Lateral biases in assumptions of lighting position

People presume that the light source in pictures comes from above, and there is some evidence that this phenomenon also demonstrates lateral biases. When investigators present multiple ambiguous stimuli or visually complex objects, people assume that the source of light is from above, and to the left. However, when single relatively simple stimuli are presented, this lateral bias is not readily observed. To investigate whether lateral bias is dependent on the number of stimuli presented, the present experiment employs a within-subjects design to present single spheres or pairs of spheres. Contrary to our expectations, leftward biases were exhibited in both the single-sphere condition and the paired-spheres condition. Single spheres that were lit from the left were more likely to be judged as convex than comparable spheres lit from the right. Similarly, when pairs of laterally lit spheres were presented and participants were asked to judge which sphere appeared to be more convex, participants chose spheres lit from the left regardless of the angle of illumination. Collectively, these results suggest that there are lateral biases in assumptions of illumination position and that these biases do not depend on the number of stimuli being perceived at any given time.