Pseudoneglect and the cross-over effect.

Several studies have found that patients with left hemi-neglect bisect long lines too far to the right, but bisect short lines too far to the left (the 'cross-over' effect). Some studies have reported that normal participants bisect long lines too far to the left, presumably reflecting an over-estimation of the left side due to the role of the right hemisphere in attention. The purpose of the present series of studies was to further study the cross-over effect in normal participants and to determine whether it may be due to perceptual or motor factors. Participants in the first study showed no cross-over effect on either the traditional line bisection task, or on the purely perceptual Landmark task. However, improvements in the Landmark task in Study 2 did lead to a significant cross-over effect. In Study 3 there was no cross-over on the traditional line bisection task even after changes were made to eliminate the 'ceiling effect' that is usually found with very short lines. Overall, the results suggest that normal participants do show a cross-over effect on a purely perceptual task, but not on the traditional manual line bisection task that includes a motor component. Possible implications for models of right hemispheric involvement in attention are discussed.