Electrocorticography (ECoG) is a specialized neurophysiological monitoring technique that directly measures the electrical activity of the brain. It provides insights into brain function by recording signals from the cerebral cortex. This method is employed in specific medical contexts to gain detailed information about brain activity.
What is Electrocorticography?
Electrocorticography involves placing electrodes directly on the surface of the brain, a method known as intracranial monitoring. This direct placement allows for precise detection of electrical signals generated by populations of neurons. The electrodes are embedded in flexible silicon rubber matrices, often as grids or strips.
This direct approach contrasts with conventional electroencephalography (EEG), where electrodes are placed on the scalp. With scalp EEG, electrical signals must travel through the skull, which can weaken and distort them due to the skull’s low conductivity. ECoG offers significantly higher spatial and temporal resolution, providing a much clearer and more detailed signal of brain activity. The recorded signals reflect the summed activity of many neurons, offering a detailed view of localized brain activity.
Why is Electrocorticography Performed?
Electrocorticography is performed when non-invasive methods cannot provide sufficient detail for complex neurological conditions. A primary application is in planning surgery for individuals with drug-resistant epilepsy, where it helps precisely identify the “epileptogenic zone”—the specific brain region where seizures originate. Locating this zone is important for surgical removal.
Beyond epilepsy, ECoG is used for functional brain mapping. This involves identifying areas of the brain responsible for specific functions like language, movement, or sensation. This mapping helps neurosurgeons avoid damaging these “eloquent cortex” regions during tumor removal or epilepsy surgery. The precision offered by ECoG is superior to non-invasive techniques, guiding surgical decisions and improving outcomes.
The Electrocorticography Procedure
The ECoG procedure begins with a craniotomy, a surgical operation where a section of the skull is removed to expose the brain’s surface. This surgery may occur under general anesthesia, but if functional brain mapping is required, the patient may be under local anesthesia so they can interact with the surgical team. Following the craniotomy, specialized electrodes are carefully placed directly onto the exposed cortical surface. The number and type of electrodes are determined by the patient’s specific seizure patterns and pre-surgical evaluations like MRI and EEG.
After electrode implantation, the patient remains in the hospital for several days, generally between three to seven days, for continuous intracranial monitoring. During this period, healthcare professionals observe the patient for spontaneous seizures, often reducing seizure medications to increase the likelihood of capturing seizure activity. If seizures do not occur naturally, methods like flashing lights or limited sleep might be used to induce them. Functional mapping tasks, such as naming objects, reading aloud, or performing movements, are conducted while the patient is awake to identify and map areas responsible for motor, sensory, and language functions. This multidisciplinary approach involves neurosurgeons, neurologists, and other specialists collaborating to gather comprehensive data.
Once sufficient data has been collected, a second surgical procedure is performed to remove the implanted electrodes. In some cases, the tissue identified as the seizure onset zone may be removed during this same surgery.
Interpreting Electrocorticography Readings
Interpreting ECoG readings involves detailed analysis of the electrical signals recorded from the brain’s surface. Neurologists and neurosurgeons examine these signals to identify specific patterns associated with neurological conditions. For epilepsy, they look for “ictal spike activity,” which is diffuse fast wave activity observed during a seizure, and “interictal epileptiform activity,” which are brief bursts of neuronal activity occurring between seizures. Identifying these abnormal electrical patterns helps pinpoint the exact seizure focus.
The collected data also guides functional mapping, where specific brain regions related to motor control, sensory perception, and language are identified. This is achieved by correlating recorded electrical activity with tasks performed by the patient or through direct electrical stimulation of the cortex. The precise localization of these eloquent areas helps surgeons plan resections to avoid damaging them during surgery. The insights gained from ECoG directly influence surgical decisions and improve treatment outcomes for individuals with complex neurological disorders.