Locked-in syndrome is the highest state of unconsciousness, where a patient, although unable or speak or move anything apart from their eyes, is completely aware of their surroundings, can comprehend and respond appropriately to speech (often by blinking) and goes through normal sleep cycles. It is a very rare condition, caused when another underlying cause paralyzes all the muscles that are responsible for voluntary movement. It is usually the result when a condition damages the brain stem while leaving the higher functions of the brain intact.