Electrocorticography (ECoG): What is it? How does it work? What is its main difference from Electroencephalography (EEG)?

Cesia M. Alvarez & Faisal R. Jahangiri

Electrocorticography, or ECoG, is an intraoperative neuromonitoring modality closely related to electroencephalography (EEG), in which direct physical contact between the electrodes in question and cortical matter is required to obtain a viable response/result [1]. Usually, a silicone cortical grid embedded with platinum disc electrodes, which could range between 4 to 64 electrodes, is placed on a specific cortical area [2]. One side of the silicone grid is numbered, indicating each electrode, and that numbered side of the electrode grid should always be facing the surgeon since it is used as a guidance tool during the cortical mapping process [2].

The main purpose of using ECoG is mapping and localizing epileptogenic cortical areas, such as treating epilepsy (identification and resection of epileptogenic cortical areas) [2]. Moreover, the neuromonitoring modalities can also map motor, sensory, and language areas through direct electrical cortical stimulation (DECS), a key factor in increasing safety rates of the surgical procedure of interest (1,2). Given that cortical matter will be stimulated, monitoring while ECoG is being applied is vital since it could provoke epileptic activity and/or after-discharges [2]. The implantation of the electrode grid is done through an awake craniotomy procedure since it requires live feedback and full exposure of the cortical matter [2].

Although EEG (electroencephalography) and ECoG (electrocorticography) are very different, there still could be some confusion between the two modalities. The main difference is the electrode placement since EEG electrodes are placed over the scalp, whereas ECoG electrodes are placed directly over cortical matter [2]. Moreover, EEG recordings aim to obtain spontaneous cortical electrical activity. In contrast, ECoG recordings focus more on the epileptogenic area localization and identifying after discharges during motor mapping of the cortical areas [2].

As previously mentioned, ECoG is mainly used for localizing epileptic cortical areas but can also serve other monitoring purposes. According to the article, Intraoperative Electrocorticography for Physiological Research in Movement Disorders: Principles and Experience in 200 Cases by Panov et al., ECoG could play a crucial role in research involving movement disorders/abnormalities [3]. According to the method section of the investigation, patients who were suffering from Parkinson's disease, dystonia, or essential tremor and were selected for DBS (deep brain stimulation) treatment were the main sample for this investigation [3]. Furthermore, the results demonstrated that ECoG offered crucial clinical advantages since it provided signals with greater amplitudes, and stimulation artifacts were easier to track. It offered a way to measure localized specific areas of neuronal spiking activity [3].

Sources:

1.       Panov, Fedor, Emily Levin, Coralie de Hemptinne, Nicole C. Swann, Salman Qasim, Svjetlana Miocinovic, Jill L. Ostrem, and Philip A. Starr. “Intraoperative Electrocorticography for Physiological Research in Movement Disorders: Principles and Experience in 200 Cases.” Journal of Neurosurgery 126, no. 1 (2017): 122–31. https://doi.org/10.3171/2015.11.jns151341.

2.       Surgical Neurophysiology – A reference guide to intraoperative neurophysiological monitoring (IONM), 2nd edition by Faisal R. Jahangiri, MD. Sirven, J.I. “Electrocorticogram (ECOG).” Essay. In Encyclopedia of the Neurological Sciences, 2nd ed., 1080–83. Elsevier, 2014.

3.       Sirven, J.I. “Electrocorticogram (ECOG).” Essay. In Encyclopedia of the Neurological Sciences, 2nd ed., 1080–83. Elsevier, 2014.