Australasian Science Magazine

By Tim Hannan

The Capgras delusion raises interesting questions about how the brain attaches emotional responses to familiar faces.

Waking up in hospital after a neurological event can be a disturbing experience. Waking up to discover that your family has disappeared and been replaced by duplicates, aliens or robots is perhaps somewhat more unsettling – for you, and for the family you no longer acknowledge as your own.

While delusions are by definition uncommonly strange beliefs, the Capgras delusion is certainly among the more bizarre. Sufferers declare that the person standing before them, while admittedly identical in appearance and behaviour to a relative or friend, is nevertheless an imposter. Named after the French psychiatrist who provided the first detailed case description in 1923, the delusion usually involves the “replacement” of a family member or close friend by a duplicate; in at least one case the missing loved one was a dog, and even inanimate objects have been believed to have been replaced by duplicates.

Capgras syndrome is one of several “delusional misidentification syndromes”, which are characterised by the presence of a single, specific delusion concerning false identity or substitution of one person for another. In contrast to psychotic disorders in which a diversity of delusions are often present, these misidentification syndromes contain a single delusion, or several delusions clustered around a single theme.

While earlier psychodynamic theories had proposed that the patient was unconsciously projecting unpleasant feelings towards a family member onto an imposter, the weight of evidence now indicates an organic cause. Most cases of Capgras occur following a brain injury or neurological illness, such as a traumatic brain injury, stroke or the onset of dementia.

How does it come about that such a strange idea can be believed? Any attempt to explain the Capgras delusion requires a sufficiently explicit model of the normal cognitive systems underlying person recognition and belief formation, as well as identification of the brain regions and systems assumed to be disrupted or damaged. However, while many aspects of cognition and brain function are well understood, delusional syndromes challenge simple theories, and drive the development of new models of cognitive functioning.

In recent years, neuropsychological studies have produced a theory that may account for the bizarre features of the Capgras delusion. The proposal is that two different brain impairments are present at the same time.

First, the patient has an abnormal perceptual experience that provides the basis for the content of the false belief. Studies have shown that, contrary to the findings with the general population, the physiological reactions of Capgras patients to familiar faces (as measured by skin response and electrophysiology) do not differ from those displayed to unknown faces.

So it seems that the patient recognises another’s face, yet experiences no emotional reaction accompanying that perception. In the absence of this “emotional confirmation”, the patient rationalises that the identity of the person cannot be the person suggested by the visual input: this looks like my wife but doesn’t feel like my wife, so this is not my beautiful wife.

The brain regions suspected to be dysfunctional include the inferior temporal cortex, which is associated with face recognition, and the amygdala, which is involved in emotional reactions to the recognised person. However, abnormal perceptual experiences resulting from these dysfunctional systems cannot be the sole cause of the Capgras delusion, as people with other visual and perceptual disorders do not form such strange conclusions.

A second factor must be present: the patient must have another problem that prevents the belief from being rejected on the grounds of its implausibility. It is therefore hypothesised that there is also dysfunction in a “belief evaluation system”, tentatively located in the right dorsolateral prefrontal cortex, which permits the patient to entertain the delusion as the best explanation of the strange perceptual experience.

Whether this two-factor explanation proves to be correct, studies of the Capgras delusion raise interesting questions about the relationships between perception and emotion, the brain and cognition, and that sense of familiarity we have when we meet those with whom we are close.