How are brain waves measured?
Electroencephalography
Electroencephalography (EEG) is an electrophysiological monitoring method to record electrical activity of the brain. It is typically noninvasive, with the electrodes placed along the scalp, although invasive electrodes are sometimes used such as in electrocorticography. EEG measures voltage fluctuations resulting from ionic current within the neurons of the brain.[1] In clinical contexts, EEG refers to the recording of the brain's spontaneous electrical activity over a period of time,[1] as recorded from multiple electrodes placed on the scalp. Diagnostic applications generally focus either on event-related potentials or on the spectral content of EEG. The former investigates potential fluctuations time locked to an event like stimulus onset or button press. The latter analyses the type of neural oscillations (popularly called "brain waves") that can be observed in EEG signals in the frequency domain.
Trance
There are four principal brainwave states that range from high-amplitude, low-frequency delta to low-amplitude, high-frequency beta. These states range from deep dreamless sleep to a state of high arousal. These four brainwave states are common throughout humans. All levels of brainwaves exist in everyone at all times, even though one is foregrounded depending on the activity level. When a person is in an aroused state and exhibiting a beta brainwave pattern, their brain also exhibits a component of alpha, theta and delta, even though only a trace may be present.