Oscillatory gamma and theta activity during repeated mental manipulations of a visual image.

Previous studies on mental manipulation have dealt with direct manipulation of a presented stimulus or a maintained image of a previously presented stimulus. Here, we investigated brain activity differences between successive maintenance and manipulation of the visual stimulus and the once transformed representations using electroencephalography (EEG) in a multi-stage sequential manipulation task. The task required the subjects to memorize a presented object which possessed four features (color, shape, direction and speed of motion) and transform the feature of the representation twice. Wavelet analysis showed strong gamma-band (>30 Hz) activity elicited in the frontal and parietal regions during two successive mental manipulation tasks of the visual stimulus. Interestingly, gamma activity in the frontal and parietal regions was stronger during the second manipulation. Our results suggest that successive mental manipulations of the once transformed representation may impose higher demand on the fronto-parietal networks. On the other hand, while the frontal theta activity was enhanced throughout maintenance and manipulation periods, the activity during maintenance of one-time manipulated representation was higher than that of the physically presented stimulus, suggesting that the frontal regions are further recruited in maintenance of manipulated images.