Illustrations of the three types of false connections in the correlation-based methods. Solid lines indicate the physical wiring between neurons, and the filled circles at the end indicate the synaptic contacts (i.e., the direction of the connections). The dotted lines are the false connections introduced by the correlation-based methods. (A) Type 1 false connections, which are due to two neurons receiving the same synaptic inputs. (B) Type 2 false connections, which are due to the propagation of correlation. (C) Type 3 false connections, which are due to unobserved common inputs.