Imagine this: 

A 70 year old married woman, who had been a home maker her entire life, was brought to the emergency one day with complaints of aggression and violence. She was brought in by the local authorities, who became involved earlier in the evening when the woman had threatened to kill her own husband. The woman was sedated and admitted for neuropsychiatric evaluation. The following morning, under calmer circumstances, the woman was interviewed. When asked why she had threatened to kill her husband, she claimed that the man wasn’t her husband at all. She told to the doctors that her husband had in fact, been replaced by an impostor. She confessed that it was the imposter that she wanted to kill. 

This is a classical history of Capgras Syndrome, also fondly known as the "delusion of doubles". In the world of psychiatry, a whole new aspect of the human body opens up to us - the mind. The problem with handling the maladies of the mind is that we don’t quite understand how to visualise these problems or think about them. Our whole approach kinda falters. So today, our goal is to understand a little bit more about this delusion and, in the process, a little more about how we process the world around us. 

So in order to understand this delusion, let us first understand how we recognise people. Now there are many theories as to how this process happens in our heads, but let us focus on the most important one. You see, when we see a person, we recognise two distinct faces of them. We recognise an external face which is how they look, what they sound like, how big their eyes are and things like that. Basically their physical features. We are able to recall this because after repeatedly seeing someones face, that gets stored in our temporal cortex. 

The next is the internal face. This is more the personality of the people that you meet. What they say, how they think, their sense of humour, etc. Basically the internal face is a reflection of how the person makes you feel and this information is stored in the limbic system. And the stronger the emotions associated with a person, the easier it is to recognise and recall the memory. Now, when you see a person, you recognise both the internal and the external faces of a person. 

But how do we know if we are recognising the internal face or the external face? You see recognising the external face was pretty easy to test. Did the face seem familiar or not? If the face was familiar then the person recognised it. If the face was not familiar then, well, obviously you didn’t recognise it. So that was pretty straight forward.

Now the internal face. Turns out, when we recognise someones personality as being familiar, your autonomic system gets activated because you have an emotional connection with that person that you know. And this activation is measured by a test known as Galvanic Skin Response (GSR) which basically measures the electrical activity of the skin. So when you recognise a person that you see, and you identify with their personality, the GSR will spike. 

So now we know, internal face and external face recognition. In people with Capgras syndrome, something very interesting happens. These patients recognise the external face just fine. But they are unable to recognise the internal face. Which means that they have a very flat GSR. The GSR of these patients is similar to the GSR that they have when they meet strangers. So the inference is this: They are able to feel emotions normally which is done in the limbic system. They are also able to recognise faces properly from their memories which is housed in the temporal lobe. But they are not able to associate the face that they recognise with the emotions associated with them. Which means that there is a problem in the connection between the temporal cortex and the limbic system.

Specifically, the problem with Capgras syndrome, has been localised to the connection between the amygdala and the infra temporal cortex. 

So as of now, this is best working hypothesis that we have as to why this delusion of doubles occurs. It has been well documented and it can occur acutely or develop over a course of weeks. The causes of Capgras syndrome are multiple. Obviously, neuroanatomically speaking, any problem in the pathway between the infra temporal cortex and then amygdala may cause it. Such a problem can occur with many neurodegenerative conditions like Parkinson’s, Alzheimer’s disease, various types of dementia and even old age. Some cases have been reported after injury to the brain, endocrinological conditions like diabetes, hypothyroidism and even migraine attacks. 

The delusion of doubles, which is now classified as a delusional misdirection syndrome, was one of the first few psychiatric disorders in which a Neuroanatomical explanation was given for the manifestations of the mind. 

So what do we take away from all this. The idea is simple. The mind as we know it is a very abstract concept. We don’t quite understand it full well because we aren’t able to rightly visualise it or manipulate it. It's very vague. But the science of medicine is fast progressing and as it moves along, it is able to give us a better grasp of how the mind is mapped onto our brain. And perhaps, some of the mysteries of the mind, we may never understand but there are numerous things yet to be fully explained and we are just getting started.