Triangular backgrounds shift the bias of line bisection performance in hemispatial neglect
- 1Department of Neurology, New York University Medical Center, New York, USA
- 2Department of Neurology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, and Memory Disorders Research Center, Boston University Medical School, Boston, MA, USA
- 3Geriatric Research, Education and Clinical Center, VA Boston Healthcare System and Department of Psychiatry, Harvard Medical School, Boston, MA, USA
- Correspondence to: Dr M P Alexander, Behavioral Neurology Unit, KS-2, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215, USA; malexand{at}caregroup.harvard.edu
- Received 26 January 2001
- Accepted 10 September 2001
- Revised 10 August 2001
Abstract
Objective: Patients with left neglect on line bisection show normal implicit sensitivity to manipulations of both the stimulus and the visual background. Three experiments were designed to define this sensitivity more exactly.
Methods: Normal controls and patients with left neglect performed a series of horizontal line bisection tasks. Independent variables were the configurations of the backgrounds for the line—rectangle, square, circle, left and right pointing isosceles triangles—and whether the background was the shape of the piece of paper or an outline drawn on a standard piece of paper. In a separate experiment different components of the triangle were outlined on a piece of paper. Deviation from true midpoint was calculated.
Results: Simply placing the target lines in a symmetric background such as a square or circle did not reliably reduce neglect. A triangle asymmetric in the horizontal plane caused a shift in bisection away from the triangle's vertex. With right pointing triangles the perceived midpoint shifted to the left of true centre (crossed over). The effects of the triangles were comparable in the patients and the controls when controlled for baseline bisection bias. The critical components of the triangles were the angular legs. This effect of background was not influenced by lesion site or by hemianopia.
Conclusions: Patients with left visual neglect remain sensitive to covert manipulations of the visual background that implicitly shift the perceived midpoint of a horizontal line. This effect is strong enough to eliminate neglect on a bisection task. The mechanism of this effect is expressed through preattentive visual capacities.
