Influence of Figure-Ground Contrast on the Implicit and Explicit Processing of Line Drawings in Patients with Left Unilateral Neglect

Abstract

The implicit and explicit processing of pairs of line drawings segregated from a ground by low luminance contrast or by high isoluminant colour contrast was assessed in a group of 13 consecutive patients with left unilateral neglect. Left-side differences between the two figures were recognised far less frequently than were right-side differences. Low luminance contrast worsened the explicit recognition of left-side differences. However, when left-side differences were not explicitly recognised, statistically significant implicit processing was found for high colour contrast trials both in the group of patients and in four individual cases. With low luminance contrast trials, implicit processing was found in only one individual case and was not found in the group of patients. A control experiment showed that normal subjects can discriminate the left or the right side of low luminance contrast stimuli with 90 accuracy, while fixating the opposite extremity of stimuli. The results of the present study show that, in patients with left unilateral neglect, implicit processing can be compromised by difficult preattentive discriminability of stimuli and are in keeping with: 1 Farah's 1994a proposal that visual awareness may be related to the quality of visual inputrepresentation in the brain; 2 Spinelli and co-workers' Spinelli, Angelelli, De Luca, & Burr, 1996; Spinelli, Burr, & Morrone, 1994 proposal that patients with left unilateral neglect may suffer from unilateral impaired functioning of the magnocellular pathway, which has high sensitivity to low luminance contrast.