Abstract
Abstract: :
Purpose: To determine and compare the time–course of VEP responses to subjective contours between normals and amblyopes. Methods: Subjective contours were generated in a display composed of an array of notched circles. When the notched circles were aligned (0°), illusory squares appeared. When the notched circles were mis–aligned (e.g. 20°, 50° and 70°), the illusory squares disappeared. VEP responses were measured under 2 stimulus conditions in both normals and amblyopes. In the test condition, alternation was between aligned (0°, Kanizsa squares) and mis–aligned states (50°) and in a control condition, alternation was between two mis–aligned states (20° and 70°) in which subject contours were not apparent. The rotation amplitudes and thus the motion cues were the same (50°) in both test and control conditions. Results: In both normals and amblyopes, the response to both test and control conditions consisted of a positive peak (P1) near 100 msec and a negative peak (N1) between 150 and 250 msec. N1 amplitude was larger after a transition to the Kanizsa squares. N1 amplitudes were largest for normal eyes, followed by the fellow eyes of amblyopes, followed by the amblyopic eyes. Amblyopic eyes showed delayed and more extended N1 responses, relative to normals or fellow eyes. Small delays of N1 were also apparent in the fellow eyes of the amblyopes, relative to normals. Conclusions: Amblyopic eyes retained some degree of sensitivity to the global organization of the stimuli. However, the significant reductions in amplitude and slowing of processing suggest that processing of subjective contours is abnormal. This residual sensitivity could reflect a sensitivity to subjective contours per se or to "salient regions" (Stanley and Rubin, 2003) defined by the symmetric arrangement of the aligned state.
Keywords: amblyopia • shape and contour • electrophysiology: clinical