Otis Singleton
The brain is an incredibly complex organ, with billions of neurons and trillions of connections. To understand its electrical activity, scientists have developed a range of techniques and technologies. One of the most well-known and widely used methods is electroencephalography (EEG), which measures electrical activity in the brain through electrodes placed on the scalp. EEG can detect changes over milliseconds and is often used to understand the brain's response to stimuli. Other methods include electrocorticography (ECoG), which requires the insertion of electrodes under the scalp, and functional magnetic resonance imaging (fMRI), which provides an unrivalled view of the localisation of brain functions. More recently, a technique using voltage-sensing molecules that fluoresce when brain cells are active has been developed, allowing researchers to see the activity of many individual neurons.
| Characteristics | Values |
|---|---|
| Name of the procedure | Electroencephalography or EEG |
| What it measures | Electrical activity of the brain |
| How it works | Detects abnormalities in brain waves |
| Time taken | 45 minutes to 2 hours |
| Preparation | No caffeine for 8-12 hours before the test |
| Other tests | Deep breathing, flashing lights, sleep study, prolonged monitoring |
| Invasiveness | Non-invasive |
| Spatial resolution | Poor |
| Temporal resolution | Superior to fMRI |
| Combined use | Can be used with fMRI, NIRS or fUS |
| Use cases | Alzheimer's disease, sleep disorders, epilepsy |
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What You'll Learn
Electroencephalography (EEG)
EEG has a superior temporal resolution compared to other methods such as fMRI, PET, and CT scans. While EEG measures the brain's electrical activity directly, other methods like SPECT, fMRI, fUS, and CT scans record changes in blood flow or metabolic activity, which are indirect markers of brain electrical activity. EEG's high temporal resolution allows it to accurately track neural dynamics in awake humans and determine the brain's electrical response to various stimuli or conditions.
EEG can be used in conjunction with other techniques such as fMRI or fUS to simultaneously record high-temporal-resolution and high-spatial-resolution data. However, there are technical challenges when combining EEG with fMRI due to the magnetic field of the MRI, which can induce currents in moving EEG electrode wires. EEG can be used with NIRS or fUS without major technical difficulties, providing valuable information about electrical activity and hemodynamics.
EEG is also useful in studying various brain functions, including sensory and information processing, memory, and the effects of substances like alcohol on brain function. It can detect abnormal brain activity associated with conditions such as epilepsy and Alzheimer's disease.
The procedure for an EEG test involves initial recording while the patient is at rest, followed by exposure to various stimuli to evoke specific brain responses. The test can take 45 minutes to 2 hours, and in some cases, prolonged inpatient or ambulatory monitoring may be required. Patients are advised to avoid caffeine and sleep deprivation before the test, and specific instructions are given based on the nature of the test and the patient's health condition.
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Electrocorticography (ECoG)
ECoG was pioneered in the 1950s by Wilder Penfield and Herbert Jasper, neurosurgeons at the Montreal Neurological Institute. They developed ECoG as part of their groundbreaking Montreal procedure, a surgical protocol used to treat patients with severe epilepsy. The procedure involves the placement of disk electrodes, typically in strips or grids containing multiple contacts, on the cortical surface. The electrodes record electrical activity from the cerebral cortex, which is then used to identify epileptogenic zones—regions of the cortex that generate epileptic seizures. These zones are then surgically removed from the cortex during resectioning, thereby destroying the brain tissue where epileptic seizures originated.
ECoG may be performed either in the operating room during surgery (intraoperative ECoG) or outside of surgery (extraoperative ECoG). ECoG is often used to identify the location of seizures in epileptic patients and to plan epilepsy surgery. Direct cortical electrical stimulation (DCES) is frequently performed alongside ECoG recording for functional mapping of the cortex and identification of critical cortical structures.
ECoG offers a temporal resolution of approximately 5 ms and a spatial resolution as low as 1-100 μm, providing a critical imaging advantage for presurgical planning. The spatial resolution of ECoG is much higher than that of EEG as the neural signal is not distorted by other tissue or bone, resulting in a higher signal-to-noise ratio.
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Functional magnetic resonance imaging (fMRI)
Functional magnetic resonance imaging or functional MRI (fMRI) is a technique for measuring and mapping brain activity that is non-invasive and safe. It is used in many studies to better understand how a healthy brain works, and it is also being used to understand how normal function is disrupted in disease.
FMRI measures brain activity by detecting changes associated with blood flow. This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases. The primary form of fMRI uses blood-oxygen-level-dependent (BOLD) contrast, discovered by Seiji Ogawa in 1990. This is a type of specialized brain and body scan used to map neural activity in the brain or spinal cord of humans or other animals by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells.
In the simplest fMRI experiment, a subject alternates between periods of performing a particular task and a control state, such as 30-second blocks of looking at a visual stimulus alternating with 30-second blocks with eyes closed. The fMRI data is then analyzed to identify brain areas in which the MR signal has a matching pattern of changes, and these areas are considered activated by the stimulus.
