Medical Mysteries and Marvels: A 48-year-old man who can walk, talk and breathe air thinks he's dead. The patient suffers from Cotard's syndrome, also known as'walking corpse syndrome,' a rare psychiatric condition that involves a person denying the existence of his own body or body parts. When a doctor hears his patient say he thinks his brain is dead, he knows something is seriously wrong. That's what happened when 48-year-old Graham went to see his general practitioner to prove his brain was "dead," according to a 2013 case study in the journal CORTEX. According to the case study, Graham knew he had the ability to think, recall and interact with people.

But the condition is so rare that it's still far from fully understood. Though it's undeniably horrific for those experiencing it, Cotard's Syndrome presents a fascinating conundrum for those studying the disorder. The condition's central contradiction -- how can someone articulate the thought that they don't exist? A 2013 case study of a Cotard's sufferer showed low activity in the brain network associated with awareness of the body. It's only one example (as with much of Cotard's Syndrome research, because the condition is so rare), but unpacking how the brains of those with the syndrome work offers hints as to how normally-functioning brains develop a sense of existence.