**7. Acknowledgment**

258 Neuroimaging – Cognitive and Clinical Neuroscience

Significant differences in performance were detected between the single and dual WM conditons (Wilcoxon signed-rank test; *Z* = 2.87, *P* < 0.01), suggesting the presence of dualtask interference, that is, degraded performance of 2 simultaneous tasks relative to a single

Time-frequency analyses of the recorded EEG data revealed enhanced theta amplitudes (4–6 Hz) of the 4 manipulation periods relative to those of the ITI in the frontal and parietal regions in both the single visual and dual WM conditions (single WM: AF3 electrode, *Z* = 3.53, *P* < 0.01; Pz electrode, *Z* = 2.04, *P* < 0.05; dual WM: AF3 electrode, *Z* = 3.71, *P* < 0.01; Pz electrode, *Z* = 3.01, *P* < 0.01). The increased frontal theta amplitudes during the dual WM conditions were significantly higher than those during the single VWM condition (AF3, *Z* = 2.24, *P* < 0.03), whereas this difference was not observed in the parietal theta activities (Pz, *Z*

In addition to the theta amplitudes, alpha amplitudes (9–12 Hz) were increased only in the parietal regions during manipulation periods in the single visual WM condition (single WM: AF3, *Z* = 1.15, *P* = 0.25, Pz, *Z* = 2.19, *P* < 0.05; dual WM: AF3 electrode, *Z* = 1.11, *P* < 0.27; Pz electrode, *Z* = 2.39, *P* < 0.02). Parietal alpha amplitudes demonstrated no significant difference between the single and dual WM conditions (Pz, *Z* = 1.78, *P* = 0.08). Moreover, enhanced parietal alpha activity was observed during the retention intervals as well as the

The EEG results concerning oscillatory amplitudes demonstrated the bottlenecks of central executive function in WM. In our recent study using single visual and auditory WM tasks, the frontal theta activity was mainly observed during the manipulation period and not the maintenance periods, whereas posterior alpha activity was enhanced both in the manipulation and maintenance periods (Kawasaki et al., 2010). Building upon those previous findings, the present study demonstrated that frontal theta activity further increased in the dual WM task in comparison to the single VWM task, whereas parietal alpha activity did not differ between the single and dual WM tasks. In this study, the dual WM task required a large amount of mental manipulation compared to the single WM task. However, the amount of visual representations to be remembered for the dual WM task was almost same that required for the single VWM task. Therefore, these results indicate that the bottlenecks for central executive function are represented by frontal theta activity, which is supported by the earlier evidence that the frontal cortex is associated with active manipulation, and the posterior regions are involved in simple maintenance (Curtis & D'Esposito, 2003; Postle et al., 1999; Rowe et al., 2000; Smith & Jonides, 1999; Wager & Smith, 2003). These results suggest that concurrent frontal theta and alpha activity is associated with the hierarchical control structures of the multiple operations

Using data from 2 EEG experiments, this study has demonstrated the brain oscillations that are related to WM capacities for visual storage and central executive function. Frontal theta and parietal alpha activities represented the storage limitations under conditions of high and low WM demands, respectively. Moreover, frontal theta activity was also related to bottlenecks in central executive function, which is necessary to perform dual WM tasks. In addition to confirming previous findings concerning regional dissociations between WM

task (e.g., psychological refractory period) (Logan & Gordon, 2001; Pashler, 1994).

= 0.68, *P* = 0.49).

**5.4 Discussion** 

involved in dual WM tasks.

**6. Conclusion** 

manipulation periods (Pz, *Z* = 0.49, *P* = 0.62).

This research was supported by Grant-in-Aid for Scientific Research on Innovative Areas (21120005 & 22118510) and Grant-in-Aid for Young Scientists (B) (23700328). We would like to thank Yoko Yamaguchi for her help in discussions for this study, Eri Miyauchi for her support of our data analyses, and Yuta Kakimoto, Ken'ichi Sawai, and Saoko Ikuno for their support of our data acquisition.
