Dynamics of the human alpha rhythm: evidence for non-linearity?

Object: For a better understanding of the physiological mechanisms responsi ble for alpha rhythms it is important to know whether nonlinear processes p lay a role in their generation. We used non-linear forecasting in combinati on with surrogate data testing to investigate the prevalence and nature of alpha rhythm non-linearity, based on EEG recordings from humans. We interpr eted these findings using computer simulations of the alpha rhythm model of Lopes da Silva et al. (1974). Methods: EEGs were recorded at O2 and O1 in 60 healthy subjects (30 males; 30 females; age: 49.28 years; range 11-84) during a resting eyes-closed sta te. Four artefact-free epochs (2.5 s; sample frequency 200 Hz) from each su bject were tested for non-linearity using a nonlinear prediction statistic and phase-randomized surrogate data. A similar type of analysis was done on the output of the alpha model for different values of input. Results: In the 480 (60 subjects, 2 derivations, 4 blocks) epochs studied, the null hypothesis that the alpha rhythms can result from linearly filtere d noise, could be rejected in 6 cases (1.25%). The alpha model showed a bif urcation from a point attractor to a limit cycle at an input pulse density of 615 pps. Non-linearity could only be detected in the model output close to and beyond this bifurcation point. The sources of the non-linearity are the sigmoidal relationships between average membrane potential and output p ulse density of the various cells of the neuronal populations. Conclusion: The alpha rhythm is a heterogeneous entity dynamically: 98.75% of the epochs (type I alpha) cannot be distinguished from filtered noise. A pparently, during these epochs the activity of the brain has such a high co mplexity that it cannot be distinguished from a random process. In 1.25% of the epochs (type II alpha) non-linearity was found which may be explained by dynamics in the vicinity of a bifurcation to a limit cycle. There is thu s experimental evidence from the point of view of dynamics for the existenc e of the two types of alpha rhythm and the bifurcation predicted by the mod el. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.