  The responses of synapses in the neocortex show highly stochastic and
nonlinear behavior. The microscopic dynamics underlying this behavior, and its
computational consequences during natural patterns of synaptic input, are not
explained by conventional macroscopic models of deterministic ensemble mean
dynamics. Here, we introduce the correlation entropy of the synaptic
input-output map as a measure of synaptic reliability which explicitly includes
the microscopic dynamics. Applying this to experimental data, we find that
cortical synapses show a low-dimensional chaos driven by the natural input
pattern.
