[Statistical efficiency of the object recognition with wire-frame stimuli]

To investigate whether internal object representations in the brain are 2D or 3D, we measured discrimination thresholds and calculated statistical efficiencies in the object recognition task based on the ideal observer analysis. Stimulus images were created by orthographic projection of 3D wire-frame objects on a fronto-parallel plane. The discrimination task with the wire-frame stimuli was performed in the monocular (Experiment 1) and the binocular (Experiment 2) conditions. The results indicate that the subjects' statistical efficiencies relative to the 2D ideal observer exceeded 100%, suggesting that the internal object representations should be 3D. In addition, the results revealed that the object type with high-level symmetry led to better discrimination in both experiments. Furthermore, the interaction between 3D cue values and the object regularity was suggested to cause the viewpoint dependency.