Modification of the convexity prior but not the light-from-above prior in visual search with shaded objects.

Studies of visual search performance with shaded stimuli, in which the target is rotated by 180 degrees relative to the distracters, typically demonstrate more efficient performance in stimuli with vertical compared to horizontal shading gradients. In addition, performance is usually better for vertically shaded stimuli with top-light (seen as convex) distracters compared to those with bottom-light (seen as concave) distracters. These findings have been cited as evidence for the use of the prior assumptions of overhead lighting and convexity in the interpretation of shaded stimuli and suggest that these priors affect preattentive processing. Here we attempt to modify these priors by providing observers with visual-haptic training in an environment inconsistent with their priors. Observers' performance was measured in a visual search task and a shape judgment task before and after training. Following training, we found a reduced asymmetry between visual search performance with convex and concave distracters, suggesting a modification of the convexity prior. However, although evidence of a change in the light-from-above prior was found in the shape judgment task, no change was found in the visual search task. We conclude that experience can modify the convexity prior at a preattentive stage in processing; however, our training did not modify the light-from-above prior that is measured via visual search.