The bisection of horizontal and radial lines: a case study of normal controls and ten patients with left visuospatial neglect.

Two experiments are reported on the bisection of horizontal and radial lines. In experiment 1, eighteen normal controls performed both tasks. In the horizontal orientation, approximately half the subjects produced transections displaced to the left of center and in half they were displaced to the right of center. With the radial orientation, all subjects save one bisected the line too far from the body. In experiment 2, ten patients with left neglect after unilateral right hemisphere damage performed the same two tasks. All patients showed left neglect, bisecting the horizontal lines too far to the right. Five of the patients also produced radial bisections that are outside normal limits; the predominant pattern is to bisect these lines too close to the body. The results show that severe left visuospatial neglect can co-occur with normal performance on the relative estimation of radial extents. However, in half the patients tested, performance was impaired in both orientations. The findings are set in the context of normative studies of visual anisotropy.