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MULTIPLE REPRESENTATIONS FOR MENTAL IMAGERY

From the point of view of cognitive psychology, proposals from the A1 community concerning the representation of knowledge used in psychological tasks are often difficult to evaluate. On the one hand, such proposals often seem designed to account for or to explain the behavior of people engaging in such tasks and, thus, constitute theories of the psychological mechanisms and representations underlying task performance. On the other hand, the proposals are frequently motivated by considerations largely outside the scope of psychological theorizing, such as programming ease and computational efficiency. Certainly the work of Marr (1982) addressed both sorts of objectives; and Marr’s proposals have been readily incorporated into psychological theory, as well as inspiring a decade of research in the fields of visual perception and cognition. Glasgow’s (1993) discussion of imagery from a computational perspective also seems directed toward both of the goals outlined above. I will confine my remarks to the usefulness of her proposals regarding representations for imagery to questions of importance to cognitive psychology. In particular, psychologists might ask how well the proposed depictive knowledge representation accommodates phenomena and experimental findings in the large literature on imagery and, more generally, visual cognition. In addition to consistency with available evidence, we might ask whether the approach provides a novel perspective capable of generating meaningful research questions. Happily, Glasgow along with many psychologists and philosophers in recent years -has concluded that the standard form of the so-called “imagery debate” is of limited interest or usefulness from theoretical, empirical, or computational standpoints. To argue whether images are represented in analog or propositional form misses a fundamental point, viz., that different sorts of underlying representations may support performance on different cognitive tasks, all of which in some sense qualify as “mental imagery.” Stated differently, an important insight of recent work in cognitive science is that multiple, taskdependent representational systems (or subsystems) may be required to explain how people accomplish various perceptual and cognitive acts. (Cognitive neuropsychology adds the suggestion that systems identifiable at the functional level may correspond to distinct underlying neural systems.) Glasgow is fully aware of this currently popular theoretical perspective. Her proposed framework, involving three distinct kinds of representations-descriptive, spatial, and visual-for mental imagery tasks, is motivated in part to account for certain findings in the psychological literature. In particular, Kosslyn’s (1987) distinction between the coding of categorical relations among parts of an object, i.e., the qualitative representation of relative location information, and the metric representation of high-precision information needed to navigate in space, can be mapped onto Glasgow’s spatial and visual modes of representation. In a similar fashion, the multiple representations described in Glasgow’s paper are compatible with evidence for dissociations between performance on visually based versus spatially based imagery tasks in patients with particular forms of brain damage (Farah et al. 1988). Another body of recent work (Cooper and Schacter 1992; Schacter ef al. 1990) has demonstrated functional dissociations between systems for representing the global structure of visual objects and for coding episodic information, including objects’ meaning,