These people claim that everyone they know is not actually that person and has been replaced by an imposter. For example, after suffering the trauma, the sufferer wakes up and sees their family in their room. But the patient says that none of them are their family, they have all been replaced by look-alikes. One is quoted as saying, “Doctor, this woman looks exactly like my mother but she isn’t, she’s an imposter.” There is even one case where a sufferer made this claim of his pet poodle. However, if they hear a familiar voice on the phone, they immediately recognize it and suffer no delusions about imposters. The disorder is linked solely to visual areas.
Why does this happen? The best hypothesis is that the connection between the visual areas of the brain and emotional response centers (the amygdala and the limbic system) has been cut by the trauma.
When you see a familiar face, you have some kind of emotional response to it. According to this hypothesis, patients with Capgras don’t feel anything, which leads them to the conclusion that the person can’t be the one they know.
The nice thing about this hypothesis (unlike Freudian ones) is that it’s testable. When shown pictures of loved ones, normal people have large galvanic skin response. Capgras suffers don’t, exactly as you’d suspect if they lacked emotional responses to visual stimuli.
What I find interesting is that when the brain receives no emotional response to a stimulus to which it used to respond, it assumes the stimulus is a fake. It’s the only conclusion that the brain can draw given the facts. The brain is quite a remarkable organ.