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What Is Electrocorticography?

By Meg Higa
Updated Mar 03, 2024
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Electrocorticography, (ECoG), is the procedure of attaching sensors called electrodes directly to a surgically exposed human brain to measure and analyze its electrical activity. External electrodes in contact with the head’s scalp can also detect the brain’s active functioning, but for diagnosing certain kinds of brain disorders, it is not precise enough. Frequent, severe and debilitating seizures from the medical condition called epilepsy is one example. A potential way to treat it is to identify and remove just the small portion of the brain that is triggering them, without damaging any critically important adjacent brain matter.

An x-ray can penetrate soft human tissue to reveal the shape of a skull, but doctors of neurology who want to see the brain inside need other technologies. One of the best methods is magnetic resonance imaging, commonly called MRI. Instead of x-ray radiation, it uses powerful magnets to change the body at the atomic level, recording their effect and displaying them in fine detail.

An image of the brain might show abnormal structures. There are, however, many brain disorders which are not caused by detectable physical things, but rather by abnormal functioning of the complex organ. To evaluate whether the brain is functioning normally, neurologists need different technologies. One of the most important of them is the simple brain map. Accumulated medical research has assigned the control of human functions — such as eyesight, language or a muscle — to very specific places of the brain.

With the map of a brain in hand, if a neurologist has a patient displaying abnormal symptoms, he knows where to look but needs technology to measure that location’s functioning state. One means is a functional MRI, which produces a brain image of areas highlighted by greater concentrations of oxygen-filled blood. The assumption is that this equates to the fuel needed by increased brain activity in such areas. Brain cells and nerve cells function by receiving, generating and transmitting electrical signals to other cells.

For a more direct representation of brain activity, a neurologist needs measurements of its electrical output. The most commonly used technology is the electroencephalogram, or EEG. A precisely mapped mesh of sensitive sensors called electrodes are attached to the scalp. Each measures brainwaves, minute voltage changes caused by electrical activity of the area of the brain directly underneath. Doctors have learned to recognize patterns in the resulting graph to be consistently indicative of certain neurological disorders, including the most common — epilepsy.

Epilepsy is characterized by uncontrollable seizures, often accompanied by loss of consciousness and muscle convulsions. They are essentially the result of an electrical overload of the brain, and can be categorized based on how the event is triggered. Many epileptics, at one time regarded as physically disabled, have been able to manage their condition with modern anticonvulsant drugs which were first introduced in the 1990s.

Among the many types of epilepsies, particularly difficult ones to treat are those which are triggered by, or originate from, a particular location of the brain. An EEG may have identified the general location, but images may have revealed no unusual abnormalities. If medication is ineffective, a neurologist may then have no recourse left but to perform a craniotomy, removing the cranium, a section of skull bone. Direct visual inspection of the exposed brain might reveal a scar, or some other probable cause.

At this stage, the neurosurgeon will normally perform an electrocorticography. Similar to an EEG, a smaller and finer mesh of electrodes will be placed directly onto the brain for a recording of the area’s electrical activity. This will pinpoint the apparent origin of epileptic episodes, also called the epileptogenic zone. The location is noted on the uppermost layer of the brain, called its cerebral cortex.

Removing the offending zone removes the trigger of seizures. The surgeon’s main objective is to surgically excise as small an amount of brain tissue as is necessary. In conjunction with electrocorticography, the entire area surrounding an epileptogenic zone might be probed with direct cortical electrical stimulation (DCES). The procedure employs a handheld pointer which delivers a mild electrical shock. Under local anesthetic and conscious, the patient might report a sensation or muscle movement from the stimulation.

Electrocorticography may identify an area about 0.4 inches (1cm) square. With DCES mapping, a surgeon can narrow this down to an even smaller region for surgical removal. In the process, he is able to avoid damaging areas of healthy brain which may be critical for normal human functions.

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