Abstract
Abstract Cerebral blood flow was measured using positron emission tomography (PET) in three experiments while subjects performed mental imagery or analogous perceptual tasks. In Experiment 1, the subjects either visualized letters in grids and decided whether an X mark would have fallen on each letter if it were actually in the grid, or they saw letters in grids and decided whether an X mark fell on each letter. A region identified as part of area 17 by the Talairach and Tournoux (1988) atlas, in addition to other areas involved in vision, was activated more in the mental imagery task than in the perception task. In Experiment 2, the identical stimuli were presented in imagery and baseline conditions, but subjects were asked to form images only in the imagery condition; the portion of area 17 that was more active in the imagery condition of Experiment 1 was also more activated in imagery than in the baseline condition, as was part of area 18. Subjects also were tested with degraded perceptual stimuli, which caused visual cortex to be activated to the same degree in imagery and perception. In both Experiments 1 and 2, however, imagery selectively activated the extreme anterior part of what was identified as area 17, which is inconsistent with the relatively small size of the imaged stimuli. These results, then, suggest that imagery may have activated another region just anterior to area 17. In Experiment 3, subjects were instructed to close their eyes and evaluate visual mental images of upper case letters that were formed at a small size or large size. The small mental images engendered more activation in the posterior portion of visual cortex, and the large mental images engendered more activation in anterior portions of visual cortex. This finding is strong evidence that imagery activates topographically mapped cortex. The activated regions were also consistent with their being localized in area 17. Finally, additional results were consistent with the existence of two types of imagery, one that rests on allocating attention to form a pattern and one that rests on activating stored visual memories.
Highlights
Visual perception occurs while a stimulus is being viewed, which leads to the creation of modality-specific internal representations; in contrast, visual mental imagery occurs when such representations are present but the object is no longer being viewed
The most striking aspect of our results is that we found greuter activation of visual cortex during image generation than during perception
Seen from the rear of the head, the X coordinate is horizontal, the Y coordinate is in depth, and the Z coordinate is vertical
Summary
Visual perception occurs while a stimulus is being viewed, which leads to the creation of modality-specific internal representations; in contrast, visual mental imagery occurs when such representations are present but the object is no longer being viewed. Visual mental images correspond to short-term memory representations that lead to the experience of “seeing with the mind’s eye.”. The recent debate has focused on the question of whether visual mental images depict information, or whether they represent informationpropositionally (and the depictiveproperties of imagery that are evident to introspection play no functional role in information processing). In this article we report three positron emission tomography (PET) experiments in which we investigatewhether visual mental imagery activates topographically mapped areas of visual cortex.Such a findingwould be good evidencethat visual imagery is a special kind of visual short-term memory representation, one that involves depictive mental representations
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