Abstracts

Rationale: High frequency (HF) oscillations ranging from 80 to 500Hz have increasing importance in the analysis of intracranial EEGs in patients with epilepsy. They occur primarily in the seizure onset zone (SOZ) and may be a good indicator for epileptogenicity. Interictal spikes which co-occur with HF oscillations or show a strong HF component may be a better indicator of epileptic irritability than spikes without a HF component. Postspike reductions of HF may reflect depression of neuronal activity. We previously developed a statistical time-frequency analysis for HF during spikes, which is based on Gabor transform. Here we evaluate various HF patterns related to spikes in different brain areas. Methods: SEEG was filtered at 500 Hz and recorded at 2000Hz. Spikes were selected according to their shape (spike/ spike-slow-wave) and location (SOZ/non-SOZ and Neocortex/ Amygdala/Hippocampus) in 15 consecutive patients. A minimum of 5 spike types from different patients were selected for each category. At least 50 spikes were averaged for each type and analyzed with time-frequency analysis, which detects power changes relative to background activity. Changes were quantified with false discovery rate controlled t-statistics and counts of significantly increased or decreased time-frequency values were compared across spike categories in the 100-250 and in the 250-500Hz bands. Results: Seventy-seven spike types were analysed, of which 39 were followed by a slow wave (table 1). 44 spike types showed an increase over 250Hz during the spike, followed by a postspike decrease in 27 cases (61%). 27 spike types showed an HF increase below 250Hz, with a postspike decreases in only five (18.5%). The following differences were observed and always stronger for frequencies above 250Hz than below 250Hz. First, spikes in the SOZ showed more HF increase during spikes and a larger postspike decrease than spikes outside the SOZ (Fig 1). In all but one patient the increase of HF during spikes was highest in the SOZ spikes. Second, spike-slow-waves showed more significantly increased HF areas and were followed by stronger postspike decreases than spikes alone. Third, HF increases during and HF decreases after the spikes were larger in hippocampal spikes than in neocortex and amygdala, with neocortical spikes showing the smallest changes. Conclusions: Increases in HF above 250 Hz show particularly clear regional differences and are very prominent in the SOZ. The postspike decrease strongly depends on the previous increase during the spike. Hippocampal spikes have the strongest HF components. Detailed analysis of HF changes during spikes may provide additional information on differing pathophysiological mechanisms of spikes and on epileptogenicity of the underlying tissue. Time-frequency analysis is a reliable tool to visualize and measure these changes.