action potential

An action potential is the brief electrical pulse a neuron fires to send a message. It is all-or-nothing: the cell either produces a full spike or none at all, the way a camera flash either fires fully or doesn't. Each one lasts only about a millisecond.

When a neuron decides to fire, a wave of electrical change races down its long output fiber, the axon, and triggers it to talk to the next cells. Information in the brain is carried mostly in the timing and rate of these spikes — fast bursts versus slow trickles mean different things. The action potential is, in this sense, the brain's basic unit of signaling.

This is ultimately what BCI electrodes are listening for. Up close, an implanted electrode can pick out the spikes of individual neurons. From farther away, on the scalp, no single spike survives; instead the electrode senses the blurred sum of countless neurons firing together, which shows up as field potentials and brainwaves.