**4.4 Discussion**

The observed VWM capacity was about 3 objects, which is consistent with many previous findings using simple visual features (Luck & Vogel, 1997). In relation to the behavioral results, the EEG results revealed that the frontal theta and parietal alpha amplitudes were sustainably enhanced during the retention interval of the DMS task. Interestingly, frontal theta activity demonstrated a positive correlation with individual WM capacity, whereas parietal alpha activity demonstrated a negative correlation.

In addition to confirming previous reports that these oscillations are involved in VWM (; Klimesch et al., 2008; Jensen & Tesch, 2002; Jensen et al., 2002), the present study was able to dissociate their functions. Frontal theta activities have been associated with central executive functions including mental manipulation and calculation tasks (Kawasaki et al., 2010) and in supporting VWM storage during high-VWM loads and demands (Curtis & D'Esposito, 2003; Kawasaki & Watanabe, 2007; Sakai et al., 2002). Parietal alpha activity has been proposed to reflect simple WM storage. Indeed, many neuroimaging studies using the DMS task with simple visual features (e.g., color) have shown that parietal activity was correlated with VWM capacity and decreased beyond the limit of VWM capacity, unlike increased frontal activity (Linden et al., 2003; Rypma et al., 2002). These results suggeted that parietal alpha activity may be involved essentially only in the maintenance of limited visual information, whereas the frontal theta activity seems to assist in VWM storage under high VWM demand, as if instead of the suppressed alpha activity.
