It’s been known since the late 19th century that the brain’s activity gives off electrical signals, and the first recording of that activity in humans was in 1924. EEG works by
attaching recording electrodes to the scalp. It is used in clinical settings to diagnose epilepsy and monitor coma patients, and until recently (when better technologies like
MRI came along) it was also used for diagnosing tumors and strokes. EEG is also a popular neuroscience research method and can provide very detailed information about the
brain’s activity while the participant performs some kind of laboratory task. EEG is only good for sensing the activity of the surface of the brain; activity lower down is just too far
away from the electrodes on the scalp to get reliable data. Recording neural activity through the skull is like listening to an argument in the apartment below yours by pressing
your ear against the floor; you might be able to hear some muffled voices, and maybe even some of the louder details,but you’ve no hope at all of hearing what’s happening in an apartment five floors below