electroencephalography (EEG)

Electroencephalography, or EEG, listens to the brain's electrical chatter using small metal electrodes placed on the scalp. When many neurons fire together, they create tiny voltage ripples that reach the surface of the head, and EEG records these as wavering lines, moment by moment. It is like pressing your ear to the outside of a stadium wall: you can hear the crowd roar and tell roughly when something exciting happens, even without seeing a single seat.

EEG's great strength is timing. It captures changes in thousandths of a second, so it follows the rhythm of thought almost as fast as it unfolds. It is also cheap, safe, portable, and completely non-invasive — nothing pierces the skin — which is why it is by far the most common signal used in brain–computer interfaces.

The catch is the skull. Bone and skin smear and weaken the signal on its way out, so EEG is spatially coarse: it can tell you roughly which region is active, but not which exact spot. The readings are also small and easily swamped by noise, so good electrode contact and careful cleanup are a constant part of the work.