Comparison of line length feature before and after brain electrical stimulation in epileptic patients

This study aims to determine whether there are any statistically significant effects in the intracranial EEG signal due to brain electrical stimulation that can be quantified by comparing the line length value computed in windows positioned before and after stimulated abnormal events versus windows before and after non-stimulated abnormal events. The line length feature has been previously demonstrated to preserve waveform dimensionality changes as the ones estimated by Katz´s fractal dimension and is a measure sensitive to variations in signal amplitude and frequency, equivalent in some ways to Teager´s energy. Brief stimulation bursts of 200 Hz were delivered in response to some detections of abnormal electrographic activity. A total of 35 epileptic patients were analyzed including 15,938 electrographic events, of which 4,584 were electrically stimulated events. The ratio and difference of the post-stimulation versus the pre-stimulation line length values were computed as comparison measures. The average line length ratios in stimulated events versus those in non-stimulated events were lower in 23 out of 35 patients, suggesting that stimulation may have had an effect on electrographic activity. Statistical analysis based on a permutation test indicated the probability of finding this difference by random chance was 5.21%, further suggesting that the line length ratio differences are most likely due to the stimulation effects on the brain that manifest in the electrographic activity.