Abstracts

High frequency activity in the intracerebral EEG of epileptic patients has been described in recent studies using microelectrodes. We report changes in high frequency activity immediately following epileptic spikes in intracerebral recordings obtained from macroelectrodes. Eight patients with intractable epilepsy were evaluated; 4 had mesial temporal epilepsy (MT) and 4 neocortical epilepsy (N). EEGs from stereotaxically placed depth electrodes were filtered at 500Hz and sampled at 2000Hz. Spikes were classified according to their localization and morphology, with 60 spikes in each type. Segments of 256ms were selected immediately following each spike (post-spike), and 2s before each spike (baseline). Log-spectral power was estimated in 4 frequency bands: slow (0-40Hz), gamma (40-100Hz), high frequency (HF, 100-200Hz), and very high frequency (VHF, 250-500Hz). Changes between post-spike periods and baselines were assessed for each channel (Wilcoxon test, p[lt]0.01). 25 types of spike were identified (16 MT, 9 N). Changes were seen in 14 types of spike in HF (10 MT, 4 N) and in 11 types in VHF (8 MT, 3 N). Changes in VHF were always associated with changes in HF. In 9 types of spike (7 MT, 2 N), there was an increase in slow activity corresponding to a visible slow wave, but also a decrease in fast activity. These changes occurred in the hippocampus and neocortex. In 6 types of spike (5 MT, 1 N), there was an increase in slow activity despite the absence of a visible slow wave and an increase in fast activity. These changes occurred mostly in the amygdala, even though the spike was minimal in that area. The same pattern was observed in the hippocampus in two types of spikes in one patient. Finally, there was one type of spike with a decrease in fast activity without changes in slow activity. Overall, changes in VHF activity were spatially more restricted than changes in HF and gamma bands. A significant proportion of spikes were followed by changes in high frequency activity, more often in mesial temporal than neocortical spikes. Slow waves following spikes were generally associated with a decrease in HF and VHF compared to baseline. Changes in high frequency were clearest in channels where the spikes were most prominent, while changes at lower frequencies were more widespread. In many instances, there was an increase in high frequencies in the amygdala, even though the spike was not prominent in this region.
This study demonstrates that high frequency activity can be recorded with macroelectrodes and has significant changes in epileptic discharges. The reduction in HF and VHF following spikes may reflect a profound inhibition that is more marked in the region of spike generation. The amygdala behaves differently from the hippocampus and neocortex, not showing this reduction in fast activity. (Supported by Canadian Institutes of Health Research
Department of Education of Basque Government.)