Histograms of links strength in the brain-heart network during different sleep stages.

<p>Group averaged links strength are obtained using the TDS measure, where each link represents the interaction of the heart with a given brain area through a specific frequency band. Links are separately grouped by brain areas (Frontal, Central or Occipital), and are arranged in order from low-frequency (<i>δ</i> and <i>θ</i>) to high-frequency (<i>γ</i>₁ and <i>γ</i>₂) bands. For a given physiologic state, the frequency profile of the TDS links strength of brain-heart interactions remains stable for all brain areas. In contrast to other brain-organ networks where the frequency profile is stable across sleep stages, the frequency profiles of links strength in the brain-heart network change with transitions across different physiologic states—for all brain areas the frequency profiles become flatter as sleep depth increases—indicating that the mechanisms driving brain-heart interactions are markedly different compared to other organs. The sleep-stage stratification observed for the links strength in the brain-heart radar-charts is consistently observed for all frequency bands and brain areas as shown in the histograms—stronger links during W and LS, and weaker links during REM and DS. There is a strong symmetry in the links strength distribution between the left and right hemispheres during all sleep stages.