EEG

 Your brain consists of 100 billion brain cells, and each of those cells talks to each other using little spikes of electricity. The overall pattern of this activity can be measured using an electroencephalography (EEG) machine.

This overall pattern is represented as a “frequency”, and that frequency can be thought of as a signature for your state of mind. We have different names for each frequency range: beta, delta, and alpha, for example.