Abstracts

MEG and EEG are two techniques used to characterize epileptiform activity in the brain. The typical events of the inter-ictal activity consist mainly of spikes and spike and waves. These events have been well defined in the EEG and these definitions have been used for many years. With the emergence of MEG, the EEG based definitions of such events have been also applied in MEG recordings without objective definitions based on specific MEG studies. Nevertheless several studies have raised the question whether the same definitions of inter-ictal events derived from EEG may also be applied to MEG. The present work has the aim of determining quantitatively the characteristics of MEG spikes and of assessing the correctness of the current clinical practice of using EEG spike definitions in the context of MEG studies. The method consisted of analysing quantitative characteristics of 120 coincident EEG and MEG epileptiform events from three patients suffering from drug resistant epilepsy. For each pair of events, the waveforms were analysed according to a morphological model (amplitudes, slopes, sharpness).
A paired t-test statistical analysis was performed on the extracted metrics. Statistical significant differences were found between corresponding EEG and MEG spike events. The MEG spikes were sharper (p[lt]0.01) and had shorter durations (p[lt]0.01). MEG spikes are statistically different from those seen in the EEG. Thus the criteria for visual detection of these events in MEG should be adjusted accordingly. These results imply that automatic spike detector algorithms should use different criteria for MEG in comparison with EEG recordings.
The present abstract is part of the work that was recently accepted for publication in the Journal of Clinical Neurophysiology. (Supported by the grant PRAXIS XXI/BD/19676/99, project POSI/EEI/12150/98 and POSI/CPS/39758/2001 from [quot]Funda[ccedil][atilde]o para a Ci[ecirc]ncia e Tecnologia[quot], Portugal and co-sponsored by the FEDER program.)