A stereo disadvantage for recognizing rotated familiar objects

832 Our visual system has developed to recognize objects in a three-dimensional (3-D) world, yet most perceptual studies display static objects on flat computer monitors. To what extent is recognition of objects affected by the presence of explicit 3-D information, such as that gained from stereoscopic disparities? Researchers who have investigated this issue, by displaying stimuli with (stereo) and without (mono) stereoscopic disparities, have generally shown an advantage in recognizing 3-D stimuli over 2-D images (e.g., Edelman & Bülthoff, 1992). In addition, stereo presentations of objects often assist in generalization to unstudied views. Humphrey and Khan (1992) reported a stereo benefit for recognition of unfamiliar objects that was larger for novel views than for studied views, although this may have been caused by a speed–accuracy trade-off. Farah, Rochlin, and Klein (1994) reported better viewpoint generalization for novel shapes when these were viewed directly (with stereoscopic disparities) than when they were presented via videotape. Burke (2005) and Bennett and Vuong (2006) have both recently reported that generalization to new views of novel objects is better when the objects are depicted in stereo than when depicted in mono, with no evidence of any speed–accuracy trade-off. In addition, Burke, Taubert, and Higman (2007) have shown a similar stereo advantage for generalizing unfamiliar faces to new viewpoints. Should these results be taken as evidence for a general stereo advantage in object recognition? One concern with such a conclusion is that most of these studies used highly similar objects, either wireclips or deformed torii (Bennett & Vuong, 2006; Burke, 2005; Edelman & Bülthoff, 1992) or faces (Burke et al., 2007). This is quite unlike normal recognition of 3-D objects, in which classifications (at least at the basic level of categorization) require discrimination among objects quite different in shape, and for which the task is relatively easy, even with 2-D depictions of stimuli. Humphrey and Khan (1992) used qualitatively different novel objects, but, as noted, the evidence for a stereo advantage in their study is weaker. We wanted to investigate whether there was any benefit of stereo depth when 2-D cues provide good information to support recognition. Recognition of rotated objects is often predicted by changes in the 2-D outline (or silhouette) of an object between study and test views (Hayward, 1998; Hayward, Tarr, & Corderoy, 1999; Lloyd-Jones & Luckhurst, 2002). For example, Hayward tested recognition of familiar objects across rotations of 0o, 60o, and 180o. Although 180o was the largest rotation, and so might be expected to produce the largest recognition cost, rotations of this size present similar (if mirror-reversed) outlines of an object when depicted in 2-D; accordingly, performance in this condition was superior to that for the 60o rotations. Depiction in 3-D introduces a new variation in outline shape when objects are rotated by 180o; a shape component close to the viewer at 0o will be far from the viewer at 180o. So, although the same component may be visible in both views, the difference in depth will be explicitly coded in 3-D; one might, therefore, expect reduced similarity in recognition performance across a 180o rotation.1 As such, stereo depth information might actually impair object recognition in this situation. We tested this hypothesis in three experiments that were similar except for the specific views of the objects presented. Each used the same basic methodology as that in Hayward (1998); using familiar objects as stimuli, participants judged whether two sequentially presented stimuli showed the same object or two different objects. A stereo disadvantage for recognizing rotated familiar objects