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In three experiments, we independently manipulated the angular disparity between objects to be compared and the angular distance between the central axis of the objects and the vertical axis in a mental rotation paradigm. There was a linear increase in reaction times that was attributable to both factors. This result held whether the objects were rotated (with respect to each other and to the upright) within the frontal-parallel plane (Experiment 1) or in depth (Experiment 2), although the effects of both factors were greater for objects rotated in depth than for objects rotated within the frontal-parallel plane (Experiment 3). In addition, the factors interacted when the subjects had to search for matching ends of the figures (Experiments 1 and 2), but they were additive when the ends that matched were evident (Experiment 3). These data may be interpreted to mean that subjects normalize or reference an object with respect to the vertical upright as well as compute the rotational transformations used to determine shape identity.

In two experiments, observers searched for a misaligned vertex in displays depicting one or more twodimensional (2-D), three-dimensional (3-D), and quasi-3-D objects that were drawn with unconnected vertices. Serial search functions were obtained for all three conditions, even after much practice. In addition, search among the 2-D objects was more efficient than search among either 3-D or quasi-3-D objects, which did not differ after practice. This finding suggests that differences in depicted dimensionality cannot explain the difference in performance between the 2-D and 3-D conditions, but differences in stimulus complexity can. Further, after practice with the 2-D displays, observers could detect a misaligned vertex as rapidly among two objects as they could with one-object displays. These findings have implications for models of object recognition that assign a critical role to vertices in early vision, as well as the notion that mechanisms underlying preattentive processing may differ from those underlying the automaticity of processing that develops with practice.

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