It's worth noting that if Capgras patients talk to a loved one on the phone, they will recognize the voice. But if that loved one enters the room, the patient will accuse his friend or family member of being an impostor; that's because hearing and sight take different pathways to reach the brain's emotional center.
Normally, we recognize faces thanks to a part of the brain called the fusiform gyrus, which is located in the temporal lobe. It processes the faces we see, and sends that information on to another part of the brain, the amygdala, which processes emotions. But in patients with Capgras, there's a disconnect between that visual center and the emotional center, explains Dr. Mariam Garuba, a New York psychiatrist
This syndrome is not a hallucination (a perception without object), since it is in the presence of a relative that the patient said see his lookalike. And this is not strictly speaking an illusion, that is a dysperception, but a frenzied construction.
With his assistant, Jean Reboul-Lachaux, Joseph Capgras described the case of Ms. M., aged 53, who claimed that all his family in a study called the illusion of the look-alikes in chronic systematized delusions and itself had several look-alikes. The term of “look-alike” was used by the patient herself.
Capgras delusion most usually occurs within a
psychiatric setting, accompanying a diagnosis of
paranoid schizophrenia; but it can also result from
neurological, toxic or other organic conditions
Current thinking as to the cause of Capgras syndrome has focused on dysfunction of the inferior temporal cortex and the amygdala. The former is involved with recognising faces and the latter with the simultaneous emotional reaction. These two structures can be damaged independently; if the ability to recognise faces remains intact but the emotional reaction which makes them familiar is absent, it is hypothesised that the conculsion drawn will be that the person in question is an identical impostor.
Joseph Capgras (1873-1950) was a French psychiatrist who first described the disorder in a 1923 paper co-authored with Reboul-Lachaux about the case of a French woman who complained that various ‘doubles’ had taken the place of people she knew. They called this ‘L’illusion des sosies’ (the illusion of doubles).
The delusions center around one of the senses—most often sight—and remain localized to that one sense. For example, a man afflicted with Capgras may recognize his wife's voice on the phone, yet believe her to be an impostor upon seeing her in person.
Sufferers of Capgras syndrome, also known as Capgras delusion, believe that a person or people close to them have been replaced with duplicates. In some cases, the afflicted may doubt their own identity, and question whether they, in whole or in part, have been replaced with an impostor.
If prosopagnosia is the result of damage to the system responsible for generating conscious face recognition, sometimes leaving an unconscious or covert mechanism intact, then Capgras delusion might arise when the reverse occurs, that is, an intact overt system, coupled with a malfunctioning covert system.