Wiktor Wise
The brain's electrical activity can be recorded using a method called electroencephalography (EEG). This technique involves placing electrodes on the scalp to detect bio signals that represent the electrical activity of neurons in the brain. EEG is typically non-invasive and provides valuable insights into brain function, including the evaluation of dynamic cerebral functioning and the study of complex brain functions such as sensory and information processing. In addition to EEG, other techniques such as electrocorticography (ECoG) and imaging methods using fluorescent molecules have also been explored to record and visualize brain electrical activity. These advancements provide a clearer understanding of how neurons work together and behave millisecond by millisecond as the brain performs its functions.
| Characteristics | Values |
|---|---|
| Name of the method | Electroencephalography (EEG) |
| Process | Electrodes are placed along the scalp to record electrical activity |
| Electrode placement | International 10-20 system or variations of it |
| Electrode number | 19 recording electrodes (plus ground and system reference) |
| Electrode gel | Conductive gel or paste |
| Electrode amplifier | Amplifies voltage between the active electrode and the reference |
| Electrode voltage | 1,000–100,000 times |
| Power gain | 60–100 dB |
| Electrode detachment | "Electrode pop" artifact |
| Related technique | MEG (captures magnetic fields generated by the brain) |
| Alternative technique | Calcium imaging |
| Fluorescent molecules | Archon1, SomArchon |
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What You'll Learn
Electroencephalography (EEG)
The electrical activity recorded by the EEG originates from neurons in the underlying brain tissue. The signals detected by the electrodes on the surface of the scalp vary depending on their orientation and distance from the source of the electrical activity. The EEG recordings primarily reflect the activity of cortical neurons near the electrodes, while deeper structures within the brain, such as the hippocampus and brain stem, do not contribute directly to the EEG signals. The value recorded by the EEG is also distorted by intermediary tissues and bones, which act like resistors and capacitors in an electrical circuit.
The EEG recordings are analysed through visual inspection of the tracing or quantitative EEG analysis. Voltage fluctuations measured by the EEG bio amplifier and electrodes allow for the evaluation of normal brain activity. The frequencies observed in a healthy human EEG range from 1 to 30 Hz, with amplitudes varying between 20 and 100 μV. These frequencies are further subdivided into different groups, including alpha (8–13 Hz), beta (13–30 Hz), delta (0.5–4 Hz), and theta (4–7 Hz).
EEG is particularly useful in the evaluation of dynamic cerebral functioning, especially in patients with suspected seizures, epilepsy, and unusual spells. It can also be used to localize the source of epileptic brain activity for resective surgery. This procedure involves implanting strips and grids of electrodes under the dura mater, and the recording is referred to as electrocorticography (ECoG) or intracranial EEG (iEEG). EEG is also used in alcohol research to study the effects of alcohol on brain function and identify people at risk for developing alcoholism.
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Electrocorticography (ECoG)
ECoG is considered less invasive than electrodes that penetrate the cortex, and it has the added benefit of a decreased risk of inflammation and a less problematic immune response and eventual encapsulation of the electrodes. The electrodes are placed on the exposed cortex, which is accessed through a craniotomy, or a surgical incision into the skull to expose the brain surface. This procedure may be performed under general anesthesia or local anesthesia if patient interaction is required for functional cortical mapping.
The ECoG recording is performed using disk electrodes placed in strips or grids containing multiple contacts on the cortical surface, either epidurally or subdurally. The electrode diameter is typically 2.3 mm of exposed surface, with a 1 cm inter-electrode distance, but layouts with higher density and smaller electrodes are becoming more common. ECoG electrode arrays typically consist of sixteen sterile, disposable electrodes made of materials such as stainless steel, carbon tip, platinum, Platinum-iridium alloy, or gold, each mounted on a ball and socket joint for easy positioning.
ECoG is often used to identify the location of seizures in epileptic patients and to plan epilepsy surgery. The cortical potentials recorded by ECoG are used to identify epileptogenic zones, or regions of the cortex that generate epileptic seizures. These zones are then surgically removed from the cortex during resectioning, destroying the brain tissue where epileptic seizures originated. ECoG also provides better spatial resolution than EEG, making it valuable for presurgical planning.
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Intracranial EEG (iEEG)
IEEG can be performed in two ways: electrocorticography (ECoG) and stereotactic EEG (sEEG). ECoG uses strips or grids of electrodes implanted in the subdural space, providing a large coverage of the cerebral cortex. sEEG, on the other hand, uses wires of electrodes that penetrate the brain to target predefined deeper sites without the need for open craniotomy.
