  A universal mechanism of emergence of synchronized low frequency brain wave
field activity is presented as a result of nonlinear coupling with flat
frequency neuronal forcing. The mechanism utilizes a unique dispersion
properties of weakly-evanescent wave--like brain surface modes that are
predicted to exist within a inhomogeneous and anisotropic physical brain tissue
model. These surface modes are able to propagate in thin inhomogeneous layers
with frequencies that are inverse proportional to wave numbers. The resonant
and non-resonant terms of nonlinear coupling between multiple modes produce
both synchronous spiking-like high frequency wave activity as well as low
frequency wave rhythms. The relatively narrow localized frequency response of
the non-resonant coupling can be expressed by terms similar to phase coupling
in oscillatory systems. Numerical simulation of forced multiple mode dynamics
shows as forcing increases a transition from damped to oscillatory regime that
is then silenced off as over excitation is reached. The resonant nonlinear
coupling results in emergence of low frequency rhythms with frequencies that
are several orders of magnitude below the linear frequencies of modes taking
part in the coupling.
