Of the many functions delegated to the human nervous system, perhaps none is more essentially human than the ability to express one’s thoughts. For persons with severe speech and motor impairments, restoration of the ability to communicate even simple needs is an important goal. Cognitively intact persons who are tetraplegic and anarthric know what they want to communicate — their brains prepare messages for delivery, but those messages are trapped. The goal in the design of brain–computer interfaces is to restore communication and mobility by harnessing voluntarily modulated brain signals to control useful external1-3 or implanted4 devices. Signals from the . . .