The computation of occluded contours in visual agnosia: Evidence for early computation prior to shape binding and figure-ground coding.
Giersch A., Humphreys GW., Boucart M., Kovacs I.
We examined whether an agnosic patient with a deficit in early visual processing, HJA, completed occluded contours. We used matching tasks with stimuli composed of three superimposed or occluded shapes. Experiments 2 and 6 required superimposed or occluded shapes to be discriminated from distractors in which the position of one shape was changed. HJA was selectively impaired with occluded relative to superimposed shapes. His performance was affected 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 completed contours to segment foreground shapes inappropriately. When asked to copy overlapping shapes (Experiment 4), HJA drew in the occluded parts as if real contours were present, at least on some occasions. These drawings and a task requiring discrimination between real and occluded contours (Experiment 7), showed a tendency to continue contours inappropriately, an insensitiviy to junctions, and impaired integration of contours into more global shapes. 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 foregroundbackground relationships between shapes are resolved. In visual agnosia, occluded contours can be computed even when there is impairment of both binding of contours to shapes and the computation of foreground-background relations in overlapping shapes.