A brief introduction to EEG and the types of electrodes

A brief introduction to EEG and the types of electrodes

Electroencefalography (EEG) is a method to record brain activity throw the capture of electric activation. This neurophysiological measurement can be acquired by non-invasive scalp electrodes. The measurement is the summation of post-synaptic neuron potentials within a large area (1 to 6 cm2) of the cortex.

EEG also has a high temporal resolution when compared with techniques as fMRI or PET. It can reach the millisecond precision and the method can be used to study spontaneous brain activity (when there is no specific movement or task) and task-related brain activity (as evoked potentials, that occur after a task or stimulus).

In this post we will explain about different types of EEG electrodes: passive, active, dry and sponge (R-NET).

Passive electrodes

Passive electrodes are usually made of silver/silver chloride (Ag–AgCl) and many systems typically use electrodes attached to individual wires. These electrodes are applied to the scalp using a conductive gel or paste, usually after preparing the scalp area by light abrasion to reduce electrode-scalp impedance.

Personal opinion: Passive electrodes are the electrodes that I take more time/work to get a good impedance. Sometimes I am not able to get the wished impedance for some electrodes.

Example of passive electrodes:  

BrainCap – with EEG hardware BrainVision BrainAmp Standard (plus) or BrainVision MOVE

BrainCap MR – with EEG hardware BrainVision BrainAmp MR (plus)

LiveCap – with EEG hardware BrainVision LiveAmp

Active electrodes

The actiCAP active electrode system makes it easier to reduce the impedance of the electrodes. This system also substantially reduces the time taken to prepare test subjects up to the start of the EEG recording.

The sensors consist of high-quality Ag/AgCl (sinter) and are perfectly suited for DC acquisition. "Active" circuits permit recordings at high transition resistances (up to 500 kOhm) and minimize ambient noise, interference due to electrical effects and artifacts due to cable movement thanks to built-in active shielding.

This technology significantly improves the signal to noise ratio even without abrasive impedance minimization and additional cleaning of the skin using alcohol or cleaning products.

The different color LEDs which are integrated in the electrode housing indicate the quality of the current electrode transition resistance. Threshold values (red, yellow, green) and additional functions can be programmed using the supplied actiCAP Control Software and displayed on a computer screen. These EEG types is highly indicated to Event Related Potential (ERP) Experiments.

Examples of actives electrodes:

ActiCAP slim and ActiCAP snap – with the hardware ActiCHamp plus, LiveAmp (all), BrainAmp Standard, BrainAmp DC and V-Amp.

For 2 more types of actives electrodes see the next type of electrodes:

Active Dry electrodes

Preparing electrodes and the electrode cap is usually a time-consuming procedure when performing EEG-measurements. Also an off-putting drawback for many test-subjects is the fact that gel remains in the hair and has to be washed off after the measurement. 

The dry electrodes and cap of actiCAP Xpress and actiCAP Xpress Twist were tailored for ease of application and optimum contact between the electrode and scalp in order to remove the necessity of electrode gel. With the flexible cap and the mushroom-shaped electrodes it is easy to establish good electrode contacts.

The exceptional feature, however, is the adjustable electrode length, that allows adapting the cap to any head geometry - even while a test subject is wearing the cap.

Examples of actives electrodes:

ActiCAP Xpress – with the hardware V-Amp.

ActiCAP Xpress Twist – with the hardware ActiCHamp Plus, LiveAmp (All) and BrainAmp DC.

Sponge based - R-NET

The R-Net electrode system consists of sponges and passive Ag/AgCl electrodes that are held in place with a durable and flexible net. Due to the gel-free application it allows for rapid preparation and high recording flexibility, which is advantageous e.g. in time-restricted EEG recording situations. Providing an optimized solution for such applications, the new R-Net complements our other active and passive, gel based and dry electrode systems.

The R-Net electrode system is available in 32, 64, 96 and 128 channel configurations (maximum number depending on the used amplifier) and in three different adult sizes (small, medium, large).

R-Net electrode system can be used with the hardwares ActiCHAmp plus, LiveAmp and BrainAmp.

Wet, dry, active and passive electrodes. What are they, and what to choose?

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References:

[Postage] Brain Products / EEG [accessed Sep 11 2018].

[PDF] A Novel Dry Active Electrode for EEG Recording. [accessed Sep 11 2018].

 How to select a suitable EEG recording Cap (English) [Brain Products Manual].

 ActiCAP Operating Instructions (English) [Brain Products Manual].

 ActiCAP Xpress Twist Operating Instructions (English) [Brain Products Manual].

 R-Net Operating Instructions (English) [Brain Products Manual].

You can find others blogs about EEG clicking here

How to select your electrode by clicking here

And how to select electrode caps here