Causes
Some of the first clues to the possible causes of the Capgras delusion were suggested by the study of brain-injured patients who had developed prosopagnosia. In this condition, patients are unable to recognize faces consciously, despite being able to recognize other types of visual objects. However, a 1984 study by Bauer showed that even though conscious face recognition was impaired, patients with the condition showed autonomic arousal (measured by a galvanic skin response measure) to familiar faces, suggesting that there are two pathways to face recognition—one conscious and one unconscious.
In a 1990 paper published in the British Journal of Psychiatry, psychologists Hadyn Ellis and Andy Young hypothesized that patients with Capgras delusion may have a "mirror image" of prosopagnosia, in that their conscious ability to recognize faces was intact, but they might have damage to the system that produces the automatic emotional arousal to familiar faces. This might lead to the experience of recognizing someone while feeling something was not "quite right" about them.
In 1997, Hadyn Ellis and his colleagues published a study of five patients with Capgras delusion (all diagnosed with schizophrenia) and confirmed that although they could consciously recognize the faces, they did not show the normal automatic emotional arousal response. The same low level of autonomic response was shown in the presence of strangers. Young (2008) has theorized that this means that patients suffering from the disease experience a "loss" of familiarity, not a "lack" of it.
In 1997, William Hirstein and Vilayanur S. Ramachandran reported similar findings in a paper published on a single case of a patient with Capgras delusion after brain injury. Ramachandran portrayed this case in his book Phantoms in the Brain, and gave a talk about it at TED 2007. Since the patient was capable of feeling emotions and recognizing faces, but could not feel emotions when recognizing familiar faces, Ramachandran hypothesizes that the origin of Capgras syndrome is a disconnection between the temporal cortex, where faces are usually recognized (see temporal lobe), and the limbic system, involved in emotions. Because the patient could not put together memories and feelings, he believed objects in a photograph were new on every viewing, even though they normally should have evoked feelings (e.g., a person close to him, a familiar object, or even himself). Ramachandran therefore believed there was a relationship between Capgras syndrome and a more general difficulty in linking successive episodic memories, since it is believed that emotion is critical for creating memories.
Most likely, more than an impairment of the automatic emotional arousal response is necessary to form Capgras delusion, as the same pattern has been reported in patients showing no signs of delusions. Ellis and colleagues suggested that a second factor explains why this unusual experience is transformed into a delusional belief; this second factor is thought to be an impairment in reasoning, although no definitive impairment has been found to explain all cases.
Capgras syndrome has also been linked to reduplicative paramnesia, another delusional misidentification syndrome. Since these two syndromes are highly associated, it has been proposed that they affect similar areas of the brain and therefore have similar neurological implications. Reduplicative paramnesia is understood to affect the frontal lobe and thus it is believed that Capgras syndrome is also associated with the frontal lobe. Many researchers debate the involvement of the frontal lobe and suggest it is the temporal and parietal lobes that cause Capgras syndrome. Even if the damage is not directly to the frontal lobe, an interruption of signals between other lobes and the frontal lobe could result in Capgras syndrome.
Capgras is such a rare and bizarre syndrome that many theories have come out to try and explain it. One view is the Freudian view. The theory is that the patient felt a strong sexual attraction to his mother (Oedipus Complex), and had strong resentment towards his father. These feelings are repressed throughout childhood, and the theory is that the blow to the head can cause the repressed latent sexual impulses to come back to the surface. The patient suddenly finds himself attracted to his mother, and thinks that it can't be his mother because of the sexual attraction, and therefore, that it must be an imposter.
Read more about this topic: Capgras Delusion