Abstracts

Coherent fast rhythms of brain electrical activity (gamma band 30-100 Hz) have been suggested to be important for temporal binding of neural activities underlying mental representations in perception and cognition. Previously, we demonstrated that fast rhythms, led from the hippocampus, intensify and show an increased coherence [italic]before[/italic] early ictal discharges caused by systemic infusion of kainic acid (KA) in rats. During and immediately after ictal discharges, gamma power and coherence were significantly decreased. A novel hypothesis was advanced that epileptic activity results from the extreme activation of an [ldquo]anti-binding[rdquo] mechanism. The current study quantifies gamma activity at pre-seizure stages and during ictal activity in humans.
We have analyzed power and multiple coherence at frequencies 1-100 Hz in intracranial recordings from the hippocampal, temporal polar subtemporal and two anterior subtemporal electrodes implanted in patients considered for surgical treatment of intractable temporal lobe epilepsy. Ten to twenty one-second EEG epochs were selected at the following stages: 1) background activity, 2) pre-seizure period immediately preceding the periodic ictal discharge and 3) ictal activity at onset. Spectral parameters were calculated within the multivariate autoregressive modeling of the EEG signal allowing the usage of shorter epoch of the EEG compared to the traditional FFT-based spectral analysis. Multiple coherence is a measure of a statistical relationship between multiple EEG channels and is an extension of pair-wise ordinary coherence. Averaged power and coherence spectra at stages 1-3 were compared at each frequency 1, 2, 3[hellip]100 Hz.
Increased values of power and multiple coherence of gamma activity were observed during 10-30 seconds immediately preceding ictal discharges in the anterior subtemporal recording sites. Maximal power increases (up to fourfold) and increases in coherence (from 0.5-0.6 to 0.8-0.9) were observed at 30-50 Hz and 80-100 Hz. Ictal discharges were most prominent in the hippocampus. During ictal discharges, power and coherence of gamma rhythms in the anterior subtemporal leads decreased to or below the background values.
These data show that 1) abnormally increased gamma activity may be the early manifestation of seizures in the temporal lobe epilepsy and 2) epileptic spikes are able to desynchronize and suppress gamma activity. These findings have implications for our better understanding of gamma activity of the brain and its relation to epileptic activity and may contribute to the development of new techniques aimed at providing earlier and more sensitive detection/prediction of seizures.