Schematics of effective information. Shown is a single subset S (grey ellipse) forming part of a larger system X. This subset is bisected into A and B by a bipartition (dotted line). Arrows indicate anatomical connections linking A to B and B to A across the bipartition, as well as linking both A and B to the rest of the system X. Bi-directional arrows indicate intrinsic connections within each subset and within the rest of the system. (Left) All connections are present. (Right) To measure EI(A→B), maximal entropy Hmax is injected into the outgoing connections from A (see Eq. 1). The resulting entropy of the states of B is then measured. Note that A can affect B directly through connections linking the two subsets, as well as indirectly via X.