NEURAL SYSTEMS SHARED BY VISUAL-IMAGERY AND VISUAL-PERCEPTION - A POSITRON-EMISSION-TOMOGRAPHY STUDY

Subjects participated in perceptual and imagery tasks while their brai ns were scanned using positron emission tomography. In the perceptual conditions, subjects judged whether names were appropriate for picture s. In one condition, the objects were pictured from canonical perspect ives and could be recognized at first glance; in the other, the object s were pictured from noncanonical perspectives and were not immediatel y recognizable. In this second condition, we assume that top-down proc essing is used to evaluate the names. In the imagery conditions, subje cts 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 visualiz ed the corresponding block letter in the grid and decided whether it w ould 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 inv olved in visual processing (the precentral gyrus); in addition, 2 were activated in perception but not imagery, and 5 were activated in imag ery but not perception. Thus, two-thirds of the activated areas were a ctivated in common. (C) 1997 Academic Press.