Impedance, Skin Potentials, Sweat

By Grace Lim for Neurotech@Davis

As you may know by now, if you are not careful in your collection of EEG data, most of what you are recording can be quite plainly crap. What I mean by crap is impedance to the electrode, which is “a combination of resistance, capacitance, and inductance and the properties of the skin, electrode gel, and the electrode can influence all three of these qualities” (Luck, 2014, Chapter 5). As the name suggests, there are many things that can impede the quality of the signal from the recording electrode. Most commonly this is the outermost layer of skin which includes oil and dead skin cells. “To assess the extent to which current can flow between the scalp and the recording electrode, it is important to measure the impedance rather than the resistance” (Luck, 2014, Chapter 5). Impedance is measured in ohms and as this measure increases, the ability to reject common mode noise decreases. This just means that the more impedance to the signal there is, the harder it is to reject the noise present from all electrode channels. The skills needed to physically reduce impedance are covered in the previous article on Intro to EEG.

Skin potentials are described by Luck as “a tonic voltage between the inside and the outside of the skin, and magnitude of this voltage changes as the impedance changes, thus if the electrode impedance goes up and down over time, this will lead to voltages that go up and down in EEG recording” (Luck, 2014, Chapter 5). This is why it is important to abrade the patch of skin the electrode will be placed on; to reduce the impedance from the skin potentials. You would do this by applying Nuprep or some other abrasive gel to the site of interest (usually the forehead for the frontal electrodes and the mastoids for your references) then wipe it off before putting the electrode on.

“The final major factor in determining the impedance between the outside of the skin and the electrode is sweat” (Luck, 2014, Chapter 5). The reasoning for this is actually quite intuitive if you are familiar with the properties of sweat; sweat contains salts making it conductive. Luck describes this issue as “the very gradual shifts caused by hydration of the skin are not ordinarily a problem, but the faster changes caused by the sweat glands can add random variance to the EEG and reduce your ability to find statistically significant effects” (Luck, 2014, Chapter 5). If you have the chance to view EEG data contaminated by sweat, it is actually obvious when it is occurring because it causes all affected channels to shift downwards. The way this is practically circumvented is making sure your participant is not hot while recording.

In addition, you can actually use this property to your advantage through the Galvanic skin response if you are interested in studying stress. What GSR is, is a measuring the skin’s change in electrical properties (sweat) while the participant is emotionally aroused. This is quite similar to what lie detector tests do! It is important to note however that GSR is not performed on the scalp, so it is not considered EEG and is instead used on someone’s hand/fingers.