Here we demonstrate how to record EEG. Not with an EEG cap but just one single channel to show how it works. Again, we need three electrodes: +,- and GND. In this case we record the potential difference between the back of the head and the mastoid to obtain a signal from the visual cortex. The GND electrode is simply attached to the other mastoid but could be placed anywhere, for the example on the forehead. Alpha waves show up when the eyes are closed but note that even this very strong feature is hard to spot in the signal.
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