Although not a “brain scan” as the term is usually used, the EEG, or electroencephalograph is a very useful way of observing human brain activity. An EEG is a recording of electrical signals produced by the brain.
Although known as early as the nineteenth century that 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 of the brain and observe changes over split seconds of time. An EEG can show what state a person is in — asleep, awake, anesthetized — because the characteristic patterns of current differ for each of these states.
Read more about the four primary brainwave states.