When you have mounted the net, it is time to check how well the electrodes are conducting the electrical potentials from the scalp. The higher the impedance, the worse they conduct, the more noise the EEG will contain. The impedance, which is similar to resistance, should be as low as possible.
To measure the impedances:
Start Net Station by double-clicking the “Net Station” icon on the desktop of the Mac. The main screen will appear.
To start a session, you must open a session template, which contains settings for different kinds of experiment. To open a session for a BCI experiment, click Session and in the template selection window, select the session called “MusicPhysio” and click Select . At the Human Neurophysiology course you should use the “HumanNeurophysiology” session.
If you want to save the EEG data in Net Station, please enter in the field “Enter Subject Identifier or Select From Table” a session name in the format <ProjectName>.<SubjectNumber>.<YYYYMMDD>.<Condition><TrialNumber>. For Mind and Machine enter a file name, e.g., MAM16.S0010.P300 although the data should be discarded after the course as they are not useful. To start the recording, click Begin Session in the Session Information window. It will first measure gains and after that the impedance measurement window should pop-up (see figure below).
select “Panels” ⇒ “Impedance” ⇒ in the window that pops up click “measure impedance”. Set the threshold in the upper left corner to 70 kOhms, for example. Above this value, electrodes are considered 'bad' and will turn red.
Make sure the screen is visible on the subject-side monitor by pressing 'select' on the black VIDMUX device between the Mac and the PC to select channel 2. To make this visible on the monitor on the subject's side, drag the window to the right outside of the Mac screen.
Make sure no more than 10 electrodes are red. Try to move aside the hair underneath bad electrodes and lightly scrub the scalp with the electrode. If this does not help, use the plastic pipet from the toolbox on the table to add a few drops of electrolyte solution to the sponge around the bad electrode.
Electrodes that are red should not coincide with any crucial locations (10-20, reference) and their values should not be out of proportion with the other ones (for example >150 kOhms).
When you are done fixing the impedances, provide the subject with earplugs and apply cling foil around the subject's head to prevent the EEG-net from drying out. More or less tightly wrap the foil around the head twice. The impedances usually improve after doing this, because the cling foil will tighten the electrode net around the subject's head, which can improve the contact with the scalp.
Double-check the impedances after having applied the cling foil.
Close the impedance measurement window by clicking Save and Close.