Figure 6.

Dynamic networks of interaction between cortical rhythms and integrated chin-muscle tone across physiological states. (A) Links in network maps represent group-averaged TDS coupling strength (section Materials and Methods 2.5.1) between each brain rhythm at a given cortical location and the chin muscle tone, after averaging over all chin EMG bands (see Figure 4 and section Materials and Methods 2.5.2), and correspond to the elements in the coarse-grained matrices shown in Figure 5A, left panels. Brain areas are represented by Frontal (Fp1, Fp2), Central (C3, C4), and Occipital (O1, O2) EEG channels, and network nodes with different colors represent seven cortical rhythms (δ, θ, α, σ, β, γ₁, γ₂) derived from the spectral power of each EEG channel. Links strength is illustrated by line thickness, and links color corresponds to the color of brain rhythms (network nodes). Shown are all links with strength %TDS ≥ 2.3%, corresponding to the significance threshold based on surrogate tests (section Method 2.4). Radar-charts centered in the hexagons represent the relative contribution from different brain areas to the strength of network links during different sleep stages. The length of each segment along each radius in the radar-charts represents the TDS coupling strength between each cortical rhythm at each EEG location and chin muscle tone. The segments are shown with the same color as the corresponding brain rhythms (network nodes). The brain-chin network interactions are mainly mediated through high frequency γ₁ and γ₂ cortical rhythms (thicker orange and red links), and are characterized with relatively symmetric links strength to all six cortical areas, as shown by the symmetric radar-chart in each hexagon, with stronger contribution from the Frontal and Central areas. Network reorganization is observed with transition across sleep stages: with overall stronger network links during wake (larger hexagon), intermediate during REM and light sleep, and much weaker interactions (smaller hexagon) during deep sleep. (B) Histograms of links strength in the brain-chin network during different sleep stages. Group-averaged links strength is obtained using the TDS measure, where each bar represents the average strength of interaction of all cortical rhythms from a given brain area (Frontal, Central or Occipital) with all chin muscle tone EMG bands. Error bars represent the standard error obtained for all subjects in the group; horizontal green lines in both panels mark a surrogate test threshold (%TDS = 2.3%; section Method 2.4) above which network interactions are physiologically significant. A pronounced sleep-stage stratification pattern is observed for the average links strength related to each cortical area, consistent for both left and right hemisphere (pair-wise comparison between sleep stages for the same brain area gives p ≤ 0.05 (MW test), except for REM vs. light sleep, and one-way ANOVA rank test comparison across all sleep stages gives p ≤ 0.001). Brain-chin muscle tone network interactions exhibit strong symmetry in links strength between the left and right hemisphere for all sleep stages (MW test, p ≥ 0.65).