Abstracts

Rationale: In the electrocorticography (ECoG) spike-like delayed responses, remote from the stimulus site, can be evoked with Single Pulse Electrical Stimulation (SPES). These responses are pathological and related to the epileptogenic zone. However the clinical use of SPES is compromised by the time-consuming and subjective visual analysis. In order to increase the efficiency and objectivity of SPES, we developed a method for automatic detection of evoked responses by SPES using the frequency characteristics of the responses. This method may also detect high frequency oscillations (HFOs, 80-500 Hz) as a SPES response. Methods: Subdural ECoG data were recorded from a temporal lobe epilepsy patient evaluated for surgery. A 32 electrode channel grid and four strips (eight channels) covered the temporal lobe. SPES (10 epochs, 1ms, 8mA, 0.2Hz) was performed stimulating over adjacent electrode pairs, recorded at a 2048Hz sampling rate. A bipolar montage of adjacent electrodes was chosen. Spikes and HFOs (ripples 80-250Hz, fast ripples 250-500Hz) were marked by an experienced ECoG reader for electrodes in- and outside the seizure onset zone. We applied a wavelet based automated time-frequency analysis, computing mean event-related changes in spectral power compared to the pre-stimulus baseline power (1). The stimulus artifact was corrected by linear interpolation. Epochs were analyzed for [-1s:1s] around the stimulus. Baseline power was computed for a pre-stimulus interval [-1s:-0.2s]. The power spectra were evaluated in the frequency range 10-520Hz. Event Related Spectral Perturbation (ERSP) with respect to baseline and bootstrap significance were calculated (1). The ERSP of single epochs was evaluated for consistency within the frequency bands of spikes, ripples and fast ripples as marked by the observer. The ERSP calculated for 10 epochs was compared with the results for a non-seizure versus seizure onset zone channel. Results: For single epochs a significant (p<0.05) ERSP increase (red) within the frequency range of 10-80Hz showed that matches with events marked by the observer as spikes. There is also a match for ripples and fast ripples (figure 1). Average ERSP of multiple epochs showed a significant increase in all frequency ranges in the seizure onset zone channel while only a slight increase in the spike frequency range was seen in the non-seizure onset zone channel (figure 2). Conclusions: With this algorithm responses evoked by SPES, spikes and HFOs, could be readily identified in the ECoG. It replaces the subjectivity of visual analysis with the objectivity of frequency decomposition combined with a measure of significance. The analysis allows efficient digital processing of multiple stimuli. This new method offers a way to study the clinical meaning of SPES responses in more detail. 1.Delorme A, Makeig S. EEGLAB: an open source toolbox for analysis of single-trial EEG dynamics including independent component analysis. J Neurosci Methods 2004; 134 (1):9-21.