Effects of stimulus asymmetry on line bisection.

Abstract

There is an upward bias in bisecting radial and vertical lines under visual guidance. We investigated whether illusory visual factors might contribute to a previously reported representational upward bias in this task. In two experiments, subjects performed a line-bisection task along the three orthogonal axes (horizontal, radial, vertical). In Experiment 1, the stimuli consisted of a line terminated at each end by two equal- or different-sized bars. In Experiment 2, the stimuli were formed by a line terminated by two equal- or different-sized directional labels: SOPRA ("top") and SOTTO ("bottom"). The results showed a bisection bias towards the smaller bar (Experiment 1) and the smaller directional label (Experiment 2) on all three spatial axes, supporting the existence of an illusory visual effect. Contrary to a previous report, the data failed to show any representational (semantic) bias related to directional label meaning. These findings suggest that perceptual/attentional factors play a significant role in normal subjects' upward bias.