Orientation invariance in visual shape perception.

To assess directly the orientation-invariance of specific shape representation stages in humans, we examined whether rotation (on the image plane or in depth) modulates the conjunction and linear non-separability effects in a shape visual search task (M. Arguin & D. Saumier, 2000; D. Saumier & M. Arguin, 2003). A series of visual search experiments involving simple 2D or 3D shapes show that these target type effects are entirely resistant to both planar and depth rotations. It was found however, that resistance to depth rotation only occurred when the 3D shapes had a richly textured surface but not when they had a uniform surface, with shading as the only reliable depth cue. The results also indicate that both planar and depth rotations affected performance indexes not concerned with the target type effects (i.e. overall RTs and magnitude of display size and target presence effects). Overall, the present findings suggest that the shape representations subtending the conjunction and linear non-separability effects are invariant across both planar and depth rotations whereas other shape representation stages involved in the task are orientation-specific.