  The relation between spontaneous and stimulated global brain activity is a
fundamental problem in the understanding of brain functions. This question is
investigated both theoretically and experimentally within the context of
nonequilibrium fluctuation-dissipation relations. We consider the stochastic
coarse-grained Wilson-Cowan model in the linear noise approximation and compare
analytical results to experimental data from magnetoencephalography (MEG) of
human brain. The short time behavior of the autocorrelation function for
spontaneous activity is characterized by a double-exponential decay, with two
characteristic times, differing by two orders of magnitude. Conversely, the
response function exhibits a single exponential decay in agreement with
experimental data for evoked activity under visual stimulation. Results suggest
that the brain response to weak external stimuli can be predicted from the
observation of spontaneous activity and pave the way to controlled experiments
on the brain response under different external perturbations.
