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. 2017 Mar 7:7:42776.
doi: 10.1038/srep42776.

Theta-alpha EEG phase distributions in the frontal area for dissociation of visual and auditory working memory

Masakazu Akiyama  1 , Atsushi Tero  2 , Masahiro Kawasaki  3   4 , Yasumasa Nishiura  5 , Yoko Yamaguchi  6
Affiliations

Theta-alpha EEG phase distributions in the frontal area for dissociation of visual and auditory working memory

Masakazu Akiyama et al. Sci Rep. .

Abstract

Working memory (WM) is known to be associated with synchronization of the theta and alpha bands observed in electroencephalograms (EEGs). Although frontal-posterior global theta synchronization appears in modality-specific WM, local theta synchronization in frontal regions has been found in modality-independent WM. How frontal theta oscillations separately synchronize with task-relevant sensory brain areas remains an open question. Here, we focused on theta-alpha phase relationships in frontal areas using EEG, and then verified their functional roles with mathematical models. EEG data showed that the relationship between theta (6 Hz) and alpha (12 Hz) phases in the frontal areas was about 1:2 during both auditory and visual WM, and that the phase distributions between auditory and visual WM were different. Next, we used the differences in phase distributions to construct FitzHugh-Nagumo type mathematical models. The results replicated the modality-specific branching by orthogonally of the trigonometric functions for theta and alpha oscillations. Furthermore, mathematical and experimental results were consistent with regards to the phase relationships and amplitudes observed in frontal and sensory areas. These results indicate the important role that different phase distributions of theta and alpha oscillations have in modality-specific dissociation in the brain.

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Conflict of interest statement

The authors declare no competing financial interests.

Figures

Figure 1
Figure 1
Task procedures of (a) auditory and (b) visual WM conditions. Subject-averaged theta (4–6 Hz) and alpha (10–12 Hz) amplitudes at frontal (AF3), temporal (P5), and parietal (Pz) electrodes under (c) auditory and (d) visual WM conditions.
Figure 2
Figure 2
Cross-histograms of probability distributions between the theta (6 Hz) and alpha (12 Hz) phases for the manipulation periods and for differences between the manipulation periods and ITI at AF3, P5, and Pz electrodes under (a) auditory and (b) visual WM conditions. (c) Z values of the CFCs between the theta and alpha phases at AF3, P5, and Pz electrodes under auditory (A) and visual (V) WM conditions. The dotted lines denote the threshold values (P < 0.01).
Figure 3
Figure 3. The arrows in the figure on the right indicate the flow of signal transduction; solid arrows indicate the directions of traveling signals, and broken arrows indicate signals that are not traveling.
The lines surrounding the lettering (solid or broken) indicate whether or not theta and alpha amplification occurs. (a) Global coupling occurs between AF3 and Pz. (b) Global coupling occurs between AF3 and P5. (c) The phase difference generated in AF3 acts as a signal for selective global coupling of Pz only. (d) The phase difference generated in AF3 acts as a signal for selective global coupling of P5 only.
Figure 4
Figure 4. A schematic diagram of the mathematical model.
The arrows in the figure indicate the flow of signal transduction.
Figure 5
Figure 5. The solid line indicates the function in the equation (5).
The green and red dot are stable or unstable point of equilibrium, respectively.
Figure 6
Figure 6. Simulation result of s0, s1, cosϕ and f(t) are shown as solid color black, red, green, and blue lines, respectively.
(a) This simulation result shows the case of Auditory WMTs. (b) This simulation result shows the case of Visual WMTs. (c) When external input is absent or cut off.
Figure 7
Figure 7. Reproduction of experimental result in mathematical simulations.
The solid line shows our simulation result.
Figure 8
Figure 8. The intensity distributions of simulation results for θ (6 Hz), α (12 Hz), β1 (18 Hz) and β2 (24 Hz) at P5 and Pz during A-MWTs (left) and V-WMTs (right), respectively.
See this image and copyright information in PMC

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