Neural Effects of Positive and Negative Incentives during Marijuana Withdrawal

According to the allostasis model of drug addiction, drug-seeking behavior is related to positive reinforcement processes in the early stages of drug use, when the user is highly sensitive to the neurological reward associated with use of the drug. Later stages of addiction are characterized by negative reinforcement processes, and in increase in motivation to use the drug in order to avoid the undesirable symptoms of withdrawal. These negative reinforcement processes, however, are not as well understood. A study published in the journal PloS One examines the effects of withdrawal symptoms on reward-motivation systems in marijuana users. The researchers recruited 59 heavy marijuana users (defined as using 4 or more times per week for at least 6 months) and 27 non-users as a control group for a monetary incentive delay task. The participants were asked to abstain from using marijuana for 72 hours before the study, and their withdrawal symptoms were measured before beginning the task. The task required participants to push a button when they saw a gray square on a computer screen, which would appear for less than half a second, under three separate conditions: 1.) a correct answer would yield a monetary reward, 2.) a wrong answer would cause a deduction in their compensation for the experiment, or 3.) money would be awarded or deducted, only the participant's reaction time would be measured. The participants were given immediate feedback about their performance throughout the task, and underwent a high-resolution MRI scan after the assessment.

While both groups performed equally well in terms of accuracy and response time, regardless of the nature of the condition (a monetary gain, loss, or neither), the MRI revealed that the marijuana users showed greater neural activity in areas of the brain related to reward in response to positive incentives as opposed to negative incentives, while for controls (non-users) the finding was the exact opposite. Additionally, negative incentives were shown to elicit a response in marijuana users from areas of the brain that underlie reward processes. The researchers also unexpectedly discovered an inverse relationship between withdrawal symptoms and reward-area response to negative stimuli. The findings suggest that not only are marijuana users more sensitive to positive incentives, but they are also less sensitive to negative incentives when experiencing stronger withdrawal symptoms. In other words, marijuana users are more motivated to receive immediate positive rewards, such as continued drug use, and less likely to have their decisions affected by possible negative outcomes, leading to a high probability of relapse. The findings of this study are expected to prove helpful for developing and improving strategies for the treatment of addiction by considering the significance of these psychological processes in addiction.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0061470

Filbey FM, Dunlop J, Myers US (2013) Neural Effects of Positive and Negative Incentives during Marijuana Withdrawal. PLoS ONE 8(5): e61470. doi:10.1371/journal.pone.0061470