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Registration of evoked potentials of the brain
Last reviewed: 23.04.2024
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Registration of evoked potentials is one of the areas of quantitative electroencephalography. The evoked potentials are short-term changes in the electrical activity of the brain that arise in response to sensory stimulation. The amplitude of single evoked potentials is so small that they are practically not isolated from the background EEG. For this reason, the method of averaging (coherent accumulation with synchronization from the moment of stimulus supply) of brain responses to a large number (from tens to hundreds) of stimuli using specialized laboratory electronic computers is used for their detection.
Types of evoked potentials depending on the nature of sensory stimuli:
- visual [flash of light or the inclusion of a formed visual image (for example, "chessboard" - a field filled with dark and light squares alternating in staggered order, the color of which changes rhythmically to the opposite one at a frequency of 1 Hz)];
- auditory and "stem" (sound click); somatosensory (electrostimulation of the skin or percutaneous stimulation of the nerves of the limbs).
Cognitive evoked potentials
More widely used is the recording of cognitive evoked potentials (endogenous evoked potentials, or "evoked potentials associated with an event"). Cognitive evoked potentials are long-latent (with a peak latency of more than 250 ms) that appear in the EEG in such an experimental situation when two types of stimuli are given to the subject. Some (which should not be looked at according to the instructions) are often served, others ("target" ones, which require either to be counted or to respond to them by pressing a button) are much rarer.
The third positive component with a peak latency of about 300 ms (P3, or P300), which occurs in response to the presentation of a "target" stimulus, has been most studied. Thus, the RZOO wave is an electrophysiological "reflection" of selective attention. Based on the data of cortical topography (maximum amplitude in the central parietal leads) and localization of the intracerebral dipole, it is formed at the level of the basal ganglia and / or the hippocampus.
The amplitude of the P300 wave decreases, and its peak latency increases with normal and pathological aging, as well as many mental disorders (schizophrenia, dementia, depression, withdrawal) associated with attention impairment. Usually a more sensitive indicator of the functional state is the magnitude of the peak latency. With successful therapy, the parameters of the P300 wave can return to normal.
In addition to P300, several types of long-latency (with a peak latency of 500-1000 msec) components of cognitive evoked potentials are described. For example, the E-wave (a wave of "waiting", or a conditional negative deviation) arises in the gap between the warning and trigger stimuli. It is also associated with cortical processes of selective attention. Premotor potentials occur before the onset of the motor reaction of the subject. They are recorded over the motor zones of the cerebral cortex. They, apparently, reflect the processes of organizing motor teams.
More recently, in the clinical and biological studies of psychiatric disorders, the evoked potentials, as well as the changes in the background EEG (primarily associated with the EEG desynchronization event) that began in response to the presentation of complex, including emotionally-colored stimuli (images of persons with different emotional expressions , pleasant and unpleasant odors). Such researches allow to come nearer to understanding of neurophysiological mechanisms of disturbances of emotional perception and reaction at a number of mental disorders.
[Interpretation of results
The averaged evoked potential is a polyphase wave complex, the individual components of which have certain amplitude ratios and peak latency values. For the majority of evoked potentials, the intracerebral location of the generators of each of the components is known. The most short-latent (up to 50 ms) components are generated at the level of receptors and stem nuclei, while medium-latent (50-150 ms) and long-latent (more than 200 ms) waves - at the level of the cortical projections of the analyzer.
Short-latency and mid-latent sensory evoked potentials have limited application in the clinic of mental disorders due to the nosologically nonspecific character of their changes. They allow to carry out objective sensorometry (for example, to distinguish the consequences of organic damage of the peripheral parts of the corresponding sensory system from hysterical disturbances of sight and hearing) by changes in the amplitude or latency of individual components.