Physiological Foundations of the Method
The propagation of nerve impulses is accompanied by continuous changes in electrical potentials across the cell membranes of vast numbers of neurons. As a result, a spatially inhomogeneous and temporally dynamic electrical field is formed within brain tissues.
EEG equipment records the potential difference. This can be recorded in two primary ways:
- Between two different points on the scalp (overlying the brain).
- Between a specific site overlying the brain and electrically neutral tissues distant from it (e.g., the earlobe).
Brain Regulatory Systems
The level of functional activity of the cerebral cortex is not constant. It is strictly controlled by specialized structures located in the brainstem and the anterior parts of the limbic system. These regulatory centers are traditionally divided into two antagonistic groups:
- Ascending activating systems. Located primarily within the midbrain reticular formation, and also including the preoptic nuclei of the forebrain. Their main task is the activation of reticulocortical and limbicocortical pathways. Excitation of these systems leads to a marked increase in the functional activity of the brain, which manifests on the EEG as desynchronization (the rhythm becomes high-frequency, low-amplitude, and irregular).
- Inhibitory (somnogenic) systems. Their centers are located in the medulla oblongata, the lower pons, and the nonspecific thalamic nuclei. Through the activation of somnogenic reticulocortical and thalamocortical systems, the level of wakefulness decreases down to the induction of sleep. On the EEG, this is reflected as synchronization: neurons unite into large populations with synchronized activity, and the rhythm becomes slow, regular, and high-amplitude.
Main Electroencephalographic Rhythms
Electrical activity corresponding to a specific functional state of the brain is called a rhythm. The classification of rhythms is based on evaluating their frequency (number of oscillations per second) and amplitude (peak-to-peak wave span).
For a healthy awake adult, two rhythms are most characteristic:
- Alpha (α) rhythm. Has a frequency of 8 to 13 Hz and an amplitude of up to 100 µV. This rhythm is best recorded in the occipital cortical regions. The primary condition for its appearance is a state of quiet, relaxed wakefulness with closed eyes.
- Beta (β) rhythm. Features a higher frequency (14–40 Hz) and lower amplitude (up to 15 µV). It arises during increased functional activity of the cortex. A characteristic phenomenon is the desynchronization reaction: if a person opens their eyes or begins solving a mental task, the alpha rhythm is instantly replaced by the beta rhythm.
Slow-wave rhythms These include the theta (θ) rhythm (4–6 Hz) and delta (δ) rhythm (0.5–3 Hz). Both rhythms are characterized by high amplitude, which can range from 40 to 300 µV. In physiological norms, these waves dominate during deep sleep and are also frequently recorded on EEGs in children and adolescents. In a healthy adult, they may be present only in negligible amounts. However, if the proportion and amplitude of θ- and δ-rhythms increase in an awake adult, this is a clear sign of pathological processes in the nervous tissue.
Recording Methodology and Lead Configurations
In routine clinical practice, EEG electrodes are placed on the patient's intact scalp and at extracranial reference points. Two main methods of potential derivation are used:
- Monopolar recording. Two electrodes are used. The active electrode is placed directly over the studied area of the cerebral cortex. The passive (reference) electrode is fixed on an electrically neutral zone—the earlobe, chin, or tip of the nose.
- Bipolar recording. This method records the potential difference exclusively between two active electrodes, both of which are placed on the scalp over brain tissues.