How the brain produces signals

To understand any BCI, it helps to know what it’s actually measuring.

Neurons and spikes

Your brain has roughly 86 billion neurons. Each one keeps a small voltage across its membrane. When enough input arrives, the neuron fires an action potential (a “spike”): a brief electrical pulse, about 1 millisecond long, that travels down its axon and releases chemical messengers to the next cells.

The rate and timing of spikes carry information. In the motor cortex, for example, some neurons fire faster when you intend to move your hand left, and others when you intend to move it right.

Signals at different scales

Scale	What’s measured	Typical sensor
Single neuron	Individual spikes	Intracortical microelectrodes
Local population	Local field potentials: summed activity near the electrode	Microelectrodes, deep brain leads
Cortical patch	Activity across millimetres of cortex	ECoG
Whole brain regions	Rhythms such as alpha and beta brain waves	EEG, MEG
Blood flow	Oxygen use following neural activity (slower, by seconds)	fNIRS, fMRI

Why this matters for BCIs

Recordings from smaller scales are richer and faster but need sensors closer to (or inside) the tissue. Recordings from larger scales are easier to get but blur many signals together. Choosing a recording method largely means choosing a scale. See Invasive vs. non-invasive.