EEG High Impedances - Common problems, why they happen, how to solve them

Typical failure patterns:

• Marginal red values (20 to 35 kΩ, gel) when channels should be acceptable.

Insufficient skin preparation.

Skin Preparation:

1. Clean the skin thoroughly with alcohol and then apply a small amount of mild abrasive paste (such as Nuprep or similar) to the area of contact with each electrode.

2. Gently rub the paste for several seconds using a cotton swab or gauze pad to reduce surface

3. Clean the excess paste.

• All channels red, gel electrodes

Usually the reference electrodes (CMS and DRL) have failed contact, or the electrode-type selector does not match the physical setup.

Less commonly, the user is connected to a Necbox without IC calibration and is looking at the legacy QI display.

• All channels red, dry electrodes

Almost always means the electrode-type selector is still set to Gel.

• Frontal channels red, posterior channels fine

Eye movement and facial muscle activity dominating the variability calculation. Asking the subject to close their eyes and relax facial muscles, then rerunning, usually fixes this.

Persistent failure of frontal channels is what bipolar current delivery specifically targeted.

• Posterior channels red with dry electrodes, especially O1, O2.

Mechanical contact pressure at the back of the head is typically lower than at the top or sides.

Dry electrode impedance is strongly pressure-dependent. May require headcap re-fitting or a different cap size.

• A single channel briefly reports ~1 MΩ immediately after a tES step.

Residual electrode polarization from the 300 μA stimulation; the 150 nA EEG IC sees this as instability. Expected behavior. Waiting 30 to 60 seconds and rerunning resolves it.

• “Too high variability” rather than a kΩ value.

The algorithm could not produce a trustworthy measurement on that channel — the signal was too noisy across the three measurement steps. The contact may or may not be physically bad; we cannot tell from the measurement alone. Common

causes are subject movement, RF interference, post-tES polarization, or biological artifacts.