. All non-control eyes, except the amblyopic eyes of deep amblyopes, showed equal or smaller (better) increment thresholds than control eyes. Deep amblyopes showed a shallow dipper curve – increment thresholds 0 – 30% lower than pedestal detection contrast. Conclusions: Deep amblyopia results in spatial summation over a larger area and reduced contrast discrimination for second-order stimuli but not for first-order stimuli (control experiment and prior studies). Therefore, our results suggest that deep amblyopia reduces neural pooling at a second-stage of cortical processing.
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