Explicit and implicit memory for rotating objects.

Although both the object and the observer often move in natural environments, the effect of motion on visual object recognition has not been well documented. The authors examined the effect of a reversal in the direction of rotation on both explicit and implicit memory for novel, 3-dimensional objects. Participants viewed a series of continuously rotating objects and later made either an old-new recognition judgment or a symmetric-asymmetric decision. For both tasks, memory for rotating objects was impaired when the direction of rotation was reversed at test. These results demonstrate that dynamic information can play a role in visual object recognition and suggest that object representations can encode spatio- temporal information. An enduring problem in visual cognition addresses how objects are represented internally for purposes of recognition. The appar- ent ease with which people recognize objects in everyday situa- tions conceals the immense complexity of the underlying mecha- nisms. Despite numerous studies of object recognition, researchers are still far from a complete understanding of the recognition processes and the underlying representations (for a recent review, see Tarr & Bulthoff, 1998). Behavioral studies of object recognition have for the most part used static visual stimuli. This tendency is echoed by noting that theories of object recognition are generally accounts of how static objects are recognized. In the real world, however, both the object and the observer often move about in the environment. Once this movement is noted, it then becomes natural to ask, "What are the effects of motion on object recognition?" The lack of investigation of this question is perhaps caused by a belief that shape is an intrinsic attribute of an object, but motion is an external attribute. This line of reasoning suggests that object recognition should not be affected by an object's motion. This belief is also supported by physiological evidence that demonstrates functional specialization in the visual system. For example, under one interpretation, the ventral stream processes shape information, whereas the dorsal stream processes spatial information, including motion (i.e., the what vs. where distinction; Ungerleider & Mishkin, 1982). Nonetheless, the perceptual literature has provided some exam- ples in which motion is important for the perception of shape. One such example is the kinetic depth effect (Wallach & O'Connell, 1953), in which motion makes the structure of the object (e.g., a wireframe cube) available to observers. A similar effect is found for biological motion, in which motion makes the perception of biological forms possible, even without any shape information (Johansson, 1973). Perhaps the most forceful argument for the importance of motion in perception comes from Gibson (1979). The theory of ecological optics proposes that visual information resides in the optic array and that invariants are extracted from dynamic optic arrays. According to Gibson, motion is paramount, for "invariants of structure do not exist except in relation to variants" (p. 87); that is, invariants are meaningful only when there is change in the optic array, and hence motion. Aside from its role in the perception of structure, motion has been shown to play a role in other visual functions. For example, Kozlowski and Cutting (1977) filmed a walking person in the dark with point-lights attached to major joints. When shown such a film, observers can reliably identify the gender of the walker on the basis of pure motion signals in the display. The direction of motion can also bias the identification of ambiguous figures (Bernstein & Cooper, 1997). Finally, research in representational momentum has clearly demonstrated that motion can bias memory for an object's spatial location (Freyd, 1987).