Cortical imaging of post-movement beta oscillations in EEG

Cortical Imaging stands for the calculation of the potential distribution on the cortical` surface, in the proposed project especially the spatial extent of short-lasting oscillations in the beta frequency band- Such beta bursts are characteristic after termination of a movement and are found in scalp EEG very close to primary sensorimotor areas. This phenomenon is very stable, found in nearly every subject and was reported with finger and foot movement in EEG as well as in MEG data. In EEG recording, the signal is "blurred" by the electrically conductive layers of scalp, skull, cerebrospinal fluids and the brain. This results in a smearing effect and in a loss of spatial resolution. There are two ways to improve the spatial resolution of EEG signals. One is the calculation of the Surface Laplacian, the other is the spatial deconvolution method. Both are "deblurring" methods and allow the calculation of the cortical potential distribution. It has to be noted that the brain`s magnetic field (MEG) has a good spatial resolution, since the MEG is little influenced by the volume conductor. MEG is, however, more expensive than EEG. is not available in each clinical - / research unit and the MEG can hardly detect the biomagnetic activity of the radially oriented pyramidal cells in the cortex. . Two deblurring methods are subject to this project: The Linear Estimation (LE) method and the Analytic Deblurring (AD) method. Both methods should be studied with spherical volume conductor and realistic head models and should be compared with the Surface Laplacian. The phenomenon under investigation is the post- movement beta synchronization. This signal is not phase-locked to the movement. Therefore. the investigation of single` not-averaged EEG trials is necessary.

The project P12407-MED intended to develop and implement methods for the calculation of the potential distribution on the cortical surface, in order to improve the neurophysiological interpretation of EEG data. For this purpose, two EEG reconstruction methods, Spline Laplace and Linear Estimation, were applied taking into account the individual head and brain shape of the investigated subjects. The main goal of this project was to investigate the cortical sources of induced, movement-related beta oscillations (14-35 Hz) in the scalp EEG. Such bursts of beta oscillations appear immediately after termination of a voluntary movement. A number of studies suggests that this "post-movement beta rebound" represents a localized synchronization of those motor cortical areas which have been active during motor preparation. There is strong evidence from neuromagnetic recordings that these beta bursts are generated in restricted cortical regions, such as the primary motor areas involved in a specific movement task. In contrast to the MEG, which is little influenced by volume conduction, EEG signals are "blurred" by the electrically conductive layers of scalp, skull, cerebrospinal fluids and the brain. In order to improve the spatial resolution, when studying the post-movement beta synchronization in EEG recordings, different "deblurring" methods, resulting in a cortical potential distribution, were investigated within this project: besides the calculation of the Surface Laplacian (SL) based on splines, spatial deconvolution methods, such as the so-called Linear Estimation (LE) method, were evaluated, using either spherical volume conductor or realistic head models. Since the induced oscillatory beta band activity is not phase-locked to the movement, the investigation of single, not- averaged EEG trials was necessary. Individual head modelling for EEG measurements is still not common since the whole procedure, including the production of the magnetic resonance images (MRIs) and the following calculations, is very time-consuming compared to standard EEG processing. The advantage of individual calculations, however, is that the obtained results can be better related to anatomical structures than in the case of spherical or "standard" head models. In order to facilitate the necessary computations, a new software tool, namely a package of graphical user interfaces in the programming language Matlab, has been developed and implemented within this project. The newly developed high-resolution ERD/ERS method has been extensively investigated in a variety of experimental tasks involving motor cortex activation, such as simple voluntary movement of different body parts, mental imagination of movement, and somatosensory stimulation. The results obtained in healthy volunteers suggest that the study of post-movement beta oscillations can be regarded as a useful tool to investigate dynamic changes of the functional state of specific sensorimotor structures. Moreover, the results of this project have also led to further studies with clinical groups, as for example, patients with movement disorders (Parkinsons disease, "Restless-legs" syndrome). These clinical studies, currently performed within the frame of the European Action COST B10 "Brain Damage and Repair", intend to improve the diagnosis of neurological disorders, which is also of interest at an European level.