Modal analysis of corticothalamic dynamics, electroencephalographic spectra, and evoked potentials - art. no. 041909

The effects of cortical boundary conditions and resulting modal aspects of continuum corticothalamic electrodynamics an explored, including feedbacks. Dispersion relations, electroencephalographic spectra, and stimulus respon se functions are calculated from the underlying physiology, and the effects of discrete mode structure are determined. Conditions under which modal ef fects are important are obtained, along with estimates of the point at whic h modal series can be truncated, and the limit in which only a single globa lly uniform mode need be retained. It is found that for physiologically pla usible parameters only the lowest cortical spatial eigenmode together with the set of next-lowest modes can produce distinct modal structure in spectr a and response functions, and then only at frequencies where corticothalami c resonances reduce dissipation to the point where the spatial eigenmodes a re weakly damped. The continuum limit is found to be a good approximation, except at very low frequencies and. under some circumstances, near the alph a resonance. It is argued that the major electroencephalographic rhythms re sult from corticothalamic feedback resonances, but that cortical modal effe cts can contribute to weak substructure in the alpha resonance. This mechan ism is compared and contrasted with purely cortical and pacemaker-based alt ernatives and testable predictions are formulated to enable experimental di scrimination between these possibilities.