What causes Capgras ("imposter") Syndrome? Capgras Syndrome is a rare disorder in which a person believes his or her loved ones have been replaced by identical-looking impostors (http://en.wikipedia.org/wiki/Capgras_delusion). What causes this condition?
There have been many proposed explanations for the condition. The explanation which seems to have gained traction states that an individual with Capgras syndrome experiences a "disconnect" between the part of the brain that recognizes faces and the part of the brain that processes emotion. Thus, the person may see someone who looks like his brother, but does not experience the feelings of familiarity, love, etc. associated with the brother (Ramachandran, 1998). In an effort to make sense of this, the person constructs a delusion that the brother has been replaced by a duplicate.

There have been studies which have found Capgras patients have reduced physiological responses associated with emotional reactions when looking at all faces, even for people they do not believe to be duplicants (Ellis, Young, Quayle, &amp; De Pauw, 1997); the authors proposed that the delusion is only constructed for loved ones because of the intensity of the emotional reaction expected during those encounters. Interestingly, individuals with Capgras syndrome often retain appropriate emotional responses to hearing the voice of the loved one (Ellis et al., 1997; Ramachandran, 1998) and can experience a range of emotions, indicating the issues is specific to visual-emotional processing.

The reason for this proposed disconnect is not clear (Brighetti, Bonifacci, Borlimi, &amp; Ottaviani, 2007; Edelstyn &amp; Oyebode, 1999). Some have suggested it is a disconnected between "processing streams" in the brain (Ellis &amp; Lewis, 2001). This theory proposes that normal face processing occurs when information from the visual cortex is filtered through two different paths in the brain. One path allows for recognition of faces, while the other path allows for recognition of emotional responses. When the input from both paths is accurate, the person correctly identifies the person they see and has the appropriate reaction to that person. If the emotional path is damaged, the person does not experience the expected emotion and so attributes this reaction to the person being replaced. (This model also proposes that if the emotional component is intact but the "face recognition component" is damaged, the person will experience prosopagnosia, also called "face blindness.") Ellis &amp; Lewis (2001) provide a nice overview of the research that led to this model. A slightly different model proposes a disconnect between the visual areas in the temporal lobes of the brain and how emotion is processed in the amygdala, but does not propose a two-path approach (Ramachandran, 1998).

While most studies focus on a unidirectional explanation for the delusion (e.g., the neurological issue leads the person to create the delusion as an explanation), Young (2008) proposed that the two processes actually interact with a feedback loop. In this model, first the person doesn't experience the expected emotion and constructs the delusion to cope; however, their strong belief feeds back to their perceptual experience so they actually begin to see the person as an impostor. That perception then feeds back into the delusion, strengthening it. Young argues this is why the delusion is so difficult to challenge (2008).

References

•   Brighetti, G., Bonifacci, P., Borlimi, R., &amp; Ottaviani, C. (2007). “Far from the heart far from the eye”: evidence from the Capgras delusion. Cognitive Neuropsychiatry, 12(3), 189-197.

•   Edelstyn, N. M. J., &amp; Oyebode, F. (1999). A review of the phenomenology and cognitive neuropsychological origins of the Capgras syndrome. International Journal of Geriatric Psychiatry, (14), 48-59.

•   Ellis, H. D., &amp; Lewis, M. B. (2001). Capgras delusion: a window on face recognition. Trends in Cognitive Sciences, 5(4), 149-156.

•   Ellis, H. D., Young, A. W., Quayle, A. H., &amp; De Pauw, K. W. (1997). Reduced autonomic responses to faces in Capgras delusion. Proceedings of the Royal Society of London. Series B: Biological Sciences, 264(1384), 1085-1092.

•   Ramachandran, V. S. (1998). Consciousness and body image: lessons from phantom limbs, Capgras syndrome and pain asymbolia. Philosophical Transactions of the Royal Society B: Biological Sciences, 353(1377), 1851-1859.

•   Young, G. (2008). Capgras delusion: an interactionist model. Consciousness and Cognition, 17(3), 863-876.

Additional Reading

•   Young, G. (2007). Clarifying" familiarity": Examining differences in the phenomenal experiences of patients suffering from prosopagnosia and capgras delusion. Philosophy, Psychiatry, &amp; Psychology, 14(1), 29-37.

•   Young, A. W., Leafhead, K. M., &amp; Szulecka, T. K. (1994). The
Capgras and Cotard delusions. Psychopathology, 27(3-5), 226-231.