Line versus representational bisections in unilateral spatial neglect.

OBJECTIVES: To clarify the mechanisms of left unilateral spatial neglect found in the bisection of lines after cueing to the left end point and to determine whether neglect occurs for the mental representation of a line.
METHODS: A new representational bisection task was developed to eliminate the influence of the right segment of the physical line that would attract attention. Eight patients with typical left unilateral spatial neglect underwent line and representational bisection tasks on a computer display with a touch panel. In the line bisection with cueing, they bisected a line after touching the left end point. In the representational bisection, the patients were presented with a line until they touched the left end point. On the blank display, they pointed to the subjective midpoint of the erased line. The performances of the two bisection tasks were compared when the length and position of stimulus lines were varied.
RESULTS: The rightward errors in the representational bisection were greater than or equivalent to those in the line bisection with cueing. The effect of line length in which the errors became greater for the longer lines was equally found in the line bisection with cueing and the representational bisection. This was confirmed in the condition where the right end point was placed at a fixed position and the line length was varied.
CONCLUSIONS: After cueing to the left end point, rightward bisection errors of patients with neglect are not caused by overattention to the right segment of the physical line. Left neglect occurs mainly for the mental representation formed at the time of cueing or seeing the whole extent of a line.