Electroencephalography

Physiological basis of the method

The spread of neural excitation is accompanied by a constant change in electrical potentials across the cell membranes of a vast number of neurons. As a result, a spatially inhomogeneous and temporally continuous electrical field is formed in the brain tissues.

EEG equipment records the potential difference. Recording can be performed in two main ways:

• Between two different points on the head surface (above the brain).
• Between a specific area above the brain and electrically neutral tissues remote from it (e.g., the earlobe).

Regulatory systems of the brain

The level of functional activity of the cerebral cortex is not constant. It is strictly controlled by special structures located in the brainstem and the anterior parts of the limbic system. These regulatory centers are traditionally divided into two antagonistic groups:

1. Ascending activating systems. Located predominantly at the level of the midbrain reticular formation, they also include the preoptic nuclei of the forebrain. Their main task is the activation of reticulocortical and limbicocortical pathways. Excitation of these systems leads to a pronounced increase in the level of functional activity of the brain, which manifests on the EEG as desynchronization (the rhythm becomes high-frequency, low-amplitude, and irregular).

1. Inhibiting (somnogenic) systems. Their centers are located in the medulla oblongata, the lower parts of the pons, and the nonspecific nuclei of the thalamus. Due to the activation of somnogenic reticulocortical and thalamocortical systems, the level of wakefulness decreases down to immersion in sleep. On the EEG, this is reflected as synchronization: neurons combine into large populations with synchronized activity, and the rhythm becomes slow, regular, and high-amplitude.

Main rhythms of the electroencephalogram

Electrical activity corresponding to a specific functional state of the brain is called a rhythm. The classification of rhythms is based on assessing their frequency (number of oscillations per second) and amplitude (peak-to-peak wave span).

For a healthy adult in a state of wakefulness, 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 regions of the cortex. The main condition for its appearance is a state of calm, relaxed wakefulness with closed eyes.
• Beta (β) rhythm. Distinguished by a higher frequency (14–40 Hz) and lower amplitude (up to 15 µV). It occurs when the functional activity of the cortex increases. 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 the delta (δ) rhythm (0.5–3 Hz). Both rhythms are distinguished by high amplitude, which can reach from 40 to 300 µV. In physiological norm, these waves dominate in deep sleep and are also frequently recorded on the EEG in children and adolescents. In a healthy adult, they may be present only in insignificant quantities. However, if the proportion and amplitude of θ and δ rhythms in a waking adult increase, this is a clear sign of pathological processes developing in the nervous tissue.

Recording technique and lead methods

In routine clinical practice, electrodes for EEG recording are placed on the patient's intact scalp skin, as well as on extracranial points. Two main methods of potential derivation are used:

1. Monopolar lead. Two electrodes are used. The active electrode is placed directly above 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.
2. Bipolar lead. This method records the potential difference exclusively between two active electrodes, both of which are installed on the head surface over brain tissues.

Briefly: answers to topical questions

What else to study on this topic

• Compressed spectral EEG analysis
• Long-term monitoring of cortical electrical activity
• Toposelective brain mapping
• Construction of rhythm amplitude and power distribution maps

Other topics in the "General Physiology of the Central Nervous System" section

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