Stereoscopic depth but not shape perception from second-order stimuli.

Depth can be seen using either linear (first-order) or non-linear (second-order) stereo micropatterns when, in the latter, contrast envelopes contain the disparity information. We examined whether a second-order mechanism can contribute to the perception of 3-D surface shape. Using a variety of different stimulus types, we show that for each, shape is easy to see with linear stimuli. Over a wide range of parameters however, none of our observers perceived shape, however faintly, from the non-linear stimuli. To explore why these elements failed, we simplified our stimulus to a step-edge in depth and measured performance while varying the number of elements. We show how performance declined when more than two non-linear elements were used. We discuss reasons for the limitation found for non-matching elements, including a dissociation for stereopsis between seeing surface shape and depth.