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Recent efforts to build computer simulation models of mental imagery have suggested that imagery is not a unitary phenomenon. Rather, such efforts have led to a modular analysis of the image generation process, with separate modules that can activate visual memories, inspect parts of imaged patterns, and arrange separate parts into a composite image. This idea was supported by the finding of functional dissociations between the kinds of imagery tasks that could be performed in the left and right cerebral hemispheres of two patients who had had their corpus collosa surgically severed. The left hemisphere in both subjects could inspect imaged patterns and could generate single and multi-part images. In contrast, although the right hemisphere could inspect imaged patterns and could generate images of overall shape, it had difficulty in generating multi-part images. the results support the computational model, and suggest a deficit in the module that arranges parts into a composite. The observed pattern of deficits and abilities implied that this module is not used in language, perception, or drawing.