Cotard's syndrome is a rare psychological disorder that leads people to believe they are truly zombielike creatures -- soulless, dead inside, and moving about in an otherwise lifeless body in a world that doesn't exist. People experiencing Cotard's syndrome aren't dissuaded by their ability to walk and talk and move their "dead" limbs -- their certainty, born of a peculiar but unshakeable belief, needs no proof. Regardless of what a physical exam turns up, the patient continues to insist his or her insides are simply not alive anymore.
The disorder was first noted in 1880, and its features haven't changed since then. People experiencing extreme clinical depression are most likely to have Cotard's syndrome, regardless of gender (though odds of suffering this delusion seem to increase with age).
Interestingly, this delusion doesn't affect the way people speak, dress, walk or behave. Their cognitive functioning seems fine; it's just that, according to them, portions of their insides have died off or their life altogether has already ended. They can drive themselves to their doctor's appointment, despite their chief complaint: Their arms are missing, their legs are lifeless flesh, their torso's been hollowed out, the universe is a mirage and their heart's long since stopped beating. Auditory hallucinations are sometimes present, as are other signs of schizophrenia. Antidepressants, antipsychotics and electroshock therapy used independently or in combination often reduce or eliminate the symptoms.