In the spring of 1947, two brothers named Homer and Langley Collyer were discovered dead in their Harlem brownstone. To reach them, police had to make their way through a barricade of 140 tons of treasure and garbage—including dress dummies, bales of newspapers, and the chassis of a Model T—that Langley had collected over decades.

Compulsive hoarding is a behavior both fascinating and tragic. Hoarders’ penchant for accumulating innumerable but often worthless objects can disrupt careers, break up families, and even kill. And there are countless cases less famous than the Collyers’. In the reality television show Hoarders, people display blithe indifference to the mounds of belongings in their houses—yet grow panicky when faced with discarding them.

That peculiar combination of nonchalance and anxiety about possessions, which David Tolin, Ph.D., adjunct associate professor of psychiatry, calls the “two basic head-scratchers” of hoarding disorder (HD), has now been captured on functional magnetic resonance imaging (fMRI) of the brain.

Tolin and his colleagues asked three groups of people—HD patients, obsessive-compulsive disorder (OCD) patients, and healthy controls—to bring in a pile of junk mail from home. Each piece was photographed, as were pieces of mail supplied by the lab. As the subjects lay in an MRI machine, they viewed photos of their own and the lab’s mail, then decided whether each item should be kept or shredded.

Compared with the other groups, HD patients experienced abnormally low brain activation in the insula (a structure within the cerebral cortex) and in the anterior cingulate cortex (ACC) when they assessed the experimenters’ possessions. By contrast, those same brain regions lit up in a hyperactive pattern when the HD patients assessed their own possessions. (Perhaps unsurprisingly, the HD patients chose to shred fewer personal items than did members of the other groups.) The results were published in the August 2012 issue of the Archives of General Psychiatry.

“That biphasic abnormality maps on really well to some of the clinical puzzles that we have in hoarding,” said Tolin, who works with HD patients as director of the Anxiety Disorders Center at Hartford Hospital’s Institute of Living, and who has appeared as a guest expert on Hoarders. “You can see that flip-flop occurring even clinically.”

While HD has long been considered a type of OCD, Tolin’s results add to a growing body of evidence that it is a distinct disorder. “The more people we talked to who had hoarding problems,” Tolin said, “the more skeptical we became that this had anything to do with OCD.” In a previous study, he found that fewer than one in five hoarders meet the Diagnostic and Statistical Manual of Mental Disorders (DSM) criteria for OCD.

In fact, HD may bear some relation to autism and anxiety disorders. The insula and ACC help a person decide whether an object is relevant to him or her; and HD patients’ low level of activation in these regions while viewing experimenters’ possessions is similar to the reaction of autistic patients to human faces. On the other hand, the hyperactive response echoes patterns seen in anxiety disorders. That, too, makes clinical sense: If every item seems relevant, then trying to decide which ones to throw away can be overwhelming.

HD will be included as a separate disorder in the upcoming fifth edition of the DSM, psychiatry’s bible. In the meantime, Tolin cautions that abnormalities in brain MRIs don’t mean that HD is untreatable. “Regardless of what’s going on in the brain,” he said, with appropriate treatment “people can overcome hoarding. They can get better.”