Illusions of Length in Spatial Unilateral Neglect

Abstract

In recent years, a number of studies investigated the processing of visual illusions in right brain-damaged patients with left unilateral neglect (N). In the more general context of exploring the complete range of processes, outside the domain of perceptual awareness, preserved in N patients, consistent evidence has been provided to the effect that subjective figures (Vuilleumier et al., 1996) and geometrical illusions (e.g., the Muller-Lyer figure and its versions: Mattingley et al., 1995; Olk et al., 2001; Ro and Rafal, 1996; Vallar et al., 2000) may be processed normally by N patients, also when the illusion arises in the contralesional left, neglected, side of the stimulus. In the majority of the reported patients the right hemispheric lesions involved the parietal, temporal, or frontal regions, sparing the occipital lobe. This finding is consistent with current knowledge concerning the neural correlates of left N (Leibovitch et al., 1998; Vallar and Perani, 1986). These results provide also indirect evidence concerning the neural correlates of these illusions, suggesting a possible role of the occipital regions. The present study aimed at exploring the neural correlates of processing of illusions of horizontal length in two subgroups of right-brain-damaged patients with left N, without (N+VFD-) and with (N+VFD+) associated evidence of left visual-field deficits, and damage sparing or involving the occipital lobe. As in our previous experiments (Vallar et al., 2000), we made use of the Brentano or combined form of the Muller-Lyer illusion, which makes one half of the line longer than the other (Coren and Gircus, 1978). The Brentano form includes both ingoing and outgoing fins together, embedding the two opposite configurations of the Muller-Lyer illusion in a single figure (see fig. 1-A).