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What is Capgras' syndrome and what is its causation?

‘After a brief conversation to warm things up and put Arthur at ease, I asked, “Arthur, who brought you to the hospital?”

“That guy in the waiting room,” Arthur replied. “He’s the old gentleman who’s been taking care of me.” “You mean your father?”

“No, no, doctor. That guy isn’t my father. He just looks like him. He’s—what do you call it?—an impostor, I guess. But I don’t think he means any harm.”1

First called “l’illusion des sosies” or “illusion of look-alikes” by the French psychiatrists Capgras and Reboul-Lachaux, Capgras’ syndrome manifests in strange confabulations regarding loved ones, as shown above.5 Historically, this bizarre delusion would be explained using psychoanalytical models; one being that past experiences had triggered an abnormal identification of someone’s face.4 However, one could latch upon a Freudian explanation (as Ramachandran explains) especially in the case of parents being the impostors. Using Freudian thought, every child is sexually attracted to his or her parent- therefore, since this new sensation is perceived as so unusual to the patient, he or she must assume that the parent is someone else. An interesting aetiology indeed: however it does not explain why Capgras’ delusion may occur in friends, pets or even inanimate objects.

Thinking through the disorder logically, one might think that the patient can clearly identify who this person is – otherwise he or she would not be recognised as the mother (or father, or friend etc.). But rather than acknowledging, “yes, she is my mother”, the patient would rather say, “my mother has been replaced by a look-alike”. Therefore, there must be some sort of disconnection between the patient’s ability to recognise visually and emotionally. So perhaps we might jump to the concept of memory loss – especially as many cases of Capgras’ occur after some sort of head injury. But this delusion is strangely specific, so memory loss is a bit too generic to blame the syndrome on.

Ellis and Young in a 1990 paper went on to scientifically prove whether there really is a deficit in emotional recognition using a technique known as skin conductance responses (SCR). This technique can measure nervous responses to stimuli. The theory behind it is that when someone views an image of a loved one, sweat glands will increase secretion. Therefore, by measuring the electrical conductivity of the skin, one can track the emotional propensity associated with an image of a loved one. As expected “Capgras patients will not show the normally appropriate skin conductance responses to familiar faces, despite the fact that these will be overtly recognised”.2

Knowing that there is a disconnection between visual and emotional recognition, one can explore the neuroanatomical basis of Capgras delusion. H.D. Ellis in a 1994 paper proposed an interesting hypothesis. Comparing the visual fields of those with Capgras’ delusion with controls, it was shown that Capgras’ patients had a poor ability to match faces in the left visual field. According to modern neuroscience, viewed images on the left crosses over to the right side of the brain during projection. Hence, there must be some sort of dysfunction in the right hemisphere – an anatomical clue in the puzzling causation of this delusion.3

There are two routes of facial recognition. The first is the one that is associated with identification (dorsal stream) and the other is the one associated with measuring the significance of the image (ventral stream). Ellis and Young proposed the idea that the dorsal stream has dysfunction, whereas the ventral remains the same. But this ventral stream is essential towards secretion of the sweat. If the ventral stream really was intact – then the SCR would not change. Clearly there is some problem within the ventral system. Delving deeper in the anatomy, there are two structures of great significance. The first being the limbic system that is associated with inducing an emotional response, and the temporal lobe responsible for visual memory. Disconnection between the temporal lobe and the limbic system would explain the delusion. Indeed, it would explain why the delusion is solely found in the visual domain. A good example of this is the Ramachandran’s case – whereby the patient would instantly recognise his father over the phone, but immediately regard him an impostor when viewed in person. The auditory domain does not use the same facial recognition pathway as the visual domain; hence no disconnection affects him over the phone.1

But of course the quest to understand this delusional misidentification is not over. The propensity to relate significant others to impostors, robots or aliens is an utterly bizarre, unexplained phenomenon. Only when we fully understand the mechanisms that underpin these perceptions can we fully understand the nature of memory, recognition and the function of the brain.

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