Abstracts

Rationale: The first studies recording both scalp and cortical EEG noted a significant loss of EEG power and loss of fidelity in the scalp recordings. Most of the subsequent work focused on the loss of fidelity of epileptiform activity and evoked potentials. Quantitative EEG methods applied to scalp EEG have contributed to the utility of continuous EEG monitoring in the critical care setting. The purpose of our study was to investigate the effects of the intact skull on background EEG rhythms of patients undergoing long term combined scalp and intracranial EEG monitoring. Methods: Twenty patients (age 18-55) from the Yale University Epilepsy Surgery Program undergoing intracranial EEG monitoring for surgical evaluation were recruited. Scalp EEG recordings were made using Grass platinum needle electrodes located over the C3, C4, O1, and O2 sites. Simultaneous recordings were obtained from subjacent subdural strip electrodes (4mm diameter platinum disks with 2.3mm exposed surface diameter) as previously reported (Zaveri et al 2011). Offline analysis was performed with custom software written in a mixture of high level languages and MATLAB. Scalp and intracranial EEG epochs, 1 hour in duration, at least 6 hours removed from a seizure, 3 or 4 days after electrode implantation surgery, during wakefulness between 9-10 AM were selected for analysis. Scalp and subdural recordings were examined for artifacts and segmented with 1-second resolution. EEG power of artifact-free EEG segments was obtained for each electrode contact studied and averaged over the epoch. Total power was calculated as the signal power between 0.5 and 35 Hz, delta 0.5-3.9 Hz, theta 4-8.4 Hz, alpha 8.5-13 Hz, and beta 13.1-25 Hz frequency bands. Results: EEG power recorded using subdural electrodes decreased with higher frequencies: delta (2.82 mV², 95%CI 1.26-6.35), theta (1.49 mV², 95%CI 0.82-2.49, alpha (0.41 mV², 95%CI 0.22-0.71), beta (0.28 mV², 95%CI 0.17-0.43). EEG power recorded using scalp electrodes decreased with higher frequencies: delta (0.41 mV², 95%CI 0.21-0.65), theta (0.12 mV², 95%CI 0.06-0.25), alpha (0.02 mV², 95%CI 0.01-0.04), beta (0.02 mV², 95%CI 0.01-0.03). Median power over the entire 0.5-35 Hz frequency band recorded from the scalp electrodes (0.58 mV², 95%CI 0.30-0.99) was 11% of the EEG power recorded using the subdural electrodes (5.14 mV², 95%CI 2.53-10.19). The ratio of EEG power recorded on the scalp divided by that recorded using subdural electrodes was attenuated further at higher frequencies: delta (0.15), theta (0.08), alpha (0.06), beta (0.06). Conclusions: The power of the background EEG recorded using subdural electrodes decreased by about 10-fold at higher frequencies. Additionally, the intact human skull attenuated EEG power at the median of all frequencies by 9-fold from 7-fold at delta up to 18-fold at the beta frequencies. The intact human cranium acts as a low-pass filter set at about 5 Hz.