Think for a moment about the people in your life whom you are closest to and most familiar with---those whom you see, talk to, and maybe share intimate moments with on a regular basis. Perhaps this would be your spouse, partner, parents, siblings, or friends. Now, try to imagine waking up tomorrow and, upon seeing one of these people, being overcome with an unshakable feeling that it is not really them you are seeing. Even though you know it sounds crazy, you can't stop yourself from thinking that this person you have known for so long has been surreptitiously replaced with an impostor---someone else who looks just like them but is a different person altogether. You know this is irrational and even absurd, but it feels so true to you that you have to believe it's what really is going on.

The sense that people we are familiar with have been replaced with look-alike impostors is the defining symptom of a rare condition known as Capgras delusion. First described in 1923 by psychiatrist Joseph Capgras and his assistant Jean Reboul-Lachaux, Capgras delusion is one of a group of disorders known as delusional misidentification syndromes that involve persistent problems in accurately identifying oneself or others. The original description of Capgras delusion involved a 53-year-old woman who had experienced the death of four of five of her children, leaving her with only a daughter. Several years after the death of her children she began to believe that her daughter and husband had been replaced by identical look-alikes. She eventually felt this was true for everyone she was close to, and she devised elaborate explanations for the duplicity that involved the existence of multiple look-alikes for each person. She believed, for example, that each day she would sometimes see (and communicate with) several different impostors who looked just like her daughter---without ever actually speaking to her "real" daughter.

Patients with Capgras delusion often don't display other major cognitive deficits and can usually appreciate how ludicrous their beliefs may seem to others. They may be able to, for instance, admit that it would be hard for them to believe if someone else described a similar experience with look-alike impersonators. For example, this interaction (from a 1979 paper on the subject) occurred between an experimenter and a Capgras patient who---after a head injury---believed his wife and five children had been replaced with look-alikes:

E. [Experimenter] Isn't that [two families] unusual?

S. [Patient] It was unbelievable! E. How do you account for it? S. I don't know. I try to understand it myself, and it was virtually impossible. E. What if I told you I don't believe it? S. That's perfectly understandable. In fact, when I tell the story, I feel that I'm concocting a story...It's not quite right. Something is wrong. E. If someone told you the story, what would you think? S. I would find it extremely hard to believe...

Despite a Capgras patient recognizing the irrationality involved, the delusion continues. Even time spent with the "impostor" doesn't dissuade the patient; in fact it only tends to strengthen the conviction that the "look-alike" is not who he or she claims to be.

Explaining the Capgras delusion

Although it is believed to stem from some neurological dysfunction, the Capgras delusion is not fully understood; several hypotheses have been proposed over the years to explain the phenomenon. Most recent hypotheses involve a deficit in the neurobiological mechanisms responsible for the recognition of familiar faces. To understand how this may lead to the development of the Capgras delusion, it can be useful to make a comparison to a disorder called prosopagnosia.

In prosopagnosia, patients have an impaired ability to recognize faces despite otherwise normal visual processing. This impairment often involves a general "face-blindness" that leads to a failure to recognize even the most familiar faces. Even though prosopagnosics are unable to overtly identify faces, however, past experiments have suggested they may experience a type of unconscious recognition when they see a familiar face. One way this has been tested has been to measure the skin conductance response (SCR) of prosopagnosic patients as they look at pictures of recognizable faces. SCR, which can detect slight changes in perspiration levels, is often used an indication of autonomic nervous system arousal and thus considered by some to be representative of a type of emotional response. An increased SCR has been observed in prosopagnosics when they look at images of people they are familiar with---even when they aren't able to identify the faces; this SCR has been interpreted as a physiological expression of unconscious recognition.

Capgras delusion is sometimes described as the "mirror-image" of prosopagnosia because Capgras patients recognize the faces of those closest to them, but their SCR is not increased upon seeing those familiar faces. Thus, it has been hypothesized that their conscious recognition is intact but their unconscious emotional response---that visceral familiarity we are used to sensing when we see those we are close to---is lacking. So, when Capgras patients are in the presence of someone they know they should have an emotional connection with, they are understandably disturbed when they don't feel any familiarity with the person. Instead they experience the same degree of autonomic arousal they would when seeing a stranger on the street.

The neurobiology underlying these unusual disruptions of familiarity is not very clear, and explanations of the mechanism responsible remain somewhat speculative. Because Capgras patients are able to recognize faces but do not display a typical emotional response to familiar faces, it has been hypothesized that there is some interruption in the pathways that connect facial recognition areas in the temporal lobe with areas of the limbic system---like the amygdala---that are involved with generating emotional responses. Although facial recognition is still functional, without the ability to activate the limbic system during facial recognition, the patient experiences a lack of emotion and familiarity.

It is thought that this dearth of familiarity is just one component of the Capgras delusion, however. Another aspect involves the pathological logic that leads to the belief that the suddenly unfamiliar person is actually an impostor. Why Capgras patients come to this specific conclusion instead of deciding that they are experiencing an abnormal neurobiological event is not very clear. It may involve an attempt to deal with the cognitive dissonance Capgras patients experience when they have a complete absence of feeling for someone they know they "should" have some emotional link with. In other words, a man would be perturbed to find he feels devoid of any familiarity towards his wife of 30 years; deciding that she must be an impostor allows him to explain his lack of emotion and perhaps reduce some of the mental strain caused by the alarming situation. The development of such an extreme and persistent delusion, however, also likely involves some neurological disruption of executive functions. For example, damage to the frontal areas of the brain, which are thought to be important in the management of rational thought, is often seen in Capgras patients and may contribute to the delusions that characterize the disorder.

There is a paucity of hard evidence to support the current hypotheses about the neurobiological bases of Capgras delusion, however. Likely due to the rarity of the disorder, many studies of Capgras patients (including the relatively few neuroimaging studies that have been published) have been case studies of just one patient. This approach, although informative, does not provide us with the type of evidence that can be used to make strong conclusions about the underlying neurobiology of the Capgras delusion. It is not surprising that research in this area has progressed relatively slowly, for Capgras delusion is far from a public health crisis; thus, answers are not pursued with the same fervor as they are in a much more prevalent disorder like Alzheimer's disease. To the neuroscientist, however, the Capgras delusion represents a fascinating opportunity to explore functions of the brain that we normally take for granted. The recognition of a spouse, for example, as someone who has been part of your life for years seems so natural and ingrained that it is difficult to believe it is dependent upon the proper functioning of neurobiological mechanisms in the same way that sight or movement might be. Capgras delusion, however, demonstrates that even our most fundamental beliefs can crumble with the dysfunction of certain brain regions.