Electrocorticography (ECoG) is a specialized, invasive neurophysiological monitoring technique used to record electrical activity directly from the brain’s surface. This method is a form of intracranial electroencephalography (iEEG) where electrodes are placed underneath the skull, making direct contact with the cerebral cortex. Unlike standard electroencephalography (EEG) which measures signals through the scalp, ECoG bypasses the skull and other tissues, offering a clearer recording of the brain’s electrical signals. The procedure is reserved for medical situations where non-invasive testing cannot provide the necessary level of detail for diagnosis and treatment planning. ECoG provides a precise electrical map of the cortex, guiding highly focused surgical interventions.
The Clinical Necessity for ECoG
Electrocorticography is used in the pre-surgical evaluation of individuals with refractory epilepsy, a condition where seizures do not respond to anti-seizure medication. Approximately one-third of all epilepsy patients experience this drug-resistant form, making them candidates for potential surgical intervention. For a successful surgery, the precise location where the seizures begin, known as the seizure onset zone (SOZ) or epileptogenic zone, must be identified with high accuracy.
ECoG allows neurosurgeons to pinpoint this zone, which is often difficult or impossible to localize using non-invasive methods like standard EEG or MRI. The data gathered helps define the boundaries of the tissue that must be removed to stop the seizures. The procedure is also used for functional mapping, which involves identifying areas of the cortex responsible for functions like movement, sensation, and speech.
Mapping these eloquent areas is performed using direct electrical stimulation through the implanted electrodes. This process ensures that the surgical removal of the epileptogenic zone avoids these functional regions, minimizing the risk of permanent neurological deficits. ECoG-guided resection has been shown to improve the chance of postoperative seizure freedom in cases of drug-resistant epilepsy.
The Procedure Implantation and Monitoring
The process of performing ECoG involves a multi-stage procedure that begins with a surgical operation to implant the electrodes. The initial surgery requires a craniotomy, where a section of the skull bone is temporarily removed to expose the brain’s surface. Electrode placement is carefully planned based on previous non-invasive imaging, such as MRI and scalp EEG results.
The electrodes are then surgically placed directly onto the cortex, typically in the form of thin, flexible silicone sheets called grids and strips, which contain an array of small platinum disc electrodes. These subdural electrodes sit on the surface of the brain, underneath the dura mater. In some cases, thin cylindrical depth electrodes are also inserted into deeper brain structures to record activity from areas like the hippocampus.
Following the implantation surgery, the patient is transferred to an Epilepsy Monitoring Unit (EMU) where continuous recording takes place over several days. During this extraoperative monitoring period, seizure medications are often reduced under medical supervision to encourage the occurrence of typical seizure events. The goal is to capture multiple spontaneous seizures to precisely localize the seizure onset zone.
Functional mapping is also performed during this time, with the patient awake, by delivering a small electrical current through individual electrodes. This stimulation temporarily disrupts the function of the underlying cortical area, allowing the medical team to identify and map the location of movement, sensation, and language centers. This detailed, real-time mapping provides the final information needed to plan the definitive resective surgery.
Signal Resolution Advantages over Scalp EEG
The primary benefit of ECoG over non-invasive methods like scalp EEG is the superior quality and resolution of the recorded electrical signal. The direct contact of the electrodes with the cerebral cortex eliminates the signal attenuation and distortion that occurs when electrical activity must pass through the skull, scalp, and other layers of tissue. The skull’s low electrical conductivity acts as a significant barrier, severely dampening the brain’s electrical signals before they reach the scalp electrodes.
ECoG provides both high spatial and high temporal resolution, making it a sensitive tool for localizing activity. The spatial resolution can pinpoint activity to within a few millimeters, allowing for the precise identification of the source of the seizure, which non-invasive EEG cannot match. Furthermore, the temporal resolution is high, capable of measuring changes in electrical activity in the millisecond range.
This high fidelity allows for the clear capture of high-frequency activity, specifically in the high gamma range (around 70–110 Hz), which is associated with localized functional brain processing. The ability to record these subtle, high-frequency signals is severely limited in scalp EEG due to filtering and noise. The resulting ECoG data provides the clearest possible data for surgical planning.
Risks and Patient Recovery
As an invasive procedure requiring a craniotomy, ECoG carries surgical risks, though major complications are relatively rare. The risks associated with the initial electrode implantation include general complications of brain surgery such as infection, bleeding inside the skull (hemorrhage), and brain swelling. Minor complications, such as urinary tract infections or deep vein thrombosis, occur more frequently but are still uncommon.
Once the required monitoring data and functional maps have been collected, a second, often less extensive, operation is performed to remove the electrode grids and strips. In many cases, the definitive surgical treatment to remove the epileptogenic zone occurs during this same second procedure. The recovery period following the removal of the electrodes and the completion of the resection is typically a matter of weeks.
The patient’s recovery includes healing from the craniotomy and managing the effects of the brain tissue removal, if resection was performed. The neurosurgical team works to ensure the patient is stable and that any new neurological deficits are addressed post-operatively. Long-term prognosis for seizure control is evaluated after the recovery period, with many patients achieving significant seizure reduction or freedom following the procedure.