Two types of image generation: evidence from PET.

Is there more than one method whereby people can generate visual mental images? Participants generated images after learning patterns in two ways. In one condition, they memorized descriptions of how segments are arranged to form patterns; in another, they memorized segments and mentally amalgamated them into patterns. In both conditions, identical stimuli cued them to form images while brain activation was monitored using PET. Comparison of the two imagery conditions revealed different activation between hemispheres when images were formed after patterns were learned by mentally combining segments versus when images were formed from stored verbal descriptions. Thus, images can be generated in at least two ways. However, this laterality difference was subtle; the majority of brain areas were activated in common across conditions. This overall similarity is counter to what would be expected if image generation is simply perceptual exploration in the absence of appropriate stimuli, as is posited by perceptual activity theory.