[Eeglablist] acceptable electrode impedance

[Евгений Машеров]

Without claiming to be a complete answer,
The standard for electrode impedance was established quite a long time ago, in the era of vacuum tube amplifiers. The electrode resistance and the amplifier's input impedance form a voltage divider, which attenuates the voltage at the amplifier's input. The relative reduction is equal to the ratio of the electrode impedance to the amplifier's impedance. Modern amplifiers have significantly higher input impedance, and this effect is insignificant.
However, checking the impedance remains advisable. High impedance may indicate a small contact area, and even slight movements can completely break contact, producing harsh artifacts. Another unpleasant effect of high electrode impedance is caused by capacitive coupling between the wires from the electrodes to the amplifier and the electrical wiring or equipment in the room. If the capacitive resistance is close in order of magnitude to the electrode impedance, interference occurs. 50 or 60 Hz power line interference is relatively easy to eliminate and is also undesirable. However, interference from various equipment can occur in the form of oscillations of a different frequency or pulses, and these are difficult to eliminate.
I believe that impedance testing is essential to prevent such artifacts. However, if the impedance cannot be adjusted to the ideal 10 kilo-ohms, recording can be done, but be careful to monitor for artifacts, which are much more likely with high impedance.

Eugen Masherov

> Dear all,
> 
> The guideline usually recommends to use <10k ohm for electrodes in eeg
> study. As far as I know, this is important to preclude artifacts and
> noise. But there are many published studies with >50k ohm.
> 
> Is it acceptable to use high-impedace electrodes if all signals look good
> by an eye? Maybe these "good" signals might not mean real good
> representations of cortical sources.
> 
> Best,
> Jinwon
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