Cooperation of Priors for the Perception of Shaded Line Drawings

It is well known that observers have a bias that the light comes from above when they look at shaded objects. Recently, we have reported a similar bias for the interpretation of line drawings, which reflects an assumption concerning the location of the observer relative to the object (Mamassian, 1995 Perception 24 Supplement, 35). We investigate here how the perception of shaded line drawings might demonstrate the interaction of these two priors.

The stimuli were images of smoothly folded surface patches, painted with parallel stripes and illuminated from the side. The sign of curvature of the stripes and the direction of illumination were balanced across stimuli to enable a separate analysis of each prior. Two levels of contour curvature and two levels of ambient illumination were used. Each image was presented at a random orientation in the image plane, and subjects were asked whether the fold appeared concave or convex.

The results were consistent with the use of the shading and contour interpretation biases by most observers. When the biases were weakened (by reducing contour curvature or by increasing ambient illumination), the stimuli became more ambiguous. Unexpectedly, the stimuli were also perceived as more ambiguous when the contour had vertical mirror symmetry. Recent work indicates that perceived depth from multiple cues is a weighted average, where the weights reflect the reliability of the individual cues (Landy et al, 1995 Vision Research 35 389 – 412). Analogously, our results are reasonably consistent with the mixture of competing priors, modulated by the degree of evidence for each.