Electric and magnetic recordings of average power within the high a band (10–12 Hz) were made over the parietal and occipital areas of the scalp while subjects were engaged in the mental imagery task of Cooper and Shepard. The subject had to determine whether an abstract probe figure was identical to a memory figure presented earlier at a different orientation, or whether it was the mirror image of the memory figure. Alpha power was found to be suppressed while the subjects were engaged in the comparison, and the duration of suppression increased with the minimum rotation angle to achieve a match. Strong correlations between suppression duration and reaction time give further evidence that the visual cortex is engaged in the process of mental imagery. Moreover, for large rotation angles of the probe figures, where the task is markedly more difficult, a shift in the spatial pattern of suppression indicates some additional activity in left occipital areas.

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