Stereo is a powerful 3D cue, but there is mixed evidence about whether stereo contributes to recognizing objects across changes in viewpoint. Some previous studies have demonstrated a stereo advantage for recognition of unfamiliar objects across viewpoints. However, in these studies, 3D structure was not well-specified by monocular information (e.g, bent paperclips). When rich monocular cues to 3D shape are available, does stereo still provide a benefit? We tested shape discrimination for smoothly-curved, solid random shapes in three viewing conditions: shading-only, stereo-only, and combined shading and stereo. Objects were simulated to have a fine homogeneous surface texture, and illumination was either ambient (stereo-only) or a diffused point light source (shading-only and combined conditions). A mirror haploscope was used to present binocular images. Observers performed same-different shape discrimination judgments for sequentially presented images. Standard and test objects were either the same orientation or differed by a rotation in depth of ?±15?°, ?±30?°, or ?±60?° around the vertical axis. To discourage a 2D strategy, pairs of comparison shapes were constructed to have the same occlusion contour when viewed from the base orientation, and light source direction was varied between standard and test objects. We found that rotation in depth markedly impaired discrimination performance in all viewing conditions, as evidenced by reduced sensitivity (da?2) and increased bias toward judging same shapes as different. We also observed a consistent benefit from stereo for all viewpoint conditions. Although the shading-only images produce a strong subjective percept of 3D shape, observers were not able to reliably discriminate shapes across changes in viewpoint (30?°: da?2 = .86, 60?°: da?2 = .46). Discrimination was significantly better with binocular viewing (30?°: da?2 = 1.34, 60?°: da?2 = .78). Our results demonstrate that shape perception for random 3D objects is highly viewpoint-dependent, and that stereo information can reduce viewpoint costs.
展开▼
