Registration of evoked brain potentials

Registration of evoked potentials is one of the areas of quantitative electroencephalography. Evoked potentials are short-term changes in the electrical activity of the brain that occur in response to sensory stimulation. The amplitude of individual evoked potentials is so small that they are practically not distinguished from the background EEG. For this reason, to identify them, the method of averaging (coherent accumulation with synchronization from the moment of stimulus delivery) of brain responses to a large number (from tens to hundreds) of stimuli is used with the help of specialized laboratory electronic computers.

Types of evoked potentials depending on the nature of sensory stimuli:

• visual [a flash of light or the inclusion of a formed visual image (for example, a “chessboard” - a field filled with dark and light squares, alternating in a checkerboard pattern, the color of which rhythmically changes to the opposite with a frequency of 1 Hz)];
• auditory and "brainstem" (audible click); somatosensory (electrical stimulation of the skin or transcutaneous stimulation of the nerves of the extremities).

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-latency (with a peak latency of more than 250 ms) waves that occur in the EEG in an experimental situation where the subject is given two types of stimuli. Some (which, according to the instructions, should not be paid attention to) are given frequently, while others ("target" ones, which require either counting or pressing a button in response) are given much less frequently.

The third positive component with a peak latency of approximately 300 ms (P3, or P300), which occurs in response to the presentation of the "target" stimulus, has been studied best. Thus, the P300 wave is an electrophysiological "reflection" of selective attention. Based on the data of cortical topography (maximum amplitude in the central parietal leads) and the localization of the intracerebral dipole, it is formed at the level of the basal ganglia and/or hippocampus.

The amplitude of the P300 wave decreases, and its peak latency increases in normal and pathological aging, as well as in many mental disorders (schizophrenia, dementia, depression, abstinence) associated with attention disorders. Usually, a more sensitive indicator of the functional state is the value of peak latency. With successful therapy, the parameters of the P300 wave can return to normal.

In addition to P300, several other types of long-latency (with a peak latency of 500-1000 ms) components of cognitive evoked potentials have been described. For example, the E-wave (the "expectation" wave, or conditioned negative deviation) occurs in the interval between the warning and trigger stimuli. It is also associated with the cortical processes of selective attention. Premotor potentials occur before the onset of the subject's motor reaction. They are recorded over the motor zones of the cerebral cortex. They apparently reflect the processes of organizing motor commands.

Relatively recently, clinical and biological studies of mental disorders have begun to study evoked potentials, as well as changes in the background EEG (primarily event-related EEG desynchronization) that arise in response to the presentation of complex stimuli, including emotionally charged ones (images of faces with different emotional expressions, pleasant and unpleasant smells). Such studies allow us to come closer to understanding the neurophysiological mechanisms of disturbances in emotional perception and response in a number of mental disorders.

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Interpretation of results

The averaged evoked potential is a polyphasic wave complex, the individual components of which have certain amplitude ratios and peak latency values. For most evoked potentials, the intracerebral localization of the generators of each component is known. The shortest-latency (up to 50 ms) components are generated at the level of receptors and brainstem nuclei, and medium-latency (50-150 ms) and long-latency (more than 200 ms) waves are generated at the level of cortical projections of the analyzer.

Short-latency and medium-latency sensory evoked potentials have limited application in the clinic of mental disorders due to the nosologically non-specific nature of their changes. They allow for objective sensometric assessment (for example, to differentiate the consequences of organic damage to the peripheral parts of the corresponding sensory system from hysterical visual and auditory disorders ) based on changes in the amplitude or latency of individual components.