Analysis of wavelet-filtered tonic-clonic electroencephalogram recordings

EEG signals obtained during tonic-clonic epileptic seizures can be severely contaminated by muscle and physiological noise. Heavily contaminated EEG signals are hard to analyse quantitatively and also are usually rejected for visual inspection by physicians, resulting in a considerable loss of collected information. The aim of this work was to develop a computer-based method of time series analysis for such EEGs. A method is presented for filtering those frequencies associated with muscle activity using a wavelet transform. One of the advantages of this method over traditional filtering is that wavelet filtering of some frequency bands does not modify the pattern of the remaining ones. In consequence, the dynamics associated with them do not change. After generation of a ‘noise free’ signal by removal of the muscle artifacts using wavelets, a dynamic analysis was performed using non-linear dynamics metric tools. The characteristic parameters evaluated (correlation dimension D2 and largest Lyapunov exponent λ1) were compatible with those obtained in previous works. The average values obtained were: D2=4.25 and λ1=3.27 for the pre-ictal stage; D2=4.03 and λ1=2.68 for the tonic seizure stage; D2=4.11 and λ1=2.46 for the clonic seizure stage.