Abstract
In humans, theta phase (4–8 Hz) synchronization observed on electroencephalography (EEG) plays an important role in the manipulation of mental representations during working memory (WM) tasks; fronto-temporal synchronization is involved in auditory-verbal WM tasks and fronto-parietal synchronization is involved in visual WM tasks. However, whether or not theta phase synchronization is able to select the to-be-manipulated modalities is uncertain. To address the issue, we recorded EEG data from subjects who were performing auditory-verbal and visual WM tasks; we compared the theta synchronizations when subjects performed either auditory-verbal or visual manipulations in separate WM tasks, or performed both two manipulations in the same WM task. The auditory-verbal WM task required subjects to calculate numbers presented by an auditory-verbal stimulus, whereas the visual WM task required subjects to move a spatial location in a mental representation in response to a visual stimulus. The dual WM task required subjects to manipulate auditory-verbal, visual, or both auditory-verbal and visual representations while maintaining auditory-verbal and visual representations. Our time-frequency EEG analyses revealed significant fronto-temporal theta phase synchronization during auditory-verbal manipulation in both auditory-verbal and auditory-verbal/visual WM tasks, but not during visual manipulation tasks. Similarly, we observed significant fronto-parietal theta phase synchronization during visual manipulation tasks, but not during auditory-verbal manipulation tasks. Moreover, we observed significant synchronization in both the fronto-temporal and fronto-parietal theta signals during simultaneous auditory-verbal/visual manipulations. These findings suggest that theta synchronization seems to flexibly connect the brain areas that manipulate WM.
Highlights
Working memory (WM) consists of a short-term maintenance system, and a central executive system that manipulates the maintained representation (Baddeley, 2007)
We used two types of WM manipulation tasks: an auditory-verbal WM task, which required the mental calculation of numbers presented through an auditory-verbal stimulus; and a visual WM task, which required the participants to move a spatial location in a mental representation in accordance with a visual stimulus
The bootstrapped results revealed that the theta (6 Hz) PSIAF3−P5 was significant during the manipulation periods compared with the corresponding values recorded during the inter-trial interval (ITI) in the auditoryverbal WM, simultaneous WM, and auditory-verbal manipulation of sequential WM (P < 0.01; Figure 2)
Summary
Working memory (WM) consists of a short-term maintenance system, and a central executive system that manipulates the maintained representation (Baddeley, 2007). The theta-range (4–8 Hz) phase synchronizations between the prefrontal cortex and the relevant cortical areas form the executive functions that link the storage systems during the manipulation of mental representations (Kawasaki et al, 2010). To investigate brain-network dynamics, we measured EEG signals during a single or dual task for two sensory modalities and analyzed phase synchronization between distant cortical areas. We conducted region-of-interest analyses, because we had previously identified the representative electrodes of prefrontal, auditory-verbal, and visual areas by analyzing EEG data from the same two single WM tasks (Kawasaki et al, 2010). Theta phase synchronization between the electrodes was observed for the relevant WM tasks
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