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Electrode locations and names are specified by the international 10-20 system (19 recording + 1 reference). An EEG voltage signal is the result of the difference in voltage between two electrodes.
Electrode location
Electrode location
There is always a reference and a ground electrode.
reference electrode: placed on the head arbitrarily. For example, it is analogous to “elevation” on a map: the reference level would be the sea, and the heights of each mountain depend on the sea level, for this reason one can choose where the reference level will be, as the delta change will always be the same.
ground electrode:t typically placed on a neutral site, such as a bone, where no electrical current flows. A couple of electrodes are also put near the eyes and chin, this is to measure the eye movements and heart rate and then be able to identify and remove this noise when analysing the data.
To keep the same universal system, odd numbers correspond to the left side of the brain, while even numbers correspond to the right side.
Galvanometers
Galvanometers
The electrodes attached to a subjects scalp connect to Galvanometers. The latter are instruments that detect and measure small electrical currents via a coil of wire that deflects a needle proportional to the current flowing through the coil.
D'arsonval galvanometer
D'arsonval galvanometer
The most commonly used type is the D’Arsonval galvanometer. Between the poles of a permanent magnet, a coil of wire is suspended from a metallic ribbon. Then a voltage is applied, which pushes electrons through the circuit, creating a current that flows through the coil.
According to Ampère’s law: a flowing current generates its own magnetic field and this field reacts with the magnetic field of the permanent magnet. This produces a torque and the coil rotates following this twisting force.
A needle (pointer in the image) is attached, this needle rotates at a certain angle to try and balance the torsion, if one measures this angle you will know the amount of current flowing through the coil. Instead of a needle, a mirror can also be attached, here the angle of torsion is measured by the deflection of a beam of light reflected from the mirror.
The Galvanometer is designed so that the deflection angle is proportional to the total charge passing through its moving coil or to a short duration voltage pulse. According to Ohm’s law, V = I*R the current (I) flowing through the coil is proportional to the voltage (V) applied, and inversely proportional to the resistance.
Electrode materials
Electrode materials
With scalp electrodes, the amplitude recorded in a normal subject in the awake state is from 10 to 100 microV (for comparison: in epilepsy amplitudes increase to 500-1500 microV).
There are different electrode types that can be used:
Metal Disc Electrodes:
Metal disc electrodes are placed at specific points on the scalp, corresponding to areas where brain signals can be best recorded. To avoid interference from head movements and to ensure stable signal recording, a conductive jelly or paste is applied between the electrode and the skin. This also prevents direct contact with the skin, minimizing the risk of signal artifacts.
Chlorided Silver Disc Electrodes:
These electrodes are made of silver coated with a layer of silver chloride. They make contact with the scalp through an electrolytic gel and a soft washer, which helps minimize motion artifacts, sweat interference, and high-pressure artifacts. This design ensures a more stable connection and precise signal recording.Dry Electrodes:
Dry electrodes are composed of layers of thick silicone rubber discs with both conductive and insulating properties. They include a metallic guard shield to prevent signal leakage. Unlike the other types, dry electrodes have a much higher contact impedance, often in the mega-ohm range, compared to the kilo-ohm range for metal and chlorided silver electrodes. This high impedance can expand their use in applications requiring minimal skin preparation.
ECoG
ECoG
If the signals are measured with electrodes directly on the cortex, it is called electrocorticogram (ECoG). This has of course the highest spatial and temporal resolution but is extremely invasive and only used during surgery.
Sensor types in Brain imaging techniques
Sensor types in Brain imaging techniques
references images:
sensor types: Peksa Janis and Dmytro Mamchur, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons
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