Electrocorticography (ECoG)

Intracranial electroencephalography

Electrocorticography is a specialized brain monitoring technique that uses electrodes placed directly on the surface of the brain to record electrical activity. This invasive procedure provides much clearer and more detailed information than standard brain wave tests, making it a valuable tool for treating severe epilepsy and advancing our understanding of how the brain works.

Table of contents

• What is Electrocorticography?
• History and Development
• How ECoG Works
• The ECoG Procedure
• Clinical Applications
• Advantages Over Other Brain Tests
• Research Applications
• Understanding the Spatial Reach

What is Electrocorticography?

Electrocorticography, commonly known as ECoG, is a type of medical test that records electrical activity directly from the surface of the brain^[1]. Unlike regular brain wave tests that use electrodes on the scalp, ECoG requires placing electrodes right on the exposed surface of the brain, which means the skull must be opened through surgery^[1]. Because of this, ECoG is considered an invasive procedure, meaning it requires a surgical incision into the body.

The electrodes used in ECoG are typically small, flat discs made of materials like platinum or stainless steel, usually about 2 to 3 millimeters in diameter^[3]. These electrodes can be arranged in different ways, such as in rectangular grids with multiple electrodes, narrow strips for specific areas, or as depth electrodes that can record from deeper brain structures^[4].

History and Development

ECoG was pioneered in the early 1950s by Wilder Penfield and Herbert Jasper, who were neurosurgeons working at the Montreal Neurological Institute^[1]. They developed ECoG as part of a groundbreaking surgical approach called the Montreal procedure, which was used to treat patients with severe epilepsy.

The technique was originally used to identify epileptogenic zones, which are regions of the brain’s outer layer that generate epileptic seizures^[1]. Once these zones were identified through ECoG recordings, they could be surgically removed to stop the seizures from occurring. Penfield and Jasper also used electrical stimulation during ECoG recordings to map important areas of the brain, such as those responsible for speech and movement, to make sure these areas were not damaged during surgery^[1].

By 1934, the first use of ECoG data during surgery provided doctors with the improved detail necessary to measure electrical activity in both surface and deep brain structures^[4]. This advancement helped overcome the limitations of scalp recordings, which had limited ability to pinpoint exactly where problems were occurring in the brain.

How ECoG Works

ECoG works by capturing electrical signals that are produced by brain cells, specifically signals called postsynaptic potentials (also known as local field potentials), which occur primarily in specialized brain cells called pyramidal cells^[1]. These electrical signals must travel through several layers of brain tissue and membranes before reaching the recording electrodes.

The reason ECoG provides much clearer signals than regular brain wave tests (called EEG) is because the electrodes are much closer to where the electrical activity is generated^[1]. In a regular EEG, the electrical signals must pass through the skull bone, which doesn’t conduct electricity well, causing the signals to become much weaker and less detailed. ECoG avoids this problem by placing electrodes directly on the brain’s surface^[1].

ECoG can capture brain activity with very high precision. It has a temporal resolution (ability to detect changes over time) of approximately 5 milliseconds and a spatial resolution (ability to pinpoint location) as low as 1 to 100 micrometers^[1]. This means it can detect very rapid changes in brain activity and can identify which specific areas of the brain are active.

The ECoG Procedure

The ECoG procedure requires surgery to access the brain’s surface^[1]. A surgeon must first perform a craniotomy, which means removing a part of the skull to expose the brain. This procedure can be performed either under general anesthesia (where the patient is completely asleep) or under local anesthesia if the patient needs to interact with doctors during the procedure^[1].

The surgery typically lasts several hours and is most commonly done under general anesthesia^[2]. The number of electrodes placed and their specific locations depend on each patient’s individual condition and what information the doctors gathered before surgery^[2].

Once the electrodes are in place, the patient remains in the hospital for three to seven days while the brain is monitored for seizure activity^[2]. In some cases, this monitoring period may be longer if no seizures occur naturally. To help trigger seizures for recording purposes, doctors may reduce seizure medications, use flashing lights, or limit the patient’s sleep^[2].

The tail of the electrode is tunneled under the skin beneath the scalp and exits 2 to 3 centimeters from where the skull was opened^[1]. It is coiled and stitched to the scalp to prevent it from being accidentally pulled out. After the monitoring period is complete, the electrodes are removed in the operating room, and sometimes the area causing seizures may be removed at the same time^[2].

