Nonlinearity in human resting, eyes-closed EEG: an in-depth case study

The question of nonlinearity in the human electroencephalogram (EEG) is imp ortant, since linear methods of EEG analysis are more well-developed and co mputationally faster than nonlinear methods. Furthermore, the presence or a bsence of nonlinearity has important theoretical implications for understan ding the nature of the brain's oscillatory activity. Using a linear summary measure as a control, we report a failure to reject the null hypothesis of a (largely) stationary linear-Gaussian process for normal, resting, eyes-c losed EEG from a single participant. We found significant evidence of nonli nearity at two occipital sites (O1 and O2) where the 8-12.5 Hz alpha rhythm was prominent. However, this element of nonlinear structure appeared trivi al, as (1) we found no evidence of time irreversibility at these loci, and (2) best-fitting linear models accounted on-average for over 94% of the var iance in the data with nonlinear modeling doing no better. Half of the rema ining variance could be accounted for by nonstationarity. While our finding s technically apply only to the one individual tested, his EEG was typical of those seen under the conditions that we employed.