Abstract EEGs were recorded from patients in early stages of Parkinson’s disease (17 patients, 9 females) and healthy controls (12 subjects, 8 females) during rest and during execution/imagining of a complex motor task. The prediction that Parkinson’s disease patients com- pared to controls would show more complex brain dy- namics during performance of a complex motor task and imagination of the movements was confirmed by meth- ods derived from nonlinear dynamics. In the resting state, analysis of correlation dimension of EEG time se- ries revealed only slight topographical differences be- tween the groups. During performance of a complex mo- tor task, however, data from Parkinson’s disease patients showed higher dimensionality than data from controls, indicating more complex EEG time series. The same dif- ference was found when subjects did not perform any motor movements but imagined the complex movements they had just performed. The data are consistent with the hypothesis that the disturbances in Parkinson’s disease result in the recruitment of superfluous cortical networks due to failed inhibition of alternative motor programs in the striatum and thus increase the complexity of cortical representation in motor conditions. Keywords Alpha · Cell assembly · Complexity · Correlation dimension · Parkinson’s disease Introduction It has been speculated that the striatum is an inhibitory system which is responsible for regulating the level of neural activity in the cortex. This assumption is motivat- ed by neuroanatomical and neurophysiological data evi- dencing that the striatum is a network with massive later- al inhibition (Wickens 1993). Because cortex and stria- tum are part of an extended neuronal loop (cortex – stria- tum – pallidum – thalamus – cortex), it may well be that lateral inhibition in the striatum has an inhibitory effect on groups of cortical neurons, too. Recent EEG data pro- vide support for this assumption. When motor move- ments are suppressed, a huge positive wave can be ob- served as a correlate of electrocortical inhibition (or dis- facilitation) (Roberts et al. 1994). This positive wave is strongly attenuated in Parkinson’s disease patients in whom striatal deficits predominate (Pulvermüller et al. 1996). This result provides evidence that striatal dys- function leads to an inability to suppress cortical activity during motor movements and supports the view that one of the normal functions of the striatum is the inhibition of cortical activity. This view leads to further predictions on electrocorti- cal responses in Parkinson’ disease patients. In particu- lar, it suggests that the motor deficits observed in these patients may result from an inability to suppress cortical activity rather than from an inability to activate cortical motor programs. If a movement is performed, one “mo- tor program”, that is, one of the cortical cell assemblies controlling motor movements (Wickens et al. 1994), should be active. If several assemblies (programs) are active at the same time, it may be impossible to perform one of the movements correctly. For movements to be executed, a critical program must be activated while competing programs must be inhibited. If Parkinson’s disease patients suffer from an inability to inhibit com- peting cortical neuron populations during motor perfor- mance, they should show evidence of enhanced and more complex cortical activity than normals when move- ments are executed. V. Müller ( ✉ ) · W. Lutzenberger · N. Birbaumer Institute of Medical Psychology and Behavioral Neurobiology, Eberhard-Karls University of Tübingen, Gartenstrasse 29, 72074 Tübingen, Germany e-mail: viktor.mueller@uni-tuebingen.de Fax: +49-7071-410925 F. Pulvermüller · B. Mohr Department of Psychology, University of Konstanz, Konstanz, Germany N. Birbaumer Dipartimento di Psicologia Generale, University of Padua, Padua, Italy Exp Brain Res (2001) 137:103–110 DOI 10.1007/s002210000638 RESEARCH ARTICLE Viktor Müller · Werner Lutzenberger Friedemann Pulvermüller · Bettina Mohr Niels Birbaumer Investigation of brain dynamics in Parkinson’s disease by methods derived from nonlinear dynamics Received: 8 May 2000 / Accepted: 2 November 2000 / Published online: 7 February 2001 © Springer-Verlag 2001