electrode impedance

Electrode impedance is a measure of how much the connection between an electrode and the skin resists the tiny electrical signal trying to pass through it. Think of it as how clogged the doorway is between the brain's faint voltage and the recording equipment: low impedance is a wide-open door, high impedance is a narrow, sticky one.

Why it matters: brain signals are already incredibly small, so if the contact resists them strongly, what reaches the amplifier is weak and easily buried in noise. That is exactly why technicians scrub the skin and use conductive gel before placing EEG electrodes — these steps lower the impedance and let the signal through cleanly.

This is also the core difficulty of "dry" electrodes, which skip the gel for convenience. Without gel, the skin contact tends to have higher impedance, so making dry electrodes that still record a clean, trustworthy signal is a genuine engineering challenge.