What are the techniques to record brain activity?
In recent decades, brain activity investigation techniques have multiplied. The parameters on which the measures are based refer to the Spatio-temporal resolution, the degree of invasiveness, and the type of information (response) that can be correlational or causative.
We list the various techniques which we will then briefly describe:
1. EEG - Electroencephalography - Standard. NON-invasive electrophysiological technique.
2. ERP (Event-Related Potential). NON-invasive electrophysiological technique.
3. Stimulation and direct recording of the cerebral parenchyma. Invasive electrophysiological technique.
4. TMS - Transcranial Magnetic Stimulation. NON-invasive magnetophysiological technique.
5. MEG - Magnetoencephalography. Magnetophysiological technique Non-invasive.
6. PET - (Positron Emission Tomography). Invasive morphological bioimaging technique
7. Static MRI (Magnetic Resonance Imaging). NON-invasive morphological bioimaging technique
8. CT - axial computed tomography. NON-invasive morphological bioimaging technique.
9. Functional fRMI - Functional Magnetic Resonance. NON-invasive morphological bioimaging technique.
10. OT - Optical Topography. NON-invasive morphological bioimaging technique.
NON Invasive electrophysiological techniques
1) The electrical activity of subcortical neurons is recorded with the EEG technique.
Through a variable number of electrodes (from 10 to 20), positioned on a headset that the subject wears, the signals (waves) are recorded in various situations (wakefulness, sleep, eyes closed), which each area of the underlying tissue emits. The instrument (which can be analog or digital) provides a continuous "tracing" (often on graph paper) of the generated waves, which testifies to brain activity. Since the signal is very weak, it is greatly amplified.
It is a NON-invasive but practical method. The application of this technique concerns the analysis of epilepsy, studies on sleep, and the assessment of brain death.
2) Then there is the ERP (Event-Related Potential) investigation technique.
This technique represents a variant of the EEG, the recording of the electrophysiological response of the subcortical neurons occurs at the presentation of stimuli.
In fact, EEG is very useful for some diseases, but alone it does not have great importance in the neuro-cognitive sciences. For these purposes, in fact, the technique that derives from the standard EEG, or ERP, is used.
It consists of an EEG during which the signals (waves) that each area of the underlying tissue emits in different situations created by visual or auditory stimuli are recorded.
For example, two peaks were identified, N400 (at 400msec from the stimulus and P600 (at 600msec from the stimulus), relating respectively to the presentations of semantic violation stimuli.
It is the observation of an effect (electrophysiological response) related to an event.
So it is a technique with very high temporal resolution and is applied to neurolinguistic research.
Invasive electrophysiological techniques
3) Another system of investigation is the stimulation and direct recording of the cerebral parenchyma.
This technique stimulates the brain area (parenchyma) electrically and directly through electrodes. It is extremely invasive in vivo technique that requires the opening of the skull, after local anesthesia of the scalp and soft parts. In the uncovered part of the brain, a grid is placed, a sheet with small disks, which will serve to conduct the electrical stimulus.
The STIMULATION technique consists of TEMPORARILY inhibiting the functioning of a group of nerve cells, while the REGISTRATION consists of the observation of behavior that the electrical stimulus produces. While the subject speaks or is about to respond to a stimulus, the small electrical discharge can block the language or inhibit the understanding of a word, etc. In the second, the subject moves a hand, a foot, etc.
It is a technique with very high spatial resolution, but obviously, it is used in very rare cases.
Before proceeding with the removal of a tumor or an outbreak of epilepsy that cannot be treated with drugs, the neurosurgeon must carefully choose the tissue to be removed.
Non-invasive magnetophysiological techniques
4) TMS - Transcranial Magnetic Stimulation
Take advantage of Faraday's law that an electric current in a stimulator produces a magnetic field, and this induces a flow of current in nearby conductors, including human tissues.
It is adopted for the correlational study of areas of the brain with certain behaviors. Through a coil positioned on the skull, magnetic fields and perpendicular electric currents can be generated, which will stimulate the nerve cells. Stimulation is capable of causing temporary "virtual lesions," that is, as in the case of direct stimulation of the parenchyma, but without such an invasive practice.
It is not an invasive technique, but you should not exceed the stimulation as it could also cause epileptic seizures. Therefore you must scrupulously follow the guidelines. It does not have a good spatial resolution, and the impulse, propagating at a depth of only 20 mm, does not make it a suitable instrument for controlling subcortical areas.
In any case, this technique allows both correlational studies and neurolinguistic studies.
5) MEG - Magnetoencephalography.
It is an appliance that must be housed in highly shielded cabins. One might think of it as logically related to the EEG technique. But when a neuron generates an action potential, therefore an electrical stimulus, it also generates by induction a magnetic field. Therefore the machine exploits this principle but poses two types of different problems. The magnetic field generated by neurons is very weak, and it is necessary to isolate it from other much more intense magnetic fields such as the earth. So, first of all, SQIDs were created, single semiconductor modules immersed in low-temperature helium for amplification. Furthermore, the machine is positioned in a highly shielded cabin, and clearly, no metal must be present.
It is possible to associate MEG with MRI morphological magnetic resonance to obtain a map of the brain and the areas that are activated, with an optimal temporal resolution. The technique gives the possibility to see the various consequential activations over time, as well as which areas and in what sequence they are activated.
