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C. Hou, M. W. Pettet, A. M. Norcia; Neural Correlates of Figure-Ground Segregation in Strabismic Amblyopia. Invest. Ophthalmol. Vis. Sci. 2008;49(13):2590.
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A number of studies suggest that the abnormal processes that underlie amblyopia may not only cause deficits in spatial vision (e.g., poor visual acuity and reduced contrast sensitivity), but also in higher-level processing (e.g., motion perception, contour integration and global image processing) operating after any initial losses in V1. Here we measured Visual Evoked Potential (VEP) responses in a texture segmentation task in patients with strabismic amblyopia to learn whether physiological abnormalities of global image segregation occur in amblyopia.
Figure-ground segmentation stimuli consisted of a 3 X 3 array of 4.5° circular figures that appeared and disappeared from a uniform background at 1 Hz. The figures and background (24° X 24°) were composed of dynamic random bars that were updated at 30 Hz. The figure regions segregated from the background on the basis of a 90 deg difference in the orientation of the figure and background textures. When there was no mis-match, the entire field appeared uniform. VEP responses time-locked to the onset and offset of the texture-defined forms were measured monocularly in 9 strabismic amblyopes with strongly reduced or absent stereopsis. Patients had constant ocular deviation or a history of prior strabismus surgery with or without anisometropia with visual acuity between 20/40 and 20/125 in one eye, with the other eye being 20/20 or better. The patients were also asked to detect small changes in the aspect-ratio of the figures during the VEP recording.
Thresholds for the shape of the figure were elevated in the amblyopic eyes: fellow eyes could detect a 12% difference in the aspect ratio of the figures, but the amblyopic eye thresholds were 25%. VEP responses to orientation-defined figure-ground segregation consisted of a positive peak (P1) near 100 ms and a negative peak (N1) between 150 and 250 ms after "figures" onset. P1 amplitudes were significantly diminished in amblyopic eyes compared with the fellow eyes but P1 latency was the same in the amblyopic and fellow eyes. Amblyopic eyes also showed reduced, delayed and more extended N1 responses, relative to fellow eyes.
Shape discrimination thresholds and VEP amplitudes for dynamic orientation-defined forms are strongly affected by amblyopia.
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