Shading and contour cooperate to modulate the perceived 3D shape of disparity-defined surfaces

Multiple cues are used to estimate 3D shape, which can mutually constrain each other. Notably, contours have a prominent effect on perceived shape from shading1, where covariations (coordinated changes) in luminance and contour create a strong impression of 3D shape. Conversely, a bumpy shading gradient can be flattened by a smooth contour in special cases where this covariation is disrupted. However, we found no such effect of contour on disparity, which is already highly specified at close distances. This is not surprising given that contour does not generally constrain disparity information. Still, disparity is expected to constrain the interpretation of shading. Here, we tested whether the effect of covariation between contour and luminance would be strong enough to override the shape specified by disparity. We started by cropping a periodic luminance pattern with either a covarying corrugated contour or a smooth contour. This effectively modulated whether the image was perceived to be a shaded 3D corrugated surface or a smooth surface with light and dark blurry stripes. We then combined these images with disparity fields that were either corrugated or smooth. Observers were asked to report the shape of the surface in a 2AFC task. Remarkably, we found that when the surface specified by disparity was smooth, the presence of covarying luminance and corrugated contour information significantly increased the rate that observers responded ‘corrugated’. This was the case despite the null effect of contour on the perceived shape from disparity when the luminance pattern was not present. Together, these findings suggest that the covariation between contour and luminance supports a 3D interpretation that is strong enough to override the otherwise powerful disparity cue. 1Todorović, D. (2014). How shape from contours affects shape from shading. Vision Research, 103, 1-10