Umair Gaines
The brain's electrical activity is moderated through neurons, which communicate via rapid electrical impulses. These impulses allow the brain to coordinate behaviour, sensation, thoughts, and emotions. To study this electrical activity, scientists have traditionally used electrodes inserted into the brain, but this technique is difficult and time-consuming. More recently, researchers have developed a light-sensitive protein that can be embedded into neuron membranes, emitting a fluorescent signal that indicates the voltage of a particular cell. This new approach is expected to provide a more informative and detailed understanding of brain function.
| Characteristics | Values |
|---|---|
| Method to record electrical activity in the brain | Electroencephalography (EEG) |
| Purpose of EEG | To record an electrogram of the spontaneous electrical activity of the brain |
| How is it done? | Electrodes are pasted onto the scalp |
| Number of electrodes | Between 16 and 25 |
| Frequency range | 1-30 Hz |
| Amplitudes | 20-100 μV |
| Frequency groups | Alpha (8-13 Hz), Beta (13-30 Hz), Delta (0.5-4 Hz), and Theta (4-7 Hz) |
| Alpha waves | Observed when a person is in a state of relaxed wakefulness |
| Beta waves | More prominent during intense mental activity |
| Theta and Delta waves | Not generally seen in wakefulness; if they are, it is a sign of brain dysfunction |
| EEG use cases | Diagnosing Alzheimer's disease, psychoses, sleep disorders, depth of anesthesia, coma, encephalopathies, cerebral hypoxia after cardiac arrest, brain death, blood flow in the brain, etc. |
| Other methods | Calcium imaging, multielectrode arrays, positron emission tomography (PET), magnetic resonance imaging (MRI), computed tomography (CT) scans |
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What You'll Learn
Electroencephalography (EEG)
EEG is used to evaluate several types of brain disorders, such as epilepsy, brain lesions, Alzheimer's disease, psychoses, sleep disorders, and more. It can also be used to monitor blood flow in the brain or neck blood vessels during surgery, and to determine the overall electrical activity of the brain, for example, to evaluate trauma, drug intoxication, or the extent of brain damage in a person who is in a coma.
The electrical activity monitored by EEG originates in neurons in the underlying brain tissue, and the recordings made by the electrodes on the scalp vary according to their orientation and distance. The value recorded is also distorted by intermediary tissues and bones, which act like resistors and capacitors in an electrical circuit. This means that not all neurons will contribute equally to an EEG signal, with the EEG predominantly reflecting the activity of cortical neurons near the electrodes on the scalp.
EEG can detect abnormal electrical discharges such as sharp waves, spikes, or spike-and-wave complexes, which are observable in people with epilepsy. The rhythmic activity detected by EEG is divided into bands by frequency, such as alpha (8–13 Hz), beta (13–30 Hz), delta (0.5–4 Hz), and theta (4–7 Hz). Alpha waves are observed when a person is in a state of relaxed wakefulness, while beta waves are more prominent during intense mental activity. Theta and delta waves are not generally seen in wakefulness – if they are, it is a sign of brain dysfunction.
EEG data can be displayed in several ways, depending on the montage, or the representation of the EEG channels. Each channel (or waveform) represents the difference between two adjacent electrodes. The display of the EEG for the reading electroencephalographer may be set up in various ways, and the EEG can be viewed in any desired display montage. The EEG is read by a clinical neurophysiologist or neurologist, and the interpretation of the data is performed by visual inspection of the tracing or quantitative EEG analysis.
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Event-related potentials (ERP)
Event-related potentials (ERPs) are a more sophisticated method of extracting specific sensory, cognitive, and motor events by using simple averaging techniques. ERPs are transient fluctuations in the brain's electrical field generated by neural activity and induced by the presentation of a visual or auditory language stimulus. ERPs can be reliably measured using electroencephalography (EEG), which measures electrical activity in the brain over time using electrodes placed on the scalp. The EEG reflects thousands of simultaneous brain processes, and to see the brain's response to a stimulus, the experimenter must conduct many trials and average the results. This causes random brain activity to be averaged out, and the relevant waveform to remain, which is the ERP.
ERPs consist of a series of positive and negative voltage deflections, which are related to a set of underlying components. Some ERP components are referred to with acronyms, such as the contingent negative variation (CNV) and error-related negativity (ERN), while most components are referred to by a letter indicating polarity (negative/positive), followed by a number indicating either the latency in milliseconds or the component's ordinal position in the waveform. For example, a negative-going peak that is the first substantial peak in the waveform and often occurs about 100 milliseconds after a stimulus is often called the N100 or N1.
