What occurs with unbalanced electrode impedances greater than 20 Kohms?

When unbalanced electrode impedances exceed 20 Kohms, it can lead to an increase in the susceptibility to noise and artifacts in the EEG signal. This is particularly true for 60Hz interference, which typically comes from electrical sources and can be more pronounced when the electrode impedances are high. The reason for the increase in 60Hz activity is that high impedance can result in a poorer grounding and filtering effect, allowing more ambient electrical noise to influence the recorded EEG signals.

In an EEG setup, ideally, the impedances should be low and balanced across all electrodes to ensure high-quality signal acquisition. When impedances are unbalanced and high, the system can become more sensitive to external electrical noise, leading to an increase in the 60Hz activity displayed on the EEG. This underscores the importance of maintaining proper electrode placement and ensuring that impedances are kept within recommended limits for optimal EEG performance.