Trystan Casey
The electrical activity of the brain can be measured in several ways, including electroencephalography (EEG), electrocorticography, voltage sensors, and fluorescent molecules. EEG is a widely used technique that involves placing electrodes on the scalp to record brain waves and detect abnormalities in electrical activity. Electrocorticography, on the other hand, requires surgical insertion of electrodes under the scalp. Voltage sensors can be combined with optogenetics to turn brain cells on and off with laser light. Fluorescent molecules, such as Archon1 and SomArchon, can be used to genetically engineer neurons and allow for imaging of electrical activity. These methods provide valuable insights into brain function, behaviour, and disorders.
| Characteristics | Values |
|---|---|
| Method | Electroencephalography (EEG) |
| Function | Records electrical activity of the brain |
| Uses | Diagnosing brain disorders, determining brain death, monitoring blood flow during surgery, studying brain functions |
| Procedure | Electrodes placed on the scalp with a conductive gel or paste |
| Number of electrodes | Between 16 and 25 |
| Time taken | Long |
| Signal-to-noise ratio | Poor |
| Compatibility | Not suitable for individuals with coarse or textured hair |
| Resolution | Temporal resolution superior to fMRI, poor spatial resolution |
| Data | Reflects activation level of various brain regions |
| Imaging technique | Voltage-sensing molecule that fluoresces when brain cells are electrically active |
| Alternative techniques | Electrocorticography (ECoG), Functional magnetic resonance imaging (fMRI), Calcium imaging |
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What You'll Learn
Electroencephalography (EEG)
EEG is a useful tool for exploring brain activity as it can detect changes over milliseconds, which is excellent for measuring the fast-propagating action potentials of neurons. It directly measures the brain's electrical activity, while other methods such as SPECT, fMRI, fUS, and PET record changes in blood flow or metabolic activity, which are indirect markers of brain electrical activity. EEG can be used in conjunction with some of these other methods, such as fMRI or fUS, to record high-temporal-resolution data alongside high-spatial-resolution data.
One of the limitations of EEG is its poor spatial resolution, which makes it difficult to pinpoint the precise location of signals in the cortex. It also cannot identify specific locations in the brain at which various neurotransmitters, drugs, etc. can be found, unlike methods such as PET and MRS. The signal-to-noise ratio of EEG is also poor, requiring relatively large numbers of subjects and sophisticated data analysis to extract useful information.
Despite these limitations, EEG has been used for many years and is a valuable tool for diagnosing disorders that influence brain activity, such as Alzheimer's disease, certain psychoses, and sleep disorders like narcolepsy. It can also be used to evaluate trauma, drug intoxication, or the extent of brain damage, and to monitor blood flow during surgery.
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Electrocorticography (ECoG)
ECoG was pioneered in the early 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.
ECoG may be performed either in the operating room during surgery (intraoperative ECoG) or outside of surgery (extraoperative ECoG). A craniotomy, or surgical incision into the skull, is required to implant the electrode grid, making ECoG an invasive procedure. The electrodes may be placed outside the dura mater (epidural) or under the dura mater (subdural).
ECoG electrode arrays typically consist of sixteen sterile, disposable electrodes made of materials such as stainless steel, platinum, or gold. These electrodes are attached to an overlying frame in a "'crown" or "halo" configuration. The diameter of each electrode is typically 2.3 mm, with a 1 cm inter-electrode distance, although smaller electrodes are increasingly used.
Direct cortical electrical stimulation (DCES) is often performed concurrently with ECoG recording for functional mapping of the cortex and identification of critical cortical structures. Electrical stimulating currents applied to the cortex are relatively low, with North America typically using 60 Hz and Europe using 50 Hz.
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Functional magnetic resonance imaging (fMRI)
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, a type of specialized brain and body scan used to map neural activity in the human brain or spinal cord, or those of other animals. This is done by imaging the change in blood flow (hemodynamic response) related to energy use by brain cells.
The discovery that MRI could be made sensitive to brain activity, as well as brain anatomy, is only about 20 years old. The essential observation was that when neural activity increased in a particular area of the brain, the MR signal also increased by a small amount. Although this effect involves a signal change of only about 1%, it is still the basis for most fMRI studies today.
In general, fMRI studies acquire both many functional images with fMRI and a structural image with MRI. The structural image is usually of a higher resolution and depends on a different signal, the T1 magnetic field decay after excitation. To demarcate regions of interest in the functional image, one needs to align it with the structural one. This is done with a coregistration algorithm that works similarly to the motion-correction one, except that here the resolutions are different, and the intensity values cannot be directly compared since the generating signal is different.
VASO is another technique used in fMRI that allows for the reliable quantification of physiological parameters such as the cerebral metabolic rate of oxygen or the oxygen extraction fraction. However, the approach suffers from some limitations compared to techniques that rely on the BOLD signal, such as lower sensitivity and reduced imaging efficiency.
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Fluorescent molecules
The development of this technique is attributed to MIT researchers, who aimed to create a more efficient and informative approach than the traditional method of inserting electrodes into the brain, which is challenging and time-consuming. The new method involves embedding a light-sensitive protein into neuron membranes, allowing for the measurement of electrical activity through fluorescent signals.
One of the key advantages of using fluorescent molecules is the ability to study neural activity with high temporal resolution. The molecules can detect changes over milliseconds, capturing the rapid electrical impulses that neurons use to communicate within the brain. This level of detail provides valuable insights into how the brain coordinates behaviour, sensation, thoughts, and emotions.
Additionally, fluorescent molecules offer the benefit of resistance to photobleaching. Photobleaching, or fading, is a common issue with fluorescent proteins due to prolonged exposure to light. By engineering molecules that are resistant to this effect, scientists can obtain more durable and reliable signals during their studies of brain activity.
The process of finding suitable fluorescent molecules is complex. Boyden and colleagues at MIT employed a robotic system to screen millions of proteins generated through directed protein evolution. This approach mimics natural evolution by creating numerous mutant genes and selecting those with the desired traits. The chosen proteins must be sensitive to voltage changes and respond quickly while also resisting photobleaching.
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Calcium imaging
One advantage of calcium imaging is its ability to longitudinally record from the same set of cells over extended periods, which can range from weeks to months. This makes it a valuable tool for studying plasticity in neuromodulation therapies, as it provides a direct visualization of neuron somas that can be used as landmarks to track the same neuron over time. The technique also allows for simultaneous and non-invasive monitoring of activity from multiple individual neurons or different brain regions.
However, calcium dynamics are relatively slow compared to voltage changes, which can blur the relationship between the optical signal and spike rates. Improvements in GEVIs (genetically encoded voltage indicators) have led to faster response times and better signal-to-noise ratios, making them more useful for neurobiological research. Nonetheless, GECIs (genetically encoded calcium indicators) and GEVIs are complementary tools, with GECIs offering a better signal-to-noise ratio and calcium sensitivity.
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Frequently asked questions
Electroencephalography, or EEG, is a widely-used method of recording electrical activity in the brain. Electrodes are placed on the scalp and the electrical activity is recorded.
EEG has poor spatial resolution and cannot be used to measure activity in the hippocampus, substantia nigra or striatum. It also takes a long time to connect a subject to EEG as the precise placement of electrodes is required.
Other methods of measuring electrical activity in the brain include electrocorticography (ECoG), functional magnetic resonance imaging (fMRI), and event-related potentials (ERP).
