A representational vertical bias.

We attempted to determine whether separable components underlie normal subjects' upward bias in bisecting vertical and radial lines under visual guidance. Twelve normal subjects indicated the midpoint of visually presented lines oriented vertically, radially, and horizontally. We placed directional labels ("TOP" and "BOTTOM") at either end of each line. Subjects showed significant bias toward the label TOP in horizontal, vertical, and "radial-down" (below eye level) conditions but not in the "radial-up" (above eye level) position. In the horizontal condition, the misbisections actually changed direction depending on whether TOP was to the left or right of midpoint. There were two biases: one toward an internally represented "top" (as suggested by the verbal labels) and another toward the upper visual field. The latter was stronger when the two were opposed. These findings suggest that normal subjects' upward bias on bisection of vertical and radial lines under visual guidance has both representational- and visual field-based components.