Effect of background motion on line bisection performance in normal subjects.
Previous studies have demonstrated that optokinetic stimulation (OKS) influences line bisection (LB) performance in normal subjects and patients with hemispatial neglect. Since subjects were required to attend to stationary targets on a moving background, prior experimental designs might have induced an illusion of target motion or induced motion (IM) in a direction opposite the background. The current study tested whether the IM affects LB performance in normal subjects and how the speed of targets also influences LB. Thirty-two right-handed normal volunteers (aged 28.0 +/- 5.3 years) were asked to bisect stationary lines with a background of horizontal OKS. These stimuli were generated by computer displayed on a large screen via a beam projector. The OKS was varied according to direction (leftward or rightward) and speed (9.4 degrees/sec or 56.1 degrees/sec), producing 4 different experimental conditions. Mean bisection errors in all conditions were compared with a control condition with no background OKS. For each condition, subjects rated the degree of IM on a 5 point scale. With fast rate OKS, subjects reported minimal IM and LB errors were in the same direction as background motion, a finding that replicates previous studies. Conversely, the slow OKS rate caused subjects to report IM and resulted in deviation of the bisection mark in a direction opposite the background OKS. While this discrepancy between the slow and fast OKS conditions might be related to motion illusion, we did not find a direct correlation between the degree of IM and bisection errors and thus reasons for these results remain unexplained.