Bottom–up and top–down functional connectivity in the conscious and the anesthetized brain. (A) Anesthesia affects top–down connectivity (red arrows) more significantly than bottom–up connectivity (blue arrows). Note that (1) a human brain has been chosen as figure background although some of the studies discussed in the text were carried out in animals; (2) the caliber of the arrows does not reflect study results quantitatively. (B) In human subjects, top–down connectivity (displayed in red) dominates bottom–up connectivity (displayed in blue) in the conscious state. This imbalance disappears upon loss of consciousness induced with a variety of general anesthetics due to the selective suppression of top–down signaling. The top panels display bottom–up and top–down connectivity separately, whereas the bottom panels represent subtractions of the individual data points to illustrate the imbalance between bottom–up and top–down connectivity. Note that the terms “feedforward” and “feedback” used in the figure refer to parieto-frontal and fronto-parietal connectivity, respectively, and are equivalent, in the present context, to the terms “bottom–up” and “top–down” used throughout this article. The induction phase of anesthesia is marked in light blue in all graphs. The data displayed in this figure were obtained using normalized symbolic transfer entropy; refer to table 1 for further details on the methods used in other studies. FB = feedback (top–down) connectivity; FF = feedforward (bottom–up) connectivity. Adapted, with permission, from Lee U. Anesthesiology 2013; 118:1264–75.¹⁹  Adaptations are themselves works protected by copyright. So in order to publish this adaptation, authorization must be obtained both from the owner of the copyright in the original work and from the owner of copyright in the translation or adaptation.