It appears that MEG can affect eye movements and heart rhythm due to strong amplification systems. For this reason, use must be limited in time.
This technique is used both in correlational studies and in neurolinguistic studies. It has been used for some time, also as a diagnostic tool for epilepsy and in the analysis of auditory disorders.
Invasive morphological bioimaging techniques
6) PET - Positron Emission Tomography.
It is used to obtain functional maps of the brain and body. In this case, an unstable short-lived half-tracer is required. It can be a radioactive isotope (an unstable atom with a different number of neutrons) of oxygen (O 15) injected into a vein in the form of water. The subject is, therefore, lying inside the tomograph and subjected to a task (visual or auditory).
The isotope after 30-60 seconds emits a positron, returning to the stable form H 2 -O 16. Each emitted positron reacts with an electron (annihilates) and generates two photons that go in opposite directions, which will be detected by the sensor ring of the instrument. The latter reconstructs a three-dimensional map of the brain (or other organs) that will be affected by the amount of radiation detected at that point.
It is considering that in the brain areas activated by the task given to the subject, there will be a greater blood supply. It is an invasive technique since it uses a radioactive element. Although it is particularly useful and with very high temporal resolutions (30-60 sec, depending on the isotope used) and spatial (very precise three-dimensional functional reconstruction), it is hardly used anymore. It is useful in the identification of tumors, lesions, and differential diagnosis of dementias.
NON-invasive morphological bioimaging techniques
7) Static MRI (Magnetic Resonance Imaging - Resonance Magnetic Static).
This technique is also called magnetic resonance tomography; it is a morphological bioimaging technique, based on magnetic resonance imaging.
The operating principle is based on subjecting the patient to a strong static magnetic field. In fact, in conditions of absence of a magnetic field, the protons have a random orientation. Instead of exposed to a magnetic field, the particles take the same direction, according to the axis of the field. If suddenly, the magnetic field is interrupted, the particles tend to summarize their random orientation and EMIT RADIO SIGNALS.
There is a part of the instrument that induces the magnetic field and a part capable of detecting the signals emitted by the protons during their random readjustment.
The technique allows you to translate the signal into gray or colored scales and identify the areas that emit it.
We have a spatial and temporal resolution (research is further improving this type of machine). It is an excellent tool for diagnosing tumors, strokes, and other injuries.
8) CT - Computerized Axial Tomography.
It is based on the different absorption of X-rays by various structures of the brain and, in particular, the differences between areas with lesions and the healthy brain are highlighted.
It allows reproducing the patient's body sections (tomography) and three-dimensional elaborations. For the production of the images, it is necessary the intervention of a data processor (computerized). Since the images produced are digital, the studied body is divided into a discrete series of volume elements (voxel), to which corresponds a unique image element (pixel), following the grayscale. The smaller the volume represented by a single pixel, the higher the spatial resolution. The adjective "axial" is currently inappropriate because the new methods no longer acquire in an axial, that is a transversal plane, which allows producing one image at a time, but a spiral technique is adopted to obtain more images in one scan. However, being an X-ray exposure, this technique cannot be used for close or prolonged periods. Its application involves the analysis of strokes, hemorrhages, and tumors.
9) fRMI - Functional Magnetic Resonance
It is always MRI but uses a SUBTRACTIVE method between a rest condition and an activation condition.It is based on the fact that an area that works more has higher blood flow and consumes more oxygen. In fact, hemoglobin exists in two forms, i.e., reduced or oxygenated. Under normal conditions at rest, without stimulation excitations, we have a normal level of reduced hemoglobin, a basal flow, therefore, a normal MRI signal. When the flow increases, the reduced hemoglobin decreases in favor of the oxygenated hemoglobin. The CVB flow (brain blood volume) increases and the MRI signal changes (increases). The paramagnetic properties of being Oxygenated and reduced are different, and you will have a different BOLD (Blood Oxygen Level Dependent Contrast) signal. This defines the ACTIVATED areas of the brain at a given moment. The scan is done in "slices" and reconstructed in a three-dimensional model.
The pixel is considered because it is this graphic volume, the three-dimensional area that will give the representation of the change in state. In short, simple stimuli are adopted since we must have a comparison with a simple task to differentiate it from the experimental task. Compared to PET, it has a better temporal resolution (in the order of 1 second), an excellent spatial resolution (less than a millimeter). If the pacemakers, teeth, and fixed metal prostheses are excluded, it is totally harmless. In recent years there are "open" machines that allow the examination also to claustrophobic patients. For this reason, fRMI is the most used tool at the moment for cognitive research. It is used both in diagnostics and in cognitive studies.
10) OT - OPTICAL TOPOGRAPHY
It provides information on oxygen consumption in brain tissues.
It is based on the different transparency of the fabrics to light in areas of the spectrum close to infrared. This type of light penetrates for a few millimeters into the tissues of the body. In infants, the tissues are particularly transparent in this light, which also passes through the bones of the skull and illuminates the cortex. By analyzing the wavelength of the light sent and that recovered from the tissues, it is possible to measure the oxygen consumption in the "illuminated" structure. Since there is no need to stand still (you wear a kind of helmet), this technique can be used for long periods of time and especially with very young children.