Neural systems shared by visual imagery and visual perception: a positron emission tomography study.

Subjects participated in perceptual and imagery tasks while their brains were scanned using positron emission tomography. In the perceptual conditions, subjects judged whether names were appropriate for pictures. In one condition, the objects were pictured from canonical perspectives and could be recognized at first glance; in the other, the objects were pictured from noncanonical perspectives and were not immediately recognizable. In this second condition, we assume that top-down processing is used to evaluate the names. In the imagery conditions, subjects saw a grid with a single X mark; a lowercase letter was presented before the grid. In the baseline condition, they simply responded when they saw the stimulus, whereas in the imagery condition they visualized the corresponding block letter in the grid and decided whether it would have covered the X if it were physically present. Fourteen areas were activated in common by both tasks, only 1 of which may not be involved in visual processing (the precentral gyrus); in addition, 2 were activated in perception but not imagery, and 5 were activated in imagery but not perception. Thus, two-thirds of the activated areas were activated in common.