What is the role of cortical signals in sleep stage scoring?

Updated: Aug 19, 2019
  • Author: Andres A Gonzalez, MD, MMM, FACNS; Chief Editor: Selim R Benbadis, MD  more...
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The EEG signal is of primary importance in interpreting polysomnographic studies. [7, 8, 9] It records electric potentials generated by the interaction between the cortex and the deeper brain structures, especially the thalamus. Two frontocortical (F3, F4), two centrocephalic (C3, C4), and two occipital (O1, O2) channels are recorded in accordance with the international 10/20 placement system. The left channels are ascribed odd numbers and the right channels are ascribed even numbers. Measurement of EEG signals is possible because of the relative difference in potential between two recording electrodes. Negative potentials, by convention, are represented by an upwardly deflecting wave. [10, 11]

These EEG sites reference all left and right electrodes to the contralateral mastoid process lead (M2 or M1). For example, C3 is referenced to M2. Common practice is to read from the right hemispheric channels, however, when a channel develops artifact or the validity of the signal is unclear comparison is made to the corresponding contralateral channel. For instance, if C4-M1 develops artifact, C3-M2 can be used for channel interpretation.

EEG tracings are classified by their frequency with Delta waves having the slowest activity at < 4 Hz, Theta waves ranging from 4-7 Hz, Alpha waves more rapid at 8-12 Hz, and Beta waves exceeding 12 Hz. Electrode placement is specific to capture specific wave types with slow waves best represented in frontal leads, spindles seen more clearly centrally, and Alpha rhythms more pronounced in the occipital region.

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