Recognizing rotated faces and Greebles: Is the inversion effect unique to faces?

The fact that faces are strongly affected by picture-plane inversion has often been cited as evidence for face-specific mechanisms. It is unclear, however, whether this “face inversion effect” is restricted to faces or results from general mechanisms that compensate for pictureplane rotations. To address this issue, in Experiment 1 observers learned upright faces and were then tested with faces across a range of orientations for comparison with previous studies using non-face objects. Experiments 2 and 3 directly compared the recognition of faces and matched novel stimuli (Greebles) in a naming task using either single or multiple trained orientations, respectively. In all three experiments, performance systematically decreased with increasing misorientation from either the upright (Experiments 1 and 2) or nearest trained orientation (Experiment 3). Taken together, these results suggest that recognizing inverted faces is difficult because faces, as a stimulus class, have several properties known to prompt orientation dependence.