Normally, the electroencephalogram (EEG) is recorded from the surface of the head with gel-based electrodes to provide a low electrode-skin impedance. If passive electrodes are used, the skin must be abraded beforehand to reduce the impedance. With active electrodes, which contain an amplifier inside, the electrode gel is injected between the electrode material and the skin. This allows the electrode system to be mounted more quickly.
One of the main advantages of gel-based active electrodes is their robust signal quality, but the main disadvantages are the long mounting time and the need to wash the cap and the user’s hair after the recording. g.tec thus developed a hybrid electrode that works with and without gel in wet or dry mode. The patented g.SAHARA hybrid electrode system consists of an 8 pin electrode made of a special conductive polymer. The pins have sufficient length to reach through the hair to the skin. The conductive polymer and the 8 pins reduce the electrode-skin impedance and give a solid contact to the skin. The electrode has additionally a hole in the middle that allows to insert electrode gel to make a wet recording. This gives more stable recordings and allows to compensate different head shapes and sizes. The electrode itself can be connected with a clip to the active electrode system on top of it.
EEG recordings are performed at frontal, central, parietal and occipital regions of the head, and therefore a mechanical system is required that holds the electrode on the skin with a constant pressure at every possible recording location. EEG electrodes are typically positioned according to the international 10/20 system. g.tec hence developed the 2nd generation of the g.GAMMAcap, with a total of 160 positions according to an extended 10/20 system, to allow a very flexible electrode montage.
The design of the g.SAHARA electrode system allows users to just plug the electrode and the clip into the cap at the desired location.
|The first and only dry electrode system that works for all frontal, central, occipital, temporal and parietal sites|
|The first hybrid active electrode available on the market|
|Allows dry and wet recordings with one electrode|
|The first and only hybrid system successfully tested with all major BCI approaches in group studies, reported in peer-reviewed papers|
|Same cost range as an active EEG electrode with gel – but works with and without gel|
|Captures the whole EEG frequency spectrum from 0.1-40 Hz|
|Mount the cap in under one minute!|
|No more need to wash the hair or cap!|
|Able to pick up frequency spectra for P300, motor imagery and SSVEP based BCIs|
|Amplification||Inside the electrode clip|
|Cables||Required 1.5 kV protection|
|Pin length||7 mm|
|Number of pins||8|
ACTIVE HYBRID ELECTRODES
Using gel sometimes entails disadvantages: long mounting time, skin abrasion (with passive electrodes) and the need to wash the hair and cap after the recording. Therefore, a dry active electrode system was developed and compared to gel based active electrodes. Studies showed that the new dry electrodes can pick up the corresponding frequency ranges of the EEG data for all three BCI approaches, and also showed that subjects’ accuracy with BCI systems was about the same for gel and dry electrodes. Nevertheless, dry electrodes are more sensitive to noise, movements of cables and electrostatic charges. Therefore, carefully mounting the cap and maintaining and a well-controlled environment are important. Nevertheless, sometimes it is necessary to work with wet electrodes to allows subject e.g. to move around during the recording. Wet electrodes make the recording more stable and are easier to do. The new g.SAHARA hybrid technology combines both worlds into one single electrode.