MONDAY, Dec. 16, 2013 (HealthDay News) -- Shrunken structures inside the brains of heavy marijuana users might explain the stereotype of the "pothead," brain researchers report.

Northwestern University scientists studying teens who were marijuana smokers or former smokers found that parts of the brain related to working memory appeared diminished in size -- changes that coincided with the teens' poor performance on memory tasks.

"We observed that the shapes of brain structures related to short-term memory seemed to collapse inward or shrink in people who had a history of daily marijuana use when compared to healthy participants," said study author Matthew Smith. He is an assistant research professor in psychiatry and behavioral sciences at the Northwestern University Feinberg School of Medicine, in Chicago.

The shrinking of these structures appeared to be more advanced in people who had started using marijuana at a younger age. This suggests that youngsters might be more susceptible to drug-related memory loss, according to the study, which was published in the Dec. 16 issue of the journal Schizophrenia Bulletin.

"The brain abnormalities we're observing are directly related to poor short-term memory performance," Smith said. "The more that brain looks abnormal, the poorer they're doing on memory tests."

The paper is provocative because the participants had not been using marijuana for a couple years, indicating that memory problems might persist even if the person quits smoking the drug, said Dr. Frances Levin, chairman of the American Psychiatric Association's Council on Addiction Psychiatry.

At the same time, Levin cautioned that the paper presents a chicken-or-egg problem. It's not clear whether marijuana use caused the memory problems or people with memory problems tended to use marijuana.

"The big $64,000 question is [whether] these memory problems predate the marijuana use," Levin said.

The study focused on nearly 100 participants sorted into four groups: healthy people who never used pot, healthy people who were former heavy pot smokers, people with schizophrenia who never used pot and schizophrenics who were former heavy pot users.

Researchers used MRI scans to study the structure of participants' brains. Both healthy and schizophrenic marijuana users showed shrinkage of regions deep in the brain that are associated with memory.