Scientists have now discovered a single lesion in a unique location of the human brain can trigger bizarre and debilitating disorders known as "delusional misidentification syndromes", scientists have discovered. These syndromes cause sufferers to believe that the people around them are not who they seem to be.

Delusional misidentification syndromes are among the most striking and least understood conditions encountered in neurology and psychiatry.

One of them, called "Capgras syndrome", is characterised by the fact people recognise their loved-ones while simultaneously experiencing them as unfamiliar. This leads them to conclude that they are impostors pretending to be their relatives. Another of these syndromes is the "Fregoli delusion" or the belief that strangers are actually loved ones in disguise.

Scientists were puzzled as to how the brain could produce such a complex disorder, and doubted that a single lesion could be to blame. Many thought that dementia, delirium or other psychiatric diseases could therefore be involved. This means that no effective treatment is currently available.

In a study published in the journal Brain, a team of scientists investigated whether this was really the case, mapping for the first time the brain injuries associated with the disorders. They showed that these syndromes actually likely arise from single lesions at unique and specific locations within the human brain.

Mapping brain lesions

The neuroscientists, from Beth Israel Deaconess Medical Center (US), first identified 17 patients who had a form of delusional misidentification syndrome. Using brain scans, they identified and mapped the lesions sustained by their brains onto a standardised brain atlas.

Analysing this data, they determined that all these lesions were functionally connected to an area of the brain called the retrosplenial cortex, which has previously been associated with the perception of familiarity. In 16 out of the 17 lesions were also connected to an area in the right ventral frontal cortex, linked to belief evaluation.

These findings were then compared to brains lesions in 15 controls with brain injuries who suffered from delusions, but not misidentification delusions. Their lesions were also connected to areas responsible for belief evaluation, but not to the retrosplenial cortex and familiarity. It suggests this is a specific brain pattern present in people with delusional misidentification syndromes.

More research needs to be done on a larger sample of patients and treatments are still a long way away, but the researchers believe gaining new insights into the disorders can already be a great help for families, by diagnosing the problem more clearly.

Lead author Ryan Darby said: "The impact on the patient's family can be heart-breaking, I've seen patients who, thinking their homes were replicas, would pack their bags every night, hoping to return to their 'real' home. Patients who believe a spouse is an imposter often lose intimacy. In these cases, even just knowing that the delusion has a name and is part of a neurological disorder can be helpful for family members."