Brain-computer interface (BCI)

A brain-computer interface (BCI) creates a direct link between the brain and an external device. It records brain activity, works out what the person intends, and turns that intent into an action, such as moving a cursor, typing text, speaking through a synthesized voice or moving a robotic arm.

You’ll also see the terms brain-machine interface (BMI), mind-machine interface and neural interface. They mean largely the same thing. “Neural interface” is the broadest: it includes devices that connect to nerves and muscles outside the brain.

The four building blocks

Every BCI, from a $200 headband to a surgically implanted chip, has the same basic parts:

1. Sensors pick up brain signals. They can sit on the scalp (EEG), on the brain’s surface (ECoG), inside brain tissue (microelectrode arrays) or even inside a blood vessel (endovascular BCI).
2. Signal processing cleans up the raw recording, removing noise from muscles, eye blinks and electrical interference.
3. A decoder, usually a machine-learning model, turns patterns of activity into a predicted intention. See Neural decoding.
4. Output and feedback. The device acts, and the user sees the result. This lets both the user and the decoder adjust and improve over time.

What BCIs can do today

• Restore communication. People with ALS or brainstem stroke have used implanted BCIs to type, and more recently to produce speech at close to conversational speed.
• Restore movement. Users control cursors, tablets, robotic arms, and even their own limbs through muscle or spinal cord stimulation.
• Treat disease. “Read-and-write” implants such as responsive neurostimulators detect epileptic seizures and stop them. See closed-loop systems.
• Support wellness and research. Consumer EEG headsets track focus, meditation and sleep, though with far less precision than medical devices.

What BCIs can’t do (yet)

Despite the headlines, no current BCI can read arbitrary thoughts or memories. Decoders are trained for narrow tasks, such as intended hand movements or attempted speech, and only work after the user cooperates during calibration. Implanted devices still need brain surgery and are only available in clinical trials. Non-invasive devices are safe and cheap but record a blurry, noisy signal.

Where to go next

• Invasive vs. non-invasive BCIs: the main trade-off in the field
• How the brain produces signals
• Who’s building BCIs: our directory of companies and labs