The computation of occluded contours in visual agnosia: Evidence for earlycomputation prior to shape binding and figure-ground coding

We examined whether an agnosic patient with a deficit in early visual proce ssing, HJA, completed occluded contours. We used matching tasks with stimul i composed of three superimposed or occluded shapes. Experiments 2 and 6 re quired superimposed or occluded shapes to be discriminated from distractors in which the position of one shape was changed. HJA was selectively impair ed with occluded relative to superimposed shapes. His performance was affec ted by the spatial separation of the occluded contours rather than the area of the occluded surface. Experiments 3 and 5 required HJA to discriminate the central shape. Making occluded contours easier to compute (by reducing their spatial separation) facilitated discrimination of a central occluded shape (in the background), although it impaired discrimination of a central occluding shape (in the foreground). Free-choice shape judgements made to the central shape (Experiment 2) showed that HJA used both real and complet ed contours to segment foreground shapes inappropriately. When asked to cop y overlapping shapes (Experiment 4), HJA drew in the occluded parts as if r eal contours were present, at least on some occasions. These drawings and a task requiring discrimination between real and occluded contours (Experime nt 7), showed a tendency to continue contours inappropriately, an insensiti viy to junctions, and impaired integration of contours into more global sha pes. The results suggest that occluded contours can be computed early on in visual processing, probably at the level where long-range mechanisms group collinear contour segments together. Our control experiment shows that HJA is not impaired in collinear contour grouping. These mechanisms are prior to processes in which contours are bound to shapes and in which foreground- background relationships between shapes are resolved. In visual agnosia, oc cluded contours can be computed even when there is impairment of both bindi ng of contours to shapes and the computation of foreground-background relat ions in overlapping shapes.