THE EFFECTS OF UNILATERAL VISUOSPATIAL NEGLECT ON PERCEPTION OF MULLER-LYER ILLUSORY FIGURES

Left visuospatial neglect after right hemisphere damage is a lateralis ed disorder of spatial perception and cognition. A study is reported o f the extent to which patients with left visuospatial neglect are susc eptible to the illusory effects normally elicited by Muller-Lyer figur es, in which inducing fins are located on one end or on both ends of a horizontal line. Seven patients with left visuospatial neglect and se ven normal controls were tested on a task of horizontal-line bisection in which stimuli consisted of plain lines of three lengths (100, 150, and 200 mm), randomly interleaved with lines with unilateral or bilat eral fins. As a group, normals made accurate bisection judgments in th e baseline (no-fins) condition, and exhibited significant illusory eff ects in both the unilateral-fin and the bilateral-fin conditions. In c ontrast, patients made substantial rightward errors in the baseline co ndition, consistent with their neglect of the left end of the line. Ho wever, as a group, patients still exhibited significant illusory effec ts with left-sided outward-projecting fins on 100 mm lines and with le ft-sided inward-projecting fins on 150 mm lines. Moreover, at least on e patient exhibited consistent illusory effects both for inward-projec ting and for outward-projecting left-sided fins at all line lengths. N ormal illusory effects in patients were also obtained with stimuli con taining unilateral right-sided and bilateral fins. The existence of su ch effects with inducing elements on the contralesional extremity of h orizontal line stimuli suggests preservation of low-level, perhaps pre attentive, perceptual mechanisms responsible for coding elementary vis ual features. The results may be relevant for an understanding of the influence of attentional factors on illusory perception.