What limits the contribution of second-order motion to the perception of surface shape?

Both motion and stereopsis can be derived from contrast as well as luminanc e defined stimuli. It is currently assumed that these two different sources of information about objects feed into one common stage. Thus it would not be expected that their role in visual perception would be different. Here we show that although motion can be carried by contrast-defined elements, s uch motion is not used to define three-dimensional (3D) surfaces. A similar effect has been reported in stereopsis; although such contrast-defined ele ments can give signed disparity signals they nevertheless do not contribute to the percept of shape. We show that the reason for this lies in the inab ility of the second order signals to cohere or bind across space/spatial sc ales rather than a characteristic of the elementary motion signals per se. (C) 2000 Elsevier Science Ltd. All rights reserved.