What best defines a brain-computer interface (BCI)?

• A. A communication system that conveys messages or control signals using only brain activity.
• B. A system that converts muscle signals into computer commands for external devices.
• C. A device that records brain activity for medical diagnostics only.
• D. A software tool for processing EEG data without user intent.

Answer: (A)

A brain-computer interface is a system that directly translates brain activity into messages or control signals for external devices, without relying on muscle output. This direct brain-to-device link is the defining feature, enabling control or communication solely through brain signals, whether via noninvasive EEG or invasive implants. Systems that rely on muscle signals (myoelectric interfaces), devices that merely record brain activity for diagnostics, or software that processes EEG data without user intent do not provide that direct brain-to-device communication.