What change detection tells us about the visual representation of shape

Many recent findings suggest that human observers are surprisingly "blind" to changes in visual displays, failing to notice when substantial scene elements are added, subtracted, or altered in successive presentations of the scene. But observers are far more sensitive to certain visual changes than others, and we suggest that which types of changes enjoy differential sensitivity can reveal a great deal about the underlying visual representations. In this study, we investigate how the human visual system represents the shape of objects by demonstrating a previously unknown influence on detection of changes in shape: the sign of contour curvature. We show that subjects are substantially more sensitive to changes in concave regions of a shape's contour than to changes in convex regions, even when these changes do not alter the number or location of parts. Further, we show that this effect is modulated by figure-ground assignment, so that changes to the same physical contour are more or less detectable, depending on the contour's perceived figural status, which determines whether the change falls in a concave or convex region. The results demonstrate a heightened sensitivity for changes at concavities that is not reducible to a sensitivity to changes in gross part structure.