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.

Gamma coherence and conscious perception.

BACKGROUND: High-frequency (e.g., gamma 30 to 50 Hz) coherent neural activity has been postulated to underlie binding of independent neural assemblies and thus integrate processing across distributed neuronal networks to achieve a unified conscious experience. Prior studies suggest that gamma activity may play a role in perceptual mechanisms, but design limitations raise concerns. Thus, controversy exists as to the hypothesis that gamma activity is necessary for perceptual awareness. In addition, controversy exists as to whether the primary sensory cortices are involved directly in the mechanisms of conscious perception or just in processes prior to conscious awareness. OBJECTIVE: To investigate the relation of gamma coherence and perception. METHODS: Digital intracranial electrocorticographic recordings from implanted electrodes were obtained in six patients with intractable epilepsy during a simple somatosensory detection task for near-threshold stimuli applied to the contralateral hand. Signal analyses were then conducted using a quantitative approach that employed two-way Hanning digital bandpass filters to compute running correlations across pairs of channels at various time epochs for each patient and each perception state across multiple bandwidths. RESULTS: Gamma coherence occurs in the primary somatosensory cortex approximately 150 to 300 milliseconds after contralateral hand stimuli that are perceived, but not for nonperceived stimuli, which did not differ in character/intensity or early somatosensory evoked potentials. CONCLUSION: The results are consistent with the possible direct involvement of primary sensory cortex in elemental awareness and with a role for gamma coherence in conscious perception.

Epileptic seizures may begin hours in advance of clinical onset: a report of five patients.

Mechanisms underlying seizure generation are traditionally thought to act over seconds to minutes before clinical seizure onset. We analyzed continuous 3- to 14-day intracranial EEG recordings from five patients with mesial temporal lobe epilepsy obtained during evaluation for epilepsy surgery. We found localized quantitative EEG changes identifying prolonged bursts of complex epileptiform discharges that became more prevalent 7 hr before seizures and highly localized subclinical seizure-like activity that became more frequent 2 hr prior to seizure onset. Accumulated energy increased in the 50 min before seizure onset, compared to baseline. These observations, from a small number of patients, suggest that epileptic seizures may begin as a cascade of electrophysiological events that evolve over hours and that quantitative measures of preseizure electrical activity could possibly be used to predict seizures far in advance of clinical onset.