**5. EEG oscillations for central executive**

This section describes the investigation of EEG oscillatory activities that represent the WM limitations for executive functions by comparing dual and single WM tasks. The dual tasks required 2 separate perceptual domains: mental manipulation with visual stimuli and the mental calculations with auditory stimuli.

### **5.1 Dual WM task for visual and auditory representations**

Fourteen healthy volunteers (10 male and 4 female; mean age = 27.92 ± 6.76 years, range 21– 41 years; 13 right-handed) with normal or corrected-to-normal visual acuity, normal hearing acuity, and normal motor performance took part in the single visual and dual WM tasks. All participants gave written informed consent, which was approved by the Ethical Committee of the RIKEN (in accordance with the Declaration of Helsinki), before the experiments were performed.

For the single VWM task, at the beginning of each trial, 5 5 gridded squares and a red circle included within one of those squares were presented on the computer screen as the visual stimulus for 1 s (Fig. 3A). The participants were required to memorize and then maintain the position of the red circle for 2 s after the visual stimulus disappeared. Awhite arrow designating a direction (up, down, right, or left) to which the participants should move the red circle in their minds was then presented at the center of screen for 1 s. The participants manipulated the mental representations for 2 s. Like the auditory working memory condition, the participants were required to repeat the mental manipulation 4 times, and then determine whether the position of the red circle which they mentally moved matched a probe visual stimulus (test display). In half of the trials, the probe stimulus matched the mental representation. In the remaining trials, the wrong probe was presented by changing only the fourth direction of movement from the initial position. The participants were asked to indicate via button press whether the probe stimulus was correct or not while the fixation point was red for 2 s. The duration of the ITI was 2 s. The size of the red circle and gridded squares was 1º 1º and 5º 5º (1º 1º per square), respectively.

For the dual WM task, the participants were asked to complete an auditory WM task simultaneously to the visual tast (Fig. 3B). When the visual stimuli described above were presented on the computer screen, a word indicating a one-digit number was simultaneously presented as the auditory stimulus through the headphones of both ears for 1 s (sample display). The auditory WM task requried the participants to memorize and maintain the presented number with rehearsal in their minds and, after a 2-s retention interval, to update the number by adding the another presented one-digit number for 2 s. After this a total of 4 incidences of auditory and visual manipulation, auditory and visual stimuli were simultaneously presented again, and participants were required to judge whether or not they were identical to the manipulated mental representation for both auditory and visual tasks (test display). In half of the trials, both the auditory and visual probe stimulus matched the mental representations. In the remaining trials, the incorrect probe for either the auditory or visual stimulus was presented, similar to the single VWM condition. The button press, duration of the inter-trial interval, and creation of the stimuli were identical to the single WM condition.


Fig. 3. Task procedure for one trial of the single visual WM (A) and dual WM (B) tasks.

#### **5.2 EEG measurements and analyses**

The same methods were used as described in Section 4.2.

### **5.3 Results**

256 Neuroimaging – Cognitive and Clinical Neuroscience

0.66 V; 6 objects: 0.45 ± 0.45 V; *Z* = 1.97, *P* < 0.05). Interestingly, frontal theta activity was significantly and positively correlated with the VWM capacity of the individual (Fz electrode: *r*(14) = 0.39, *P* < 0.05), whereas the parietal alpha activity was negatively

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

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

This section describes the investigation of EEG oscillatory activities that represent the WM limitations for executive functions by comparing dual and single WM tasks. The dual tasks required 2 separate perceptual domains: mental manipulation with visual stimuli and the

Fourteen healthy volunteers (10 male and 4 female; mean age = 27.92 ± 6.76 years, range 21– 41 years; 13 right-handed) with normal or corrected-to-normal visual acuity, normal hearing acuity, and normal motor performance took part in the single visual and dual WM tasks. All participants gave written informed consent, which was approved by the Ethical Committee of the RIKEN (in accordance with the Declaration of Helsinki), before the experiments were

For the single VWM task, at the beginning of each trial, 5 5 gridded squares and a red circle included within one of those squares were presented on the computer screen as the visual stimulus for 1 s (Fig. 3A). The participants were required to memorize and then maintain the position of the red circle for 2 s after the visual stimulus disappeared. Awhite arrow designating a direction (up, down, right, or left) to which the participants should move the red circle in their minds was then presented at the center of screen for 1 s. The participants manipulated the mental representations for 2 s. Like the auditory working

correlated with the VWM capacity (Poz electrode: *r*(14) = -0.44, *P* < 0.05).

parietal alpha activity demonstrated a negative correlation.

VWM demand, as if instead of the suppressed alpha activity.

**5.1 Dual WM task for visual and auditory representations** 

**5. EEG oscillations for central executive** 

mental calculations with auditory stimuli.

performed.

**4.4 Discussion** 

All participants performed all the WM tasks with high accuracy rates (mean accuracy rate (± s.d.), 97.3 ± 4.7% and 91.1 ± 7.1% for visual and dual WM conditions, respectively).

Human Oscillatory EEG Activities Representing Working Memory Capacity 259

functions, the present study further suggests important roles for these brain oscillations, which reflect different local synchronizations within specific cell assemblies, in the WM

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

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**7. Acknowledgment** 

**8. References** 

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

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* = 0.68, *P* = 0.49).

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

#### **5.4 Discussion**

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 involved in dual WM tasks.

#### **6. Conclusion**

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 functions, the present study further suggests important roles for these brain oscillations, which reflect different local synchronizations within specific cell assemblies, in the WM process: theta for manipulation and alpha for maintenance.
