The bonnet contains electrodes that pick up impulses from the brain.
The EEG shows the electrical impulses of the brain.
Although not a "brain scan" as the term is usually used, the EEG, or electroencephalograph, deserves mention as one of the first -- and still very useful -- ways of non-invasively observing human brain activity. An EEG is a recording of electrical signals from the brain made by hooking up electrodes to the subject's scalp. These electrodes pick up electric signals naturally produced by the brain and send them to galvanometers (instruments that detect and measure small electric currents) that are in turn hooked up to pens, under which graph paper moves continuously. The pens trace the signals onto the graph paper.
Although it was known as early as the nineteenth century that living brains have electrical activity, an Austrian psychiatrist named Hans Berger was the first to record this activity in humans, in the late 1920s.
EEGs allow researchers to follow electrical impulses across the surface of the brain and observe changes over split seconds of time. An EEG can show what state a person is in -- asleep, awake, anaesthetized -- because the characteristic patterns of current differ for each of these states. One important use of EEGs has been to show how long it takes the brain to process various stimuli.
A major drawback of EEGs, however, is that they cannot show us the structures and anatomy of the brain or really tell us which specific regions of the brain do what.