Leftwards error in bisecting the gap between two points: stimulus quality and hand effects.

Subjects attempted to bisect the gap between two LEDs; these differed from each other in brightness, stimulus duration, onset/offset (a) synchronies, and in continuous or flashing mode of presentation. There was a systematic bias to the left, especially with the left hand, indicating a motor contribution to this perceptual task. A brief, faint or flashing LED attracted the subjective midpoint, perhaps because of increased processing demands; indeed the general leftwards bias was attributed to a greater visuospatial processing capacity in the right hemisphere. Leftwards bias was rather greater when central fixation was maintained, indicating that hemispatial mechanisms may be more important than anatomical connectivities. The technique may be useful for understanding and treating unilateral neglect.