FMRI has several limitations. Firstly, it does not provide access to detailed information about the brain's activity at a microscopic level. While it can determine the likely source of cognitive function within the brain, it does not have the spatial and temporal resolution to capture the complex, rapid dynamics of individual neurons or small groups of neurons. Secondly, fMRI data is susceptible to noise from various sources, requiring the use of statistical procedures to extract the underlying signal. Finally, fMRI has a lower temporal resolution compared to other techniques such as EEG, which can detect changes over milliseconds, making it less suitable for tracking neural dynamics in real-time.
Despite these limitations, fMRI remains a widely used technique for studying brain activity due to its ability to provide a macroscopic view of brain function and its non-invasiveness. Researchers continue to work on improving the spatial and temporal resolution of fMRI, making it a valuable tool in the field of neuroscience.
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Positron emission tomography (PET)
PET scans differ from other nuclear medicine examinations in that they detect metabolism within body tissues, whereas other types of nuclear medicine examinations detect the amount of a radioactive substance collected in body tissue in a certain location to examine the tissue's function. A tiny amount of a radioactive substance, called a radiopharmaceutical (or radionuclide/radioactive tracer), is used during the procedure to assist.
The radionuclides used in PET scans are made by attaching a radioactive atom to chemical substances that are used naturally by the particular organ or tissue during its metabolic process. For example, in PET scans of the brain, a radioactive atom is applied to glucose (blood sugar) to create a radionuclide called fluorodeoxyglucose (FDG), because the brain uses glucose for its metabolism. FDG is widely used in PET scanning. Other substances may be used, depending on the purpose of the scan. If blood flow and perfusion of an organ or tissue are of interest, the radionuclide may be a type of radioactive oxygen, carbon, nitrogen, or gallium.
PET scans can be used to evaluate the brain after trauma to detect hematoma (blood clot), bleeding, and/or perfusion (blood and oxygen flow) of the brain tissue. PET can also be used to detect the spread of cancer to other parts of the body from the original cancer site.
PET is a quantitative molecular imaging technique that allows in-vivo analysis of neurotransmitter systems in the brain by detecting γ-ray photons resulting from the annihilation of positron-emitting radionuclides. The principle of PET is based on the annihilation coincidence detection of positron-emitting radionuclides. PET detects the γ-ray photons resulting from annihilation of positron and electron resulting from radioactive β-decay of radiolabelled compounds. Following the positron decay, two 0.511 MeV photons are emitted at a certain angle and simultaneously detected by scintillation crystals. The light of the scintillation crystals is further converted into electrical signals, which are adequately processed and reconstructed to deliver image data.
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Intracortical Encephalogram electrodes
Electroencephalography (EEG) is a widely used method to record the electrical activity of the brain. The electrodes used in this method are placed on the scalp and are non-invasive. However, in certain cases, electrodes may need to be inserted near the surface of the brain, under the dura mater. This is where intracortical encephalogram electrodes come into play.
Intracortical encephalogram (ICE) electrodes are a type of intracranial electroencephalography (iEEG) or electrocorticography (ECoG) electrode. ECoG is an invasive procedure that involves placing electrodes directly on the exposed surface of the brain, also known as the cerebral cortex, to record electrical activity. The placement of these electrodes requires a craniotomy, a surgical incision into the skull. The ECoG procedure was pioneered in the 1950s by Wilder Penfield and Herbert Jasper, neurosurgeons at the Montreal Neurological Institute.
ICE electrodes are flexible and transparent, with a standard spacing of 1 cm between grid electrodes and a diameter of 5 mm for individual electrodes. These electrodes are designed to be lightweight and flexible to ensure that normal brain movements do not cause injury. One of the key advantages of ICE electrodes is their ability to be slid underneath the dura mater, allowing access to cortical regions not directly exposed by the craniotomy. This flexibility in placement provides greater coverage of the brain and improves the spatial resolution of the recordings.
ICE electrodes have been used in conjunction with machine learning techniques to predict epilepsy seizures in rats. By analysing intracortical encephalogram signals, researchers have developed models that can predict seizures with impressive accuracy. This technology has significant implications for the development of brain-computer interfaces and the advancement of epilepsy treatment in humans. Furthermore, ICE electrodes can be used in tandem with sub-dural electrodes to discriminate and discretize artifacts from epileptiform and other severe neurological events.
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Frequently asked questions
EEG stands for Electroencephalography or Electroencephalogram. It is a procedure that detects abnormalities in brain waves or electrical activity.
An EEG measures the brain's electrical activity directly by placing electrodes on the scalp. It can detect changes over milliseconds.
EEG has a 'temporal resolution' far superior to fMRI (~1 ms vs. 1 sec). Because of this, EEG can be used to more accurately track neural dynamics in awake humans.
The primary limitation of EEG is its poor spatial resolution, much poorer than for fMRI. It is also difficult to know precisely where in the cortex signals arise.