The iEEG signal is highly localized and provides anatomically precise information about the selective engagement of neuronal populations at the millimetre scale. It also offers insights into the temporal dynamics of their engagement at the millisecond scale. By monitoring multiple nodes of a given network simultaneously, iEEG can reveal functional interactions within and across networks during different stages of neural computation.
IEEG is particularly useful in epilepsy management. Before epilepsy surgeries, iEEG is employed for function mapping and epileptogenic foci localization. It helps to precisely define the epileptogenic zone (EZ) when non-invasive data are inconclusive. iEEG can also be used to predict cognitive and motor outcomes following resection.
Furthermore, iEEG has the ability to measure signals within a specific frequency range, including delta, theta, alpha, beta, and gamma oscillations, as well as the broadband signal. This allows for studying the interaction and coupling of activity across different frequencies and neural systems.
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Fluorescent molecules
In 2018, researchers at MIT engineered a light-sensitive protein that can be embedded into neuron membranes. When the electrical activity of a neuron increases, the protein emits a fluorescent signal that indicates how much voltage the cell is experiencing. This technology was first tested on mouse brain tissue, zebrafish larvae, and the worm Caenorhabditis elegans. The latter two organisms are transparent, so it is easy to expose them to light and image the resulting fluorescence. The researchers also demonstrated that this protein can be used in conjunction with optogenetic proteins that are commonly used to stimulate or silence neuron activity.
The protein, called Archon1, can be genetically inserted into neurons, where it becomes embedded in the cell membrane. When the neuron's electrical activity increases, the molecule becomes brighter, and this fluorescence can be seen with a standard light microscope. Researchers at Boston University and MIT have shown that they can see the activity of many more individual neurons than ever before as they fire inside the brains of mice. With Archon1, it is also possible to measure very small fluctuations in activity that occur even when a neuron is not firing a big spike in electrical activity. This could help neuroscientists study how small fluctuations impact a neuron’s overall behavior, which has been very difficult to do in the past.
A new molecule called SomArchon has also been developed, which accumulates specifically in the center of neuron cell bodies, preventing interference from the long, tendril-like axons of neighboring neurons.
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Calcium imaging
The experimental protocol for calcium imaging involves four components:
- Induction of calcium indicator expression through the injection of an AAV9 viral vector into the striatum.
- Placement of a Gradient Refractive Index (GRIN) lens above the striatum at the target recording location.
- Induction of a Parkinsonian phenotype by injecting 6-OHDA into the substantia nigra pars compacta.
- Placement of a bipolar stimulating electrode within the subthalamic nucleus (STN) for electrical stimulation.
In one study, calcium imaging was performed on cells labelled with GCaMP, a genetically encoded calcium indicator used to detect neural activity. A GRIN lens was implanted into the striatum to capture miniature microscopy videos, which were then analysed to determine changes in striatal activity during stimulation.
Another study used calcium imaging to investigate the impact of electrical stimulation on neural activity in a pathological mouse model. This involved combining chronic optical recordings of hundreds of cells with stimulation over extended periods. The results demonstrated the feasibility of using calcium imaging to understand the mechanisms of deep brain stimulation therapies.
Furthermore, calcium imaging has been employed in studies of electrical stimulation in Parkinson's disease models. By stimulating neural activity and tracking the position of the animal, researchers can assess behavioural changes and extract spatial and temporal properties of neural activity.
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Frequently asked questions
Electroencephalography (EEG) is a method used to record the electrical activity of the brain.
EEG uses electrodes placed along the scalp to record electrical activity. The electrodes are connected to differential amplifiers that amplify the voltage. The recordings are then interpreted by visual inspection or quantitative EEG analysis.
EEG is limited by the fact that the electrical activity of the brain must pass through multiple biological filters before being detected, which can reduce signal amplitude and spread out the activity. Additionally, other biological and environmental electrical artifacts can interfere with the accurate identification of normal and pathological patterns.
Yes, there are alternative methods such as electrocorticography (ECoG), subdural EEG (SDE), intracranial EEG (iEEG), and stereotactic EEG (SEEG). Additionally, researchers have been developing imaging techniques that use fluorescent molecules or proteins that light up in response to voltage changes in the brain. These techniques aim to provide a clearer picture of brain cell activity.
