Curvature discontinuities are cues for rapid shape analysis

Using the visual search method, we show that stimuli that contain curvature discontinuities (i.e., points where the second derivative along an image c ontour is not defined) are easily found among stimuli containing only smoot h changes in curvature. Curved stimuli that lack curvature discontinuities, however, are difficult to find among distracters that have them. These res ults suggest that the visual system detects and analyzes abrupt changes in curvature in the image quickly to extract vital information about the 3-D s tructure of the visual environment.