  The dynamic behavior of scalp potentials (EEG) is apparently due to some
combination of global and local processes with important top-down and bottom-up
interactions across spatial scales. In treating global mechanisms, we stress
the importance of myelinated axon propagation delays and periodic boundary
conditions in the cortical-white matter system, which is topologically close to
a spherical shell. By contrast, the proposed local mechanisms are multiscale
interactions between cortical columns via short-ranged non-myelinated fibers. A
mechanical model consisting of a stretched string with attached nonlinear
springs demonstrates the general idea. The string produces standing waves
analogous to large-scale coherence EEG observed in some brain states. The
attached springs are analogous to the smaller (mesoscopic) scale columnar
dynamics. Generally, we expect string displacement and EEG at all scales to
result from both global and local phenomena. A statistical mechanics of
neocortical interactions (SMNI) calculates oscillatory behavior consistent with
typical EEG, within columns, between neighboring columns via short-ranged
non-myelinated fibers, across cortical regions via myelinated fibers, and also
derive a string equation consistent with the global EEG model.
