1 Introduction Shading is a strong cue of depth perception (Ramachandran 1988a, 1988b; Todd and Mingolla 1983). For example, a top-lit disk tends to be perceived as convex, whereas a bottom-lit disk tends to be perceived as concave (figure 1). The perception of distinct three-dimensional elements is supposed to be based on two constraints; ‘‘single light sources’’and ‘‘overhead light sources’’ (Kleffner and Ramachandran 1992; Ramachandran 1988a). We can understand the former constraint in terms of the fact that we cannot perceive disks as being simultaneously convex or concave.The latter constraint is evident when the figure is rotated by 908. The depth of the three-dimensional elements in the rotated figure is ambiguous because the ‘overhead light source’ is, in a sense, lost. This phenomenon also suggests that the perception of shape from shading is linked Perceptual grouping in shape from shading Perception, 2004, volume 33, pages 601 ^ 614 Takahiro Kawabe, Kayo Miura Behavioral and Health Sciences, Graduate School of Human-Environmental Studies, Kyushu University, 6-19-1, Hakozaki, Higashi-ku, Fukuoka-city, 8128581 Japan; e-mail: kawabe@psycho.hes.kyushu-u.ac.jp Received 22 August 2002, in revised form 18 February 2004 Abstract. We report on the reversal of asymmetry in visual-search tasks with shaded items. Previous studies have suggested that the target of a bottom-lit disk among distractors of top-lit disks is detected in a rapid and parallel manner, but not vice versa. However, in this study, we have shown that the compound items of top-lit disks were searched more quickly than those composed of bottom-lit disks where the items had to be segregated from their background. By modulating the inter-element distances, we confirmed that the reversal of search asymmetry cannot be due to the grouping of items. Further, we showed that the regions of the top-lit disks were perceived as figure more consistently than those of bottom-lit disks. The results indicate that the boundary assignment to the compound items of the top-lit disks enhances the segrega- tion of search items from the background, and that the search mechanism may access the relatively higher representation that includes figure ^ ground relations. DOI:10.1068/p3438 Figure 1. Two shaded disks.The left one, which is lit from above, is often perceived as being convex. The right one, which is lit from below, is often perceived as being concave.