Clinical Applications

The primary clinical use of ECoG is in treating patients with severe epilepsy that does not respond to medications^[2]. When medications fail to control seizures, doctors may use imaging techniques and regular brain wave tests to try to locate where seizures are coming from in the brain. However, these methods don’t always provide precise enough information.

In these cases, patients may undergo ECoG to gather better information about their seizures^[2]. The electrodes can be paddle-shaped devices that sit on the brain’s surface or cylindrical devices that go into the brain, and sometimes a combination of both types is used. These procedures are often called subdural grid placement, depth electrode placement, or stereoelectroencephalography (sEEG)^[2].

ECoG is extensively used to find the seizure focus in patients with drug-resistant epilepsy^[3]. The brain area responsible for seizures can then be surgically removed. Accurate identification of the area generating seizures and its removal requires understanding exactly how far the ECoG signal spreads, so doctors know precisely which tissue to remove.

Beyond epilepsy treatment, ECoG is crucial for identifying functional areas of the brain during neurosurgical procedures^[4]. During surgery, the information gathered creates a map showing where seizures occur as well as other important brain areas such as those controlling speech and movement^[2]. This helps surgeons avoid damaging these critical areas.

Advantages Over Other Brain Tests

ECoG offers several important advantages over standard brain wave tests performed from outside the skull. The most significant advantage is much better image quality and detail. The spatial resolution of ECoG is much higher than EEG, which is a critical advantage when planning surgery^[1].

Because the electrodes are placed directly on the brain, ECoG is less affected by common sources of interference that plague scalp-based recordings, such as muscle movements and eye blinks^[7]. This makes the recordings much cleaner and easier to interpret, especially during activities like speaking, which would normally create a lot of interference in scalp recordings.

Another advantage is that ECoG can record a particular type of brain signal called high gamma power (also called “broadband activity”), which occurs at frequencies between 70 and 190 Hz^[7]. This signal has a very good signal-to-noise ratio and is likely to reflect the combined activity of groups of neurons directly underneath the electrode, making it directly related to increases and decreases in local brain activity.

ECoG combines adequate timing precision and location precision with lower risks of medical complications compared to other invasive methods^[4]. This balance makes it particularly valuable for clinical applications.

Research Applications

Beyond its clinical uses in treating epilepsy, ECoG is increasingly used to study brain function and cognitive processes in humans^[5]. Since the mid-twentieth century, ECoG has provided researchers with an intimate view into how the human brain works^[4].

ECoG is valuable for brain research because it provides both high temporal resolution and high spatial specificity^[4]. Researchers can use ECoG to study how different parts of the brain process information, how brain areas communicate with each other, and how brain activity relates to thoughts, feelings, and behaviors.

The technique is also of growing importance in developing brain-machine interface technologies^[3]. These are systems that allow people to control external devices, like computer cursors or robotic arms, directly with their brain activity. ECoG signals have been successfully used in such interfaces, where the signals are converted into computer-driven outputs like cursor movement, mouse clicking, letter generation, or movement of a robotic arm^[6].

One advantage for research is that patient volunteers often enjoy participating in sensory and cognitive experiments, as it helps alleviate boredom during extended hospital stays^[7]. Patients often see the value of contributing to the advancement of scientific knowledge and medical treatments.

Understanding the Spatial Reach

An important question for both clinical treatment and research is understanding the spatial spread of ECoG, which means knowing how much brain tissue around an electrode actually contributes to the signal it records^[3]. This is crucial for accurately estimating which brain tissue is involved in generating seizures or in cognitive processes.

Research has shown that ECoG is surprisingly local in its reach^[3]. Studies in the visual areas of monkey brains found that the spatial spread of ECoG has a diameter of approximately 3 millimeters, which is only about 3 times larger than the spread of signals recorded from much smaller electrodes^[3]. This was unexpected, as researchers thought ECoG might capture signals from a much larger area.

Using depth electrodes, the local field potential gives a measure of a population of neurons in a sphere with a radius of 0.5 to 3 millimeters around the tip of the electrode^[1]. This relatively small area of influence means that ECoG can provide quite precise information about which specific brain regions are active.

These findings validate the use of ECoG in clinical practice and basic research^[3]. The fact that ECoG is a local signal means it can provide a useful tool for clinical applications, cognitive neuroscience studies, and brain-machine-interfacing applications^[5].