The most famous ERP component is the P300, which shows when using an oddball paradigm, or when a series of standard stimuli are presented with a deviant stimulus from time to time. This is a positive peak that shows between 200 and 700 ms after the deviant stimulus. Another example is the N170, which reflects the neural processing of faces and is elicited by presenting images of faces along with other types of images. It is generated only with images of faces, and the increased negativity occurs between 130 and 200 ms after the stimulus is presented.
ERPs are also used in neurolinguistics research, with components such as the Early Left Anterior Negativity (ELAN), the N400, and the P600/SPS. The analysis of ERP data is increasingly supported by machine learning algorithms.
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Brain cell activity imaging
One traditional method of measuring electrical activity in the brain is through electroencephalography (EEG). EEG involves recording the spontaneous electrical activity of the brain using electrodes placed along the scalp or, in some cases, through surgical placement, known as electrocorticography. While EEG provides valuable information, it has limitations. The electrical signals measured by EEG are distorted by intermediary tissues and bones, making it challenging to capture the activity of all neurons, especially those deeper within the brain.
To overcome these limitations, researchers have developed advanced imaging techniques. One notable technique involves the use of voltage-sensing molecules that fluoresce when brain cells are electrically active. This method, reported in Nature, offers an incredibly detailed view of brain cell activity. Researchers from Boston University and the Massachusetts Institute of Technology (MIT) have successfully used this approach in mice brains, allowing them to observe the activity of numerous individual neurons simultaneously.
Another innovative approach is the development of light-sensitive proteins that can be embedded in neuron membranes. These proteins emit fluorescent signals that indicate the voltage levels within the cells. This technique, pioneered by MIT researchers, allows for the study of neuron behaviour at a millisecond timescale. It provides a more comprehensive understanding of how neurons communicate and work together in larger circuits.
Additionally, functional magnetic resonance imaging (fMRI) has emerged as a leading technique for studying whole-brain function in humans. fMRI is based on changes in brain circulation and metabolism associated with neuronal activity. It detects alterations in blood flow, oxygen consumption, and glucose utilisation, which are intricately linked to cellular activity in the brain. The development of molecular fMRI further enhances this technique by utilising chemical or genetically encoded probes that bind to specific molecular and cellular targets, providing insights into the distinct molecular hallmarks of neural activity.
In conclusion, brain cell activity imaging has progressed significantly with the development of advanced imaging techniques. These methods provide a clearer understanding of how electrical activity is moderated in the brain, offering valuable insights into neuron behaviour and overall brain function.
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Intracranial electrodes
There are several types of intracranial electrodes, including subdural electrodes, depth electrodes, and laminar electrodes. Subdural electrodes are placed directly on the surface of the brain and cover a large area of the cortex, including regions important for cognitive functions such as language, sensory processing, attention, and memory consolidation. Depth electrodes are placed at precise locations in the brain that are potential sites of seizure origin, such as the amygdala and hippocampus. Laminar electrodes, on the other hand, allow for simultaneous recording of neuronal activity in different cortical layers, and are useful for investigating the laminar profile of brain events.
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Calcium imaging
One advantage of calcium imaging is that it provides optical measurements of brain activity, which are non-invasive and allow for simultaneous monitoring of activity from many individual neurons or different brain regions. This is in contrast to traditional methods of measuring electrical activity in the brain, such as inserting electrodes into the brain, which are labor-intensive and typically only allow for recording from one neuron at a time.
Overall, calcium imaging is a valuable technique for studying neural activity and understanding brain function, but it also has some limitations and challenges that need to be considered.
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Frequently asked questions
Brain cells function using electrical impulses, which allow the brain to coordinate behaviour, sensation, thoughts, and emotion.
Electroencephalography (EEG) is a common method to record the electrical activity of the brain. It involves placing electrodes on the scalp to detect electrical charges that result from brain cell activity.
Traditional methods of measuring electrical activity in the brain, such as EEG, can only record the activity of one neuron at a time and are labour-intensive. Other methods, such as MRI and CT scans, do not provide direct measures of electrical activity.
Electrical brain stimulation, such as transcranial direct current stimulation (tDCS), can be used to temporarily alter brain activity without surgery. This can be beneficial for people with brain damage or disorders, as well as for improving emotion regulation, attention, and memory in healthy individuals.
