Comparision: active versus passive electrodes

Comparison of EOG/EMG-artifacts, cable and head movements

g.GAMMAsys and g.EEGcap are high-performance active and passive electrode systems for non-invasive electrophysiological derivations such as EEG, EMG, ECG, EOG and other signals. Active electrodes have an additional ultra-low noise pre-amplifier located inside the electrode. The system is designed for use with biosignal amplifiers, like g.BSamp, g.USBamp and with g.MOBIlab+. Electrodes are made of sintered Ag/AgCl and work in a frequency range of 0 - 100 kHz. The active electrodes can be used with all g.tec EEG caps as an alternative to the passive standard electrodes.

The system avoids or reduces artifacts and signal noise resulting from high impedance between the electrode(s) and the skin (e.g. 50/60 Hz coupling, artifacts caused by electrode or cable movements, distorted signals or background noise). No modifications are required for the biosignal amplifier which can be used with or without the active electrode system.

Electrode Mounting

The compare active and passive electrodes active electrodes were mounted on positions F1 (channel 1), C1 (channel 2), O1 (channel 3) with g.GAMMAgel (no abrasion) and passive electrodes were mounted on positions F2 (channel 4), C2 (channel 5) and O2 (channel 6) with abrasive gel. Active and passive electrodes are located next to each other to allow a better comparison. The ground electrode was located on position Fpz. The active electrodes were referenced against the right ear. The passive electrodes are referenced against the left ear. Five conditions were compared: (i) eye movements, (ii) biting, (iii) cable artefacts, (iv) active head movements by the person himself and (v) passive head movements done by a second person.

Eye Movement

Channels closer to the eyes (1 and 4) show higher EOG artefacts than central and occipital channels.

Both passive and active electrodes show a similar EOG contamination which is also clear because both pick up the same source signal.

Biting

Biting produces EMG contamination almost equally on all channels.

No difference between active and passive electrodes because both pick up the same source signal.

Cable Artefacts

Cable artefacts are produced by touching or shaking the cables.

Active electrodes are almost unaffected while the passive electrodes show large movement artefacts.

Active Head Movements

Active head movements produce fewer artefacts with active electrodes compared to passive ones. Artefacts for both electrodes can occur because of skin-electrode movements.

Passive electrodes are mostly affected by the cable movements initiated by the head movements.

Passive Head Movements

Passive head movements have lower accelerations than active head movements and therefore the artefacts are smaller and mostly visible with passive electrodes.