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Transformations of visuospatial mental imagesHere are some simple instructions: "Close your eyes.Imagine you are standing in front of a pedestal facing a bust of William James. Imagine now that you have walked around the pedestal and are now viewing the bust from the side. Now, imagine that a motor is turning the bust, allowing you to view it from a range of angles. Finally, imagine that you reach out, pick up the bust, and turn it upside-down to look at the bottom."Most people report that they can follow such instructions, and doing so often results in phenomenal experiences that are vivid and faithful to reality-though the degree that this is the case varies among individuals (Isaac and Marks, 1994). The experiences evoked by instructions such as these are transformations of visuospatial images. They may not require explicit intentions to produce: Humans and other animals appear to construct and transform visuospatial representations to solve a range of everyday reasoning problems, including navigation, the making and using of tools, and construction. But what is the computational and neurophysiological nature of the representations involved? How are those representations transformed during explicit and implicit visuospatial imagery?There is now a body of theory and evidence speaking to each of these questions. The goals of this article are, first, to introduce a framework for thinking about visuospatial image transformations, and, second, to interpret the available data within that framework to provide current best answers to both questions. To do so, we first provide an overview of the phenomena constituting visuospatial imagery and then describe a framework for relating image transformations to the representations on which they operate. The rest of the article characterizes different types of image transformations and the relations among them. The article's major empirical claim is that two sorts of visuospatial transformations can be dissociated: transformations in which the representations of individual objects are updated relative to other spatial representations and transformations in which one's personal perspective is updated. These visuospatial transformations are characterized by different patterns of behavioral performance, different neural correlates, and different psychometric properties. 96Authors' Note: We would like to thank Ty Fagan for his assistance with the meta-analysis. We also thank Sarah Creem-Regehr, Nora Newcombe, Amy Shelton, Barbara Tversky, and Maryjane Wraga, whose thoughtful comments were extremely helpful in revising the article. The research of our laboratory described here was supported in part by the McDonnell Center for Higher Brain Function.

Human spatial reasoning may depend in part on two dissociable types of mental image transformations: object-based transformations, in which an object is imagined to move in space relative to the viewer and the environment, and perspective transformations, in which the viewer imagines the scene from a different vantage point. This study measured local brain activity with event-related fMRI while participants were instructed to either imagine an array of objects rotating (an object-based transformation) or imagine themselves rotating around the array (a perspective transformation). Object-based transformations led to selective increases in right parietal cortex and decreases in left parietal cortex, whereas perspective transformations led to selective increases in left temporal cortex. These results argue against the view that mental image transformations are performed by a unitary neural processing system, and they suggest that different overlapping systems are engaged for different image transformations.

Motor imagery is a complex cognitive operation that requires memory retrieval, spatial attention, and possibly computations that are analogs of the physical movements being imagined. Likewise, motor preparation may or may not involve computations that are analogs of actual movements. To test whether motor imagery or motor preparation activate representations that are specific to the body part whose movement is imagined or prepared, participants performed, imagined, and prepared hand movements while undergoing functional MRI scanning. Actual hand movements activated components of the motor system including primary motor and somatosensory cortex, the supplementary motor area, the thalamus, and the cerebellum. All of these areas showed strong lateral organization, such that moving a given hand activated the contralateral cortex and ipsilateral cerebellum most strongly. During motor imagery and motor preparation, activity throughout the motor system was much reduced relative to overt movement. However, significant lateral organization was observed during both motor imagery and motor preparation in primary motor cortex, the supplementary motor area, and the thalamus. These results support the view that the subjective experience of imagined movement is accompanied by computations that are analogs of the physical movement that is imagined. They also suggest that in this regard motor imagery and motor preparation are similar.

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