A study of plane bisection in four cases of visual neglect.

We report four case studies of bisection performance in patients with severe left neglect consequent upon right hemisphere lesion. The stimuli to be bisected range from a horizontal line through horizontally oriented rectangles to a square; the stimuli are displayed on a VDU and bisections made by moving a 'mouse' that controls a cursor line on the screen. For one of the patients, both the starting position of the cursor (left or right) and the shape of the stimulus have a major effect upon bisection accuracy. One patient shows an effect of starting position but not stimulus geometry; two patients show an effect of stimulus geometry but not starting position. For 3 out of 4 patients, the estimated midpoint of a line is thus not equivalent to that of a plane with the same horizontal extent. A putative 'attentional' interpretation of the results is provided.