In a dramatic contrast to the reputation of marijuana as a non-violent drug that doesn't cause any long-lasting effects to users, a new study from Chicago's Northwestern University links pot use to notable brain dysfunction and abnormal behaviors.

Led by researchers at the Northwestern University Feinberg School of Medicine, the study was the first to target key subcortical brain regions of chronic marijuana users with Magnetic Resonance Image technology in an effort to link abnormalities in those regions with working memory impairments.

Working memory is the ability to remember and process information in the moment and, when needed, transfer it to long-term memory. A poor working memory predicts poor academic performance and everyday functioning.

The findings were published Dec. 16 in the journal Schizophrenia Bulletin. The research was supported by grants from the National Institute of Mental Health and National Institute of Drug Abuse, under the National Institutes of Health.

Previous studies, explained Northwestern researchers, evaluated the effects of marijuana on the cortex, and directly compared chronic marijuana use in otherwise healthy individuals -- and individuals with schizophrenia.

The research found that the younger individuals were when they started chronically using marijuana, the more their brains were shaped abnormally. The findings also suggested the cerebral regions related to memory might be more susceptible to the effects of marijuana if abuse of the drug starts at an earlier age.

"The study links the chronic use of marijuana to these concerning brain abnormalities that appear to last for at least a few years after people stop using it," said lead study author Matthew Smith, an assistant research professor in psychiatry and behavioral sciences at Northwestern. "With the movement to decriminalize marijuana, we need more research to understand its effect on the brain."

Teens who were heavy marijuana users -- in other words, smoking pot daily for at least three years -- showed abnormal changes in their working-memory brain structures and performed poorly on memory tasks, according to the new research.

The brain abnormalities and memory problems were observed during the individuals' early 20s, two years after they stopped smoking marijuana. That lag seemed to demonstrate long-term effects of chronic use.

The memory-related structures in the brains of chronic users showed signs of shrinking and collapse inward, a possible result of decreases neurons.

The research further showed the marijuana-related brain abnormalities are are similar to schizophrenia-related brain abnormalities -- emphasizing the work of scientists from Northwestern and other institutions to prove changes in brain structure may lead to changes in brain functions.

Smoking marijuana has been linked in prior research to developing schizophrenia. And, of the 15 marijuana smokers in the study who reportedly have schizophrenia, 90 percent started using the drug heavily before they developed the mental disorder.

"The abuse of popular street drugs, such as marijuana, may have dangerous implications for young people who are developing or have developed mental disorders," said the paper's senior co-author John Csernansky, chair of psychiatry and behavioral sciences at Northwestern's school of medicine, as well as Northwestern Memorial Hospital. "This paper is among the first to reveal that the use of marijuana may contribute to the changes in brain structure that have been associated with having schizophrenia."

Chronic marijuana use could augment the underlying disease process associated with schizophrenia, Smith noted. "If someone has a family history of schizophrenia, they are increasing their risk of developing schizophrenia if they abuse marijuana," he said.

Marijuana is the most commonly used illicit drug in the United States, with young adults having the highest and fastest growing level of use.