  An inhomogeneous anisotropic physical model of the brain cortex is presented
that predicts the emergence of non--evanescent (weakly damped) wave--like modes
propagating in the thin cortex layers transverse to both the mean neural fiber
direction and to the cortex spatial gradient. Although the amplitude of these
modes stays below the typically observed axon spiking potential, the lifetime
of these modes may significantly exceed the spiking potential inverse decay
constant. Full brain numerical simulations based on parameters extracted from
diffusion and structural MRI confirm the existence and extended duration of
these wave modes. Contrary to the commonly agreed paradigm that the neural
fibers determine the pathways for signal propagation in the brain, the signal
propagation due to the cortex wave modes in the highly folded areas will
exhibit no apparent correlation with the fiber directions. The results are
consistent with numerous recent experimental animal and human brain studies
demonstrating the existence electrostatic field activity in the form of
traveling waves (including studies where neuronal connections were severed) and
with wave loop induced peaks observed in EEG spectra. The localization and
persistence of these cortical wave modes has significant implications in
particular for neuroimaging methods that detect electromagnetic physiological
activity, such as EEG and MEG, and for the understanding of brain activity in
general, including mechanisms of memory.
