Line bisection and rebisection: the crossover effect of space location

To investigate the relationship between the bisection test and the severity of behavioral hemineglect and to verify if this test can predict the behavioral hemineglect.

Thirty stroke patients with left hemiparesis were divided into 4 groups according to the Catherine Bergego Scale, which assessed the behavioral hemineglect: severe unilateral neglect (UN), moderate UN, mild UN, and lack of UN. Eleven healthy subjects served as age-matched control subjects. In the bisection test, 18 lines were presented on the left, middle, and right of an A4 paper, respectively. The subjects were asked to place a short cross mark in the exact middle point of each line on the paper using their right hand. The middle 6 lines in the above bisection test were extracted on another sheet of A4 paper for the rebisection test. The subjects were asked to divide a line into 4 segments by successive bisections. The proportion of the right part to the length of line for bisecting was calculated.

In the bisection test, the main effect of space was significant in every group except the mild neglect group. The crossover effect of space location was found in the severe UN group, the group without UN, and the controls. In the severe UN group, the patients bisected the left and middle lines with rightward bias (<50%) but bisected the right lines with leftward bias (>50%). In the group without UN and the controls, the subjects bisected the left lines with leftward bias (>50%) but bisected the middle and right lines with rightward bias (<50%). Almost the same results were seen in the rebisection test.

This study showed that if the spatial crossover effect occurred in the right space condition, it was strongly supported that this patient had moderate to severe behavioral hemineglect. The crossover effect of the space location was explained by a new model.