Movement and ERD/ERS
One characteristic feature of the brain is its ability to generate rhythmic potentials or oscillatory activity. Already in 1949 Jasper and Penfield discovered this fact and discussed the relationship between alpha and beta rhythms and their functioning in relation to underlying neural networks. The frequency of brain oscillations depends both on membrane properties of single neurons and the organization and interconnectivity of networks to which they belong (Lopes da Silva, 1991). Such a network can either comprise a large number of neurons controlled, for example, by thalamo-cortical feedback loops or only a small number of neurons interconnected, for example, by intra-cortical feedback loops. Coherent activity in large neuronal pools can result in high amplitude and low frequency oscillations (e. g. alpha band rhythms), whereas synchrony in localized neuronal pools can be the source of gamma oscillations (Lopes da Silva and Pfortscheller, 1999). The dynamic of such a network can result in phasic changes in the synchrony of cell populations due to externally or internally paced events and lead to characteristic EEG patterns. Two such pattern types are observed, the event-related desynchronization, or ERD, in form of an amplitude attenuation and the event-related synchronization, or ERS, in form of an enhancement of specific frequency components (Pfortscheller and Lopes da Silva, 1999a, Pfortscheller and Lopes da Silva, 1999b).