Do surface DC-shifts affect epileptic hippocampal EEG activity?

Abstract

Despite considerable research on EEG-feedback of slow cortical potentials (SCPs) for seizure control in epilepsy, the underlying mechanisms and the direct effects on intracerebral pathological activity within the focal area remain unclear. Intrahippocampal EEG recordings from four patients with temporal lobe epilepsy and implanted electrodes were analyzed with regard to spike activity and power in 10 frequency bands (0.5-148Hz) during SCP feedback based on surface recordings (position Cz). Trials with positive, negative and indifferent SCPs were contrasted. Three of the four patients showed changes in spike activity during SCPs, but these were inconsistent between patients, and resulted in increased and decreased activity in both positive and negative SCPs. Spectral analysis revealed that in all patients, positive surface shifts showed a bi-hemispheric higher power in the high-frequency activity above 40Hz. Two patients showed a higher power also during negative shifts, both in high-frequency activity and one in most other frequency bands. Feedback-related power effects did not differ between focal and non-focal side. The inconsistent change in spiking activity and the lack of decrease of power in pathology associated frequency bands during SCPs show that these SCPs do not decrease pathological activity within the epileptic focus. A possible relation of higher power in high-frequency activity during positive SCPs to cognitive processes, such as memory functions, is discussed.