Abstracts

Rationale: High frequency oscillations (HFO) are pathologic markers of the seizure onset zone (SOZ) in focal epilepsy. HFO have also been shown to be physiologic markers of cognition. Pathologic HFO (pHFO) differ from physiologic HFO (nHFO) demonstrating higher spectral amplitude and longer duration. The mechanisms of pHFO generation remain unclear. We sought to differentiate the characteristics of HFO generated from normal cortex (NC) and SOZ during a motor task. Methods: Intracranial EEG was recorded from four patients undergoing evaluation for epilepsy surgery. An automated HFO detector marked and characterized all HFO occurring during a motor task. The task consisted of individual finger movements made to command. The center frequency, peak spectral amplitude, and duration of HFO were compared (Wilcoxon Rank Sum tests) between electrodes in NC and those in the independently determined SOZ and irritative zone (IZ). To look for areas of cortical activation during the task we performed density-based clustering using the frequency and latency from movement of the HFO detections as dimensions. In electrodes with clusters, we compared HFO rates and spectral amplitude distributions in and out of clusters between NC and SOZ. Results: HFO occurred at a higher rate in SOZ (median 3.97/s) than in NC (3.00/s, p=2.0 x 10^-5). There was no difference in rate between NC and IC. Compared to NC, HFO in SOZ were of higher median amplitude (4.32 v 4.28, p < 10^-15), lower median frequency (365 v 397, p < 10^-15), and longer median duration (12.2 v 11.2 milliseconds, p < 10^-15). Recordings from SOZ were significantly more likely to demonstrate clusters (71%) than those from NC (38%, p= 5.7 x 10^-5). In electrodes with clusters, the rate of HFO detection was higher within clusters in SOZ (median 1.77/s) than in NC (median 1.54/s, p=3.9 x 10^-09), but not outside of the clusters. HFO were separated into groups exceeding threshold spectral amplitude z-scores of 5, 6, 7, 8, and 9. SOZ demonstrated a higher rate of HFO at each threshold level than NC (>5: p=4.19 x 10^-05, >6: p=2.31 x 10^-08, >7: p=3.6 x10^-10, >8: p= 4.36 x 10^-07, >9: p=3.7 x 10^-18). Conclusions: During a motor task, cortex in the SOZ generates a population HFO of higher spectral amplitude, longer duration and lower frequency than NC. HFO in SOZ show greater task-induced patterned firing and clustering. These data suggest that pHFO may arise from pathologic cortex during physiologic processing.