Abstracts

Rationale: Pathological high frequency oscillations (HFOs), ripple (R:80-250 Hz) and fast ripple (FR: >250 Hz), have been recently recorded with macroelectrodes as a possible biomarker of epileptogenesis. However, analysis of spontaneous interictal HFOs needs long sampling time and skillful technique. We have focused on the early responses (cortico-cortical evoked potentials: CCEPs) to 1Hz single-pulse cortical stimulation in order to trace cortico-cortial networks involved in brain functions and seizure propagation. By using CCEP [early negative (N1) and late negative (N2) components] as a dynamic measure of cortical excitability, we also have investigated modulation of cortical excitability at and around the focus (Matsumoto et al., 2005; Iwasaki et al., 2010). Our objective is to clarify the HFO correlates of CCEPs and its relation to human epileptogenesis. Methods: We employed 8 patients with intractable partial epilepsy who underwent invasive presurgical evaluation with chronic implantation of subdural electrodes (IRB #443). Repetitive single-pulse electrical stimulation at 1 Hz was applied to 1) the seizure onset zone (SOZ) and 2) the normal cortex of the same cortical architecture (nSOZ) in each patient. After carefully removing epochs mixed with spikes, CCEPs were recorded from the surrounding cortices within the SOZ (in case of 1) and nSOZ (in case of 2) by off-line averaging time-locked to the stimulus onset. We applied short-time Fourier transform as time-frequency analysis to obtain induced responses at the timing of N1 (HFO-N1) and N2 (HFO-N2). We then calculated the logarithmic power spectra of HFO-N1 and HFO-N2 and analyzed power change in reference to the baseline activity for the following 3 frequency bands - below R (