In 2011, Dr. Carol W. Berman shared a story about one of her patients, a 24-year-old graduate student named Janet. One day Janet returned home from school to find a strange man in her apartment wearing her husband’s pajamas. She threatened to call the police, which baffled the man who insisted he was her husband.

Janet admitted that he looked like her husband, but she knew in her gut that it wasn’t him. Then she had a startling realization: Her husband had been replaced by a stranger who looked exactly him.

The man really was her husband. Janet was later diagnosed with Capgras syndrome, a disorder in which a person holds a delusion that a close friend, family member or even a pet has been replaced with an impostor.

Such an experience is understandably terrifying. The impostor may look and act like your loved one — and even know intimate details of your life — but there’s no emotion or sense of familiarity. The person you love appears to be gone.

What causes it?

Capgras syndrome, which is named after the French psychiatrist who first described it, often occurs in people who have psychotic disorders, but more than a third of people experience it following head trauma.

While scientists aren’t positive what causes it, they suspect Capgras is the result of a breakdown of communication between the part of the brain that processes visual information for faces and the part that manages emotional responses.

In 1990, psychologists Hadyn Ellis and Andy Young hypothesized that people with Capgras might have damage to the part of the brain that controls the automatic emotional response to a familiar face. Neuroscientist Vilayanur Ramachandran reported similar findings in 1997, noting that the disorder could be caused by a structural problem with the amygdala, the part of the brain that processes emotional responses.

When we see someone we know, a part of the brain known as the fusiform gyrus identifies the person’s face and informs the amygdala. In people with Capgras syndrome, this connection between visual and emotional recognition doesn’t work correctly, so they see a familiar face but they don’t associate it with any emotions.

In 2007, Ramachandran gave a TED talk on Capgras and explained what happens when he measures a Capgras patient's galvanic skin response in reaction to a photo of the patient's mother.

"The galvanic skin response is flat. There's no emotional reaction to his mother because that wire going from the visual areas to the emotional centers is cut. So his vision is normal because the visual areas are normal, his emotions are normal — he'll laugh, he'll cry, so on and so forth — but the wire from vision to emotions is cut and therefore he has this delusion that his mother is an impostor."

However, studies show that the syndrome seems to affect patients only when they’re looking at the person they believe to be an impostor. Often, when hearing that person’s voice, the patient can recognize the voice and even experience an emotional response.

Ramachandran says this makes sense because our visual system and auditory system have different pathways to the amygdala.

"If this patient then goes and (his) mother phones from an adjacent room ... and he picks up the phone, and he says, 'Wow, mom, how are you? Where are you?' There's no delusion through the phone," he explains. "Then, she approaches him after an hour, he says, 'Who are you? You look just like my mother.'"

Some studies have found that for blind people with Capgras, the delusion extends to a loved one’s voice; however, when they touch the person they think is an impostor, they experience familiarity.

Is it similar to face blindness?

Capgras syndrome is often compared to prosopagnosia, or face blindness, but the two are quite different.

People with prosopagnosia have difficulty perceiving faces. They may not recognize the faces of loved ones — or even their own faces in severe cases — but they’re typically able to develop coping mechanisms to aid in recognition. They may rely on voice recognition, focus on a particular identifying feature such as a birthmark, or remember what outfit a person was wearing.

Some people are born with face blindness, but others acquire it after brain damage.

However, people with Capgras are able to perceive faces normally — they’re just unable to associate that face with any sense of familiarity.