Last update: September 16, 2022
By BrainMatters

Electroencephalography (EEG) is a method that makes it possible to record, from the scalp, whether electrical potential differences exist in the brain. Electrodes are placed on the head when an EEG is made. These electrodes are usually attached to a cap so that they are at a fixed distance from each other, which makes it easier to compare signals. To ensure that the potential differences are passed on properly, a conductive gel is applied to the head. No conclusions can be drawn from a raw EEG signal, as there is a lot of noise in the signal. In order to filter out this noise, ERPs are used. 

The basis of an EEG signal is a long story. There are two different forms of electrical activity in the brain: action potentials and postsynaptic potentials. An action potential travels from the cell body at the beginning of the axon to the synaptic cleft at the end of the axon. At the end of the axon, neurotransmitters are released into the synaptic cleft.

Neurotransmitters bind to receptors on the dendrites of the postsynaptic cell. These bindings cause the ion channels of these cells to open or close. Ions with a certain electrical potential are admitted through ion channels. This causes the electrical charge of the entire dendrite to rise or fall when the supply of certain ions starts or stops.

Electrodes on the skull are not able to measure action potentials. This is due to the timing of the potentials and the location of the axons. When an action potential is generated, an electric current flows along one point on the axon. The next moment this current disappears at this point of the axon and enters at the next location on the axon. This happens until the synaptic cleft is reached. If two action potentials would fire at exactly the same moment, and would reach the synaptic cleft at exactly the same moment, then (only then) the potentials will be added up. So there would be twice as much voltage measured. However, when the action potentials occur just after each other, one axon has finished firing while the other is still firing. This results in the stopped axon inhibiting the signal from the other axon. The voltage would then be too small to be measured. Neurons rarely if ever fire exactly at the same time, and EEG & ERP can therefore only be used to measure postsynaptic potentials.

A postsynaptic potential lasts up to hundred(s) of milliseconds (an action potential lasts only 1 millisecond). These potentials fire constantly and only reach the dendrites and the cell body. In contrast to an action potential, a postsynaptic potential has no fixed firing ratio.

When an excitatory neurotransmitter is released by the presynaptic cell, a positive electricity flows into the postsynaptic cell. The electrical voltage thus dissipates from the synaptic cleft in the dendrites/cell body. This makes the cell positively charged and the area around the dendrites (the synaptic cleft) negative. The electrical charge must also leave the cell to return to equilibrium. This happens at the cell body, creating a positive charge again just outside the cell body. This creates a temporary dipole (negative just outside the dendrites and positive just outside the cell body).

The dipole of one cell can hardly be measured, but under some circumstances it is possible that the different dipoles sum up. These conditions are as follows:

  • The dipoles must exist simultaneously (this probability is greater than for action potentials because these PS potentials are longer)
  • Thousands or millions of neurons must be involved
  • The neurons must have the same orientation
  • The neurons must have the same input (excitatory, inhibitory)

Author: Myrthe Princen (translated by Thomas von Rein)

Image: Marcel Loeffen

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