Home > Conscious > Chapter 4 > 4.2. The Principles of EEG

 

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Figure 4-5. An EEG recording setup. [Source: Wikipedia]

Electroencephalography (EEG) is a method that records electrical activity of the brain. It is typically noninvasive. In the method, several electrodes are placed along the scalp, measuring voltage fluctuations at various locations (Figure 4-5). Brain activity is associated with ion flow along axons or dendrites. Displacement of these ions causes voltage fluctuations which, after amplification, can be displayed in the EEG. Since the cerebral cortex is near the scalp, the brain activity observed by EEG mainly originates from the cerebral cortex. Furthermore, the axons and dendrites of pyramidal neurons are longer than other types of neurons, and perpendicular to the scalp, their ion flow will cause greater voltage fluctuations on the scalp. Hence, the brain waves observed by EEG originate mainly from the pyramidal neurons in the cortex (Figure 4-6).

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Figure 4-6. The voltage fluctuations caused by pyramidal neurons. Top left is the result of EEG; middle panel shows changes in local field potentials (LFP); the bottom left displays the oscillation of a single neuron. [Source: Adjamian, 2014 ]

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Figure 4-7. The magnetic field generated by neural activity. [Source: Wikipedia]

In addition to voltage fluctuations, the ion flow can also generate magnetic fields (Figure 4-7). Magnetoencephalography (MEG) takes advantage of this principle to identify the exact location of brain abnormality. Superconducting quantum interference device (SQUID) is used to detect the weak magnetic signals (Figure 4-8), but it must be placed in a room isolated from external magnetic fields.

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Figure 4-8. A person undergoing an MEG. [Source: Wikipedia]