Brain-machine interfaces (BMIs) enable users to interact with their environment solely through the power of thought. A variety of tasks elicit specific signatures in brain activity that represent a user’s mental intent in terms of motor control, decision processing, attention, and more.Neural signals from relevant areas of the brain can be recorded during such tasks—either invasively, via microelectrodes or ECoG, or non-invasively, via EEG—and decoded to convey the user’s cognitive state. Through closed-loop visual feedback systems with external devices (e.g., robotic prosthetics), users are provided with a direct representation of their brain output and of the message being conveyed. As training progresses, users can learn to modulate or alter their brain state in order to communicate successfully with the outside world. See page 292.