The Capgras delusion (or Capgras syndrome) is a disorder in which a person holds a delusional belief that a friend, spouse or other close family member, has been replaced by an identical-looking impostor. This disconnection creates a sense that the observed face is not the person's it purports to be, and therefore lacks the familiarity that should be associated with it. If it is a relative's face, it is experienced as an impostor's; if the sufferer sees their own face they may feel no association between it and their sense of self, resulting in a sense that they do not exist.
Prosopagnosia (sometimes known as face blindness) is a disorder of face perception where the ability to recognize faces is impaired, while the ability to recognize other objects may be relatively intact. Few successful therapies have so far been developed for affected people, although individuals often learn to use 'piecemeal' or 'feature by feature' recognition strategies. This may involve secondary clues such as clothing, hair color, body shape, and voice. Because the face seems to function as an important identifying feature in memory, it can also be difficult for people with this condition to keep track of information about people, and socialize normally with others.
The Cotard delusion or le délire de négation ("negation delirium"), is a rare neuropsychiatric disorder in which a person holds a delusional belief that he or she is dead, does not exist, is putrefying or has lost his/her blood or internal organs. Rarely, it can include delusions of immortality.
ALIEN HAND SYNDROME
Alien hand syndrome (anarchic hand or Dr. Strangelove syndrome) is an unusual neurological disorder in which one of the sufferer's hands seems to take on a mind of its own. Alien hands can perform complex acts such as undoing buttons, removing clothing, and manipulating tools. Sometimes the sufferer will not be aware of what the alien hand is doing until it is brought to his or her attention, or until the hand does something that draws their attention to its behavior. At times, the hands appear to be acting in opposition to each other. For example, one patient was observed putting a cigarette into her mouth with her intact, 'controlled' hand (her right, dominant hand), following which her alien, non-dominant, left hand came up to grasp the cigarette, pull the cigarette out of her mouth, and toss it away before it could be lit by the controlled, dominant, right hand.
ENVIRONMENTAL DEPENDENCY SYNDROME
A disorder in which the patient looses the capacity to keep his own identity constant, as he adapts himself excessively to variations in the social contexts, violating his own identity connotations in order to favor a role which the environment proposes. In one famous case, the patient, when with doctors, assumed the role of a doctor; when with psychologists he said he was a psychologist; at the solicitors he claimed to be a solicitor. The patient doesn't just make these claims, he actually plays the roles and provides believable accounts for how he came to be in these roles. To investigate further, doctors used actors to contrive different scenarios. At a bar, an actor asked the patient for a cocktail, prompting him to immediately fulfill the role of bar-tender, claiming that he was on a two-week trial hoping to gain a permanent position. Taken to the hospital kitchen, the patient quickly assumed the role of head cook, and said he had to concoct special meals for diabetic patients. He maintains these roles until the situation changes. However, he didn't take on the part of a laundry worker at the hospital laundry, perhaps because it was too far out of keeping with his real-life career as a politician.
Synesthesia is a condition in which one type of sensory stimulation creates perception in another sense. In one common form of synesthesia, known as grapheme, color synesthesia or color-graphemic synesthesia, letters or numbers are perceived as inherently colored, while in ordinal linguistic personification, numbers, days of the week and months of the year evoke personalities. In spatial-sequence, or number form synesthesia, numbers, months of the year, and/or days of the week elicit precise locations in space (for example, 1980 may be "farther away" than 1990), or may have a (three-dimensional) view of a year as a map (clockwise or counterclockwise).