People who use marijuana for many years respond differently to natural rewards than people who don't use the drug, according to a new study.

Researchers found that people who had used marijuana for 12 years, on average, showed greater activity in the brain's reward system when they looked at pictures of objects used for smoking marijuana than when they looked at pictures of a natural reward — their favorite fruits.

"This study shows that marijuana disrupts the natural reward circuitry of the brain, making marijuana highly salient to those who use it heavily," study author Dr. Francesca Filbey, an associate professor of behavioral and brain science at the University of Texas at Dallas, said in a statement. "In essence, these brain alterations could be a marker of transition from recreational marijuana use to problematic use." [11 Odd Facts About Marijuana]

In the study, researchers looked at 59 marijuana users who had used marijuana daily for the past 60 days, and had used the drug on at least 5,000 occasions during their lives. The researchers wanted to see whether the brains of these long-term marijuana users would respond differently to picures of objects related to marijuana use than they did to natural rewards, such as their favorite fruits, compared with people who did not use marijuana.

The researchers asked the marijuana users how they normally smoked the drug — for example, whether they used joints, bongs or pipes. The researchers also asked the marijuana users what their favorite fruits were, according to the study, published in May in the journal Human Brain Mapping.

Then, the researchers showed the participants pictures of the respective marijuana paraphernalia they normally used, as well as pictures of their favorite fruits, while scanning the participants' brains. [10 Things You Didn't Know About the Brain]

It turned out that when the people were shown the pictures of the marijuana paraphernalia, they showed greater responses in the parts of the brain associated with reward, compared with when they were shown the fruit pictures.

For comparison, the researchers also conducted the same experiment in a group of 70 people who did not use marijuana, and found that those people did not show greater brain responses when they were shown pictures of random marijuana paraphernalia, compared with when they were shown pictures of their favorite fruits.

The findings may shed light on how addiction changes the brain.

"Addiction is a great public health problem, the roots of which are embedded in brain circuits," said Dr. Ashesh Mehta, director of Northwell Health's Laboratory for Human Brain Mapping in Manhasset, New York, who was not involved in the study. The new study showed that "a circuit that includes brain areas that are known to be involved in a number of forms of addiction was more active when chronic marijuana users saw pictures that reminded them of marijuana compared to other types of pictures," Mehta added.

The findings "add to the growing body of research showing this particular circuit" is involved in addiction, and point toward possible ways to treat it, Mehta told Live Science.

The researchers also looked at the relationship between certain problems linked to marijuana use, such as family issues, and the people's brain responses to the pictures of marijuana paraphernalia. It turned out that the more problems that people's marijuana use was related to, the more their brains responded to the marijuana cues.

"The greater the number of problems they had indicating potential cannabis use disorder, the greater the alterations are in their response to the cues," Filbey told Live Science.

Originally published on Live Science.

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