A common problem in measurement setups where the subject is allowed to move around is the movement artefact. Movement artifacts originate from two different phenomena:
• Movement of the cables
• Movement of the electrode
The movement artifacts from the cables can be solved by using active shielding. The origin of the movement artefact is caused by the changing capacitive coupling because the cables are moving around in an electrical field. With active shielding, this capacitive coupling is eliminated and the cable can move around freely in space.
Movement artifacts from the electrode are very often measured. They are caused by changes in pressure, which is exerted on the gel layer of the electrode. That change in pressure (caused by pulling on the cable or pushing on the electrode for example) modifies the double layer between the tissue and the metal (often with gel in between) of the electrode itself. This double layer is responsible for the offset of the electrode, so the offset of the electrode is modified by any change in pressure that is exerted on the surface of the interface. Factors that influence the amount of electrode movement artifact are: weight and size. The smaller and lighter the electrode, the smaller the artefact. Because TMSi uses passive electrodes we can design extremely small and lightweight electrodes, reducing the electrode movement artefact considerably.
A good example of this is a special type of electrode, the so-called micro electrodes. When an electrode is used with very small amount of gel and a very small surface for the measurement, the movement artifact is reduced considerably. The micro electrodes have much higher impedance than the standard ECG electrodes, so using this type of electrode a specialized TMSi amplifier system with a very high input impedance is required. The cable pulling on the electrode can also easily be avoided by applying a small ‘’strain relief’’ (sticker or band) near the electrode.