The perception of surface orientation from multiple sources of optical information.

An orientation matching task was used to evaluate observers' sensitivity to local surface orientation at designated probe points on randomly shaped 3-D objects that were optically defined by texture, lambertian shading, or specular highlights. These surfaces could be stationary or in motion, and they could be viewed either monocularly or stereoscopically, in all possible combinations. It was found that the deformations of shading and/or highlights (either over time or between the two eyes' views) produced levels of performance similar to those obtained for the optical deformations of textured surfaces. These findings suggest that the human visual system utilizes a much richer array of optical information to support its perception of shape than is typically appreciated.