A NORMAL BIAS TOWARD A PICTORIALLY DEFINED TOP IN LINE BISECTION

Background: We set out to determine whether separable visual and repre sentational components underlie normal subjects' upward adn distal bia ses in bisecting vertical and radial lines under visual guidance. Meth ods: Thirty-four normal subjects were asked to bisect lines oriented h orizontally, vertically, and radially. Human silhouette figures were p laced at either end of each lie. These figures were presented upright or upside down in order to pictorially define a ''top'' to each line i ndependent of the actual top of the visual field. Results: Although su bjects erred toward the top of the visual field, they also demonstrate d a significant bias toward the heads of the figures for lines in all spatial orientations. Conclusions: This result supports the existence of two biases: one toward the upper visual field, and another toward a n internally represented ''top'' as suggested pictorially. These findi ngs provide further support for the hypothesis that normal subjects' u pward and distal biases on bisection of vertical and radial lines unde r visual guidance have both representational and visual-based componen ts.