DURATION OF EEG AND MEG-ALPHA SUPPRESSION INCREASES WITH ANGLE IN A MENTAL ROTATION TASK

Electric and magnetic recordings of average power within the high or b and (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 f igure was identical to a memory figure presented earlier at a differen t orientation, or whether it was the mirror image of the memory figure . Alpha power was found to be suppressed while the subjects were engag ed in the comparison, and the duration of suppression increased with t he minimum rotation angle to achieve a match. Strong correlations betw een 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 ma rkedly more difficult, a shift in the spatial pattern of suppression i ndicates some additional activity in left occipital areas.