Do Methamphetamine Users Have Hyperactive Responses To Money?

Substance use disorders have brought enormous medical, social, and economic costs to the world. Amphetamine-type stimulants are the second-most widely used illicit drug worldwide.  The number of methamphetamine (MA) abusers have increased rapidly in recent years. Understanding the risks of individuals’ substance abuse and the neural mechanisms of substance users are important for the prevention and intervention of addiction.

Reward processing has an important impact on individual behavior and decision-making. In particular, anticipation of a reward facilitates approach behavior, whereas anticipation of punishment facilitates avoidance behavior. Many researchers believe that substance users are highly responsive to drug or drug-related stimuli. But for the monetary reward, the results are still inconsistent.

The reward processing is usually subdivided into two stages: the reward anticipation and the reward outcome. Reward anticipation refers to the individual’s approaching behavior and motivational state to reward after the presentation of the reward clue, usually related to the mesolimbic dopaminergic system. The reward outcome reflects the intrinsic hedonic enjoyment after receiving the consumption of the reward, usually related to the mesolimbic opioid system. With its high temporal resolution, the event-related potential (ERP) technique can separate the two substages in reward processing and understanding the neural dynamics of the reward process.

In the current study, 21 MA use disordered individuals were selected from a compulsory addiction rehabilitation center according to the selection criteria, and 22 healthy control (HC) individuals who matched in terms of age and education but had no history of drug use were also recruited. The participants were asked to complete a laboratory-based simple gambling task, which could investigate the differences in the processing of monetary reward. The electroencephalographic (EEG) activity was recorded during the experiment. In this task, the participants completed a game similar to gambling. Participants were asked to earn as many points as they could, and told that the total points they earned would be related to their extra bonus In each trial, the center of the screen displayed two options (9 and 99), and the subjects were asked to choose 9 or 99. Each option had a 50% chance of winning or losing. Choosing option 99 yielded a gain of 99 or a loss of 99, which represented a high-risk option; choosing option 9 yielded in a gain of 9 or a loss of 9, which represented a low-risk option. The response was followed by a central fixation for 2000 ms, and then the feedback was presented for 1000ms. After the experiment was over, the total points for the gambling task were presented.

Although there was no difference in the ratio of high-risk choices between the MA group and the HC group, MA-use disordered individuals made more risky choices following a loss outcome on a previous trial compared to the HC group. Importantly, we discovered a difference in the neural responses in the reward anticipation and reward outcome stages. Specifically, in the reward anticipation stage, we found that the MA group had an enhanced stimulus-preceding negativity (SPN) compared to the HC group. During the reward outcome stage, MA-use disordered individuals showed an enhanced feedback-related negativity (FRN) for the losses versus gains as compared to HC. Furthermore, an enhanced P300 was observed under the gain condition, but not under the loss condition, in MA-use disordered individuals as compared to HC.

Our results indicate that MA-use disordered individuals exhibit a sensitized neurological response to monetary rewards. This study helps to elucidate the neural mechanisms of reward processing for methamphetamine-use disordered individuals.