You can obtain low impedances by preparing the subject and headcap correctly.
The quality of the connection between EEG electrodes and the scalp can be determined through an impedance measurement. High impedances can indicate bad signal quality (low signal to noise ratio). Besides low impedances, it is also important to have similar impedances for all electrodes. An EEG amplifier attenuates any noise that is identical in the active and reference channels. However, when there are varying impedances, the amount of noise (e.g. 50 Hz noise from electrical activity in the recording environment) can differ per electrode and will therefore not be attenuated.
The impedances can be checked before starting a recording of the EEG signals. To obtain low impedances with a gel headcap, start by introducing enough gel between the electrode and scalp with a syringe. Next, push the hair aside by rotating the syringe. This rotating movement will also evenly distribute the gel and improve the electrode-skin connection. In the following instructional video, it is shown how to gel an EEG headcap.
With TMSi's EEG only amplifier, APEX, it is also possible to record live impedances during a recording. In this way, the quality of the electrode-skin connection can always be inspected.
After finishing the EEG measurement, it is necessary to clean the cap (after every use) to prevent that any gel residue is left in the electrodes. Electrode gel dries and blocks the contact between the skin and the electrode during the next measurement. Read more about how to clean an EEG headcap here.