Abstracts

Rationale: Direct cortical recordings have revealed local low-amplitude high-frequency activity at epileptic seizure onset. The non-invasive recording of these is challenging as a relatively strong signal is needed for scalp EEG and MEG. We analyzed ictal MEG data of five epilepsy patients to identify and localize fast (beta-gamma) trains at seizure onset. Methods: MEG seizures were recorded with a whole-head MEG device (Elekta Neuromag®). The data were sampled at 300 Hz or 600 Hz. To track possible head movement during epileptic events, the head position of two patients was monitored continuously using 154-166-Hz signal from four coils on the scalp. For visual analysis, the data were band-pass filtered at 0.3-90 Hz and clinical seizures were inspected.Results: The typical ictal events of all five patients started with visible fast oscillations over a few seconds. Oscillations at about 25-45 Hz were analyzed with equivalent current dipoles. The sources were in the premotor cortex, in the prefrontal cortex, close to the supplementary motor cortex, in the lateral temporal neocortex, and in the right occipital medial cortex, consistent with the ictal semiology or other diagnostic data in each patient.Conclusions: We conclude that 25-45 Hz ictal onset activity can be recorded by MEG and, probably, represents a relatively local circuit, which helps to identify the epileptogenic region.