Abstract
Abstract: :
Purpose: Amblyopic observers often have difficulty detecting contours that are defined by chains of Gabor patches (Kovacs et al., ARVO, 1996;; Vision Res, 1999; Hess et al., 1997 Vision Res.). Here we asked whether deficits in contour integration could be detected using the Visual Evoked Potential. Methods: VEPs were recorded in a mixed group of 11 amblyopic observers (acuities between 20/32 and 20/250) and in 8 normal adults. The observers were presented with circular Gabor-defined contours (2 cpd, spaced at 3 wavelengths) presented in pattern appearance/disappearance mode. In one condition the Gabor elements all lay tangent to the implicit circular contour (collinear). In a second condition, the Gabor patches had a constant 60 deg orientation offset with respect to the contour (pinwheel) and in a third condition the patches were randomly oriented with respect to the contour. Results: The VEP waveform from the fellow eye of the amblyopes was similar to that of normal controls consisting of a positive peak at 100 msec, a negative peak at 120 msec followed by a broad positive peak at 220 msec. Significant differences between collinear and non-collinear contours were first seen at the P100 peak, with the largest differences occurring at the transition to the P220 peak. Random and pinwheel contours yielded similar potentials, indicating that collinearity (or near collinearity) is needed to produce the difference potential. The waveforms in the amblyopic eye were typically quite different from those in the fellow eye, often being of longer latency (except for P100) and lower amplitude. However, group average waveforms were different between collinear and random contours in the amblyopic eyes. Individually, 10 of 11 amblyopic eyes failed to show a significant difference, but 6 of 11 fellow eyes did. Of these 6, 5 did not have strabismus and 4 had measurable stereopsis. Of the 5 fellow eyes that failed to show a signiificant difference potential , two had measurable stereopsis. Conclusions: Contour specificity is present, but weaker in amblyopic eyes.
Keywords: amblyopia • spatial vision • visual cortex