Abstracts

Time changes in the frequency content of EEG signals have traditionally been depicted using short-time Fourier transforms (STFT). However, the fixed length of the analyzing window of the STFT limits the bandwidth of its useful results. A fixed window cannot accurately resolve low-frequency events whose periods are longer than the window duration, nor can it describe the transient behavior of high-frequency events. Multi-resolution transforms, such as the Morlet wavelet transform, address this problem by scaling their analyzing functions to accommodate the different wavelengths of different frequencies. In the present study, we introduce a novel application of the Morlet transform, in which time-frequency Morlet spectra of epileptiform discharges are depicted with both amplitude and phase information shown on the same plot. Using this technique, the phase of an event is denoted by color, and its amplitude by color intensity., Selected scalp and intracranial recordings from five patients with either focal or multifocal epileptiform discharges were used. These recordings were analyzed with a multi-resolution time-frequency algorithm (the Morlet transform) from 0-500 Hz across all electrodes. The amplitude and phase information from each channel was merged in a unique visualization to allow for time and spatial localization of phase changes., The Morlet transform provided clear visualization of the frequency components, in time, of the electrographic events. In addition, this technique allowed easy visual comparison of EEG phenomena between different electrodes. Some of the spectral attributes of these events were not immediately apparent on conventional review of the EEG. Furthermore, much of the phase relation information seen using the Morlet transform, was not visualized using visual inspection, particularly at higher frequencies., The application of the Morlet transform to EEG is novel and can potentially be a useful tool for visualizing clinically relevant features in EEG., (Supported by American Epilepsy Society, Canadian Institutes of Health Resarch, Alberta Heritage Foundation for Medical Research.)