Mental imagery of high- and low-resolution gratings activates area 17.

Some, but not all, previous neuroimaging studies of visual mental imagery have found that Area 17 (primary visual cortex) is activated when people visualize objects. The present study was designed to test the hypothesis that the necessary degree of resolution of the mental image is a determining factor in whether Area 17 is activated during imagery. Eight male subjects visualized and compared sets of stripes that required high or low resolution to resolve, while their brains were scanned using 15O(CO2) positron emission tomography (PET). When imagery in general (visualization of high- and low-resolution gratings stimuli combined) was compared to an auditory baseline condition where subjects did not visualize, Area 17 was activated. However, region of interest (ROI) and statistical parametric mapping (SPM) analyses revealed no difference between imagery conditions using high- and low-resolution stimuli. These results indicate that the resolution of the stimuli alone does not necessarily determine whether Area 17 will be activated during visual mental imagery.

Squinting with the mind's eye: effects of stimulus resolution on imaginal and perceptual comparisons.

Subjects either viewed or visualized arrays that were divided into four quadrants, with each quadrant containing a set of stripes. In two experiments, one array contained only relatively narrow (high-resolution) stripes, and one contained only relatively thick (low-resolution) stripes. The subjects compared sets of stripes in different quadrants according to their length, spacing, orientation, or width. When the subjects visualized the arrays, they required much more time to compare high-resolution patterns than low-resolution patterns; when the subjects saw the arrays, they evaluated both types of arrays equally easily. In addition, the results from the third experiment provide strong evidence that people use imagery in this task; in one condition, the subjects evaluated oblique sets of stripes, and in another condition, they evaluated vertical and horizontal stripes. In both imagery and perception, the subjects made more errors when evaluating oblique stimuli; in imagery, they also required more time to evaluate oblique stimuli. The results suggest that additional effort is required in imagery to represent visual patterns with high resolution. This finding demonstrates that, although imagery and perception may activate common brain regions, it is more difficult to represent high-resolution information in imagery than in perception.

The role of area 17 in visual imagery: convergent evidence from PET and rTMS.

Visual imagery is used in a wide range of mental activities, ranging from memory to reasoning, and also plays a role in perception proper. The contribution of early visual cortex, specifically Area 17, to visual mental imagery was examined by the use of two convergent techniques. In one, subjects closed their eyes during positron emission tomography (PET) while they visualized and compared properties (for example, relative length) of sets of stripes. The results showed that when people perform this task, Area 17 is activated. In the other, repetitive transcranial magnetic stimulation (rTMS) was applied to medial occipital cortex before presentation of the same task. Performance was impaired after rTMS compared with a sham control condition; similar results were obtained when the subjects performed the task by actually looking at the stimuli. In sum, the PET results showed that when patterns of stripes are visualized, Area 17 is activated, and the rTMS results showed that such activation underlies information processing.