One January day in 2007, a terrifying idea seized a 45-year-old wife and mother in Omaha, Neb.: Her husband and teenage sons were not, in fact, her husband and teenage sons. Strangers who happened to be identical to her family members had taken over her home, and to fend them off, she armed herself with a fireplace poker, called her neighbors -- and 911.

It sounds like something out of the 1956 sci-fi film "Invasion of the Body Snatchers," but the Nebraska mom actually was suffering from something called Capgras delusion, a rare psychiatric disorder in which a patient believes her friends or family members are not who they say they are -- and that the real people have been replaced by identical-looking impostors.

Normally, we recognize faces thanks to a part of the brain called the fusiform gyrus, which is located in the temporal lobe. It processes the faces we see, and sends that information on to another part of the brain, the amygdala, which processes emotions. But in patients with Capgras, there's a disconnect between that visual center and the emotional center, explains Dr. Mariam Garuba, a New York psychiatrist who treated the Nebraska woman when she was admitted to an Omaha, Neb., emergency room three years ago. (Garuba wrote about the unusual case, referring to the patient only as "Ms. A," in a clinical psychiatry journal last year.)

In other words: Ms. A knew that these people standing in front of her looked, talked and acted like her husband and her children, but they didn't make her feel the way she usually did when she saw them.

It's worth noting that if Capgras patients talk to a loved one on the phone, they will recognize the voice. But if that loved one enters the room, the patient will accuse his friend or family member of being an impostor; that's because hearing and sight take different pathways to reach the brain's emotional center. (Extra credit: Watch neurologist V.S. Ramachandra deliver a fascinating speech on Capgras and other brain disorders at a 2007 conference.)

In some cases, that disconnect that is thought to cause Capgras is brought on by a head injury; in others, it's related to an existing psychiatric or neurological disorder. Ms. A falls in the latter group, as a longtime bipolar disorder patient who'd also been diagnosed with multiple sclerosis. Although she'd taken drugs to treat the bipolar disorder in the past, she was taking none in January 2007; she was also not taking any medication for her MS. Her physicians, including Garuba, believed the Capgras delusion occurred because of a relapse of Ms. A's MS. She was treated with antipsychotics, and after a few days, she gradually stopped believing that her doctors were trying to poison her; after nearly a month in the hospital, she stopped believing that her family members were impostors.

The rare disorder is named for Joseph Capgras, a French psychiatrist who was the first to write about the delusion in 1923, after treating a woman who became convinced that her husband and others she knew were actually body doubles. Similar cases to Ms. A's in recent years include a 24-year-old woman who, after some complications with pneumococcal pneumonia led to epileptic seizures, began to believe that some of the ICU physicians had been replaced by impostors. And in the UK, a 42-year-old woman claimed that while she was in the ICU for pneumonia in 1999, each of her family members except for her mother were replaced by aliens.

Of course, we don't know exactly how these patients voiced their suspicions, but it must have at least carried the spirit of this quote from the 1950s "Body Snatchers" trailer: "Listen to me! Please, listen! If you don't -- if you won't -- if you fail to understand -- then the same incredible terror that's menacing me will strike in you!"

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