Effects of stimulus asymmetry on line bisection

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.