Fig 2. Second-order cumulant contributions to plasticity.
A. The cross-covariance Cij between the presynaptic neuron j and the postsynaptic neuron i is obtained by summing over all the possible α- and β-paths from every possible source neuron k in the network. Each path is calculated via the corresponding weights in the connectivity matrix and the EPSC function (see Eq 3). B. Same as A but for the auto-covariance Cii of the postsynaptic neuron i (see Eq 4). In this case, γ is the second index to sum over the path from the source neuron k to the postsynaptic neuron i. It should be noted that the main difference between the α- and γ-path is given by the time dependence of the EPSC function, here represented in the Fourier domain for convenience.