The complex interaction of normal biases in line bisection.

To better understand neglect it is necessary to understand the inherent biases of normal subjects. We attempted to determine whether the biases present in normal subjects' line bisection are based on visual and representational biases or whether misbisection is simply due to perceptual/illusionary biases toward the narrower end of the stimulus. Normal subjects' bisected lines presented in four orientations: horizontal, vertical, and radial (both above and below eye level). Arrowheads were chosen as the stimulus placed at each end of the line. Misbisection toward the narrow end of the arrow would support the hypothesis that bisection bias is based on illusionary factors. However, in all four orientations subjects erred toward the wider end of the arrow. This suggests that the visual and representational biases that underlie normal subjects' upward bias on bisecting vertical and radial lines cannot be explained solely on the basis of a perceptual/illusionary bias.