Methamphetamine increases motivation through brain processes separate from euphoria

A study published in the journal Psychopharmacology has found that the increase in motivation people experience from methamphetamine is separate from the drug’s ability to produce a euphoric high. The findings suggest that these two common effects of stimulant drugs likely involve different underlying biological processes in the brain. This research indicates that a person might become more willing to work hard without necessarily feeling a greater sense of pleasure or well-being.

The researchers conducted the new study to clarify how stimulants affect human motivation and personal feelings. They intended to understand if the pleasurable high people experience while taking these drugs is the primary reason they become more willing to work for rewards. By separating these effects, the team aimed to gain insight into how drugs could potentially be used to treat motivation-related issues without causing addictive euphoria.

Another reason for the study was to investigate how individual differences in personality or brain chemistry change how a person responds to a stimulant. Scientists wanted to see if people who are naturally less motivated benefit more from these drugs than those who are already highly driven. The team also sought to determine if the drug makes tasks feel easier or if it simply makes the final reward seem more attractive to the user.

“Stimulant drugs like amphetamine are thought to produce ‘rewarding’ effects that contribute to abuse or dependence, by increasing levels of the neurotransmitter dopamine. Findings from animal models suggest that stimulant drugs, perhaps because of their effects on dopamine, increase motivation, or the animals’ willingness to exert effort,” explained study author Harriet de Wit, a professor at the University of Chicago.

“Findings from human studies suggest that stimulant drugs lead to repeated use because they produce subjective feelings of wellbeing. In the present study, we tested the effects of amphetamine in healthy volunteers, on both an effort task and self-reported euphoria.”

For their study, the researchers recruited a group of 96 healthy adults from the Chicago area. This group consisted of 48 men and 48 women between the ages of 18 and 35. Each volunteer underwent a rigorous screening process that included a physical exam, a heart health check, and a psychiatric interview to ensure they were healthy.

The study used a double-blind, placebo-controlled design to ensure the results were accurate and unbiased. This means that neither the participants nor the staff knew if a volunteer received the actual drug or an inactive pill on a given day. The participants attended two separate laboratory sessions where they received either 20 milligrams of methamphetamine or a placebo.

During these sessions, the participants completed a specific exercise called the Effort Expenditure for Rewards Task. This task required them to choose between an easy option for a small amount of money or a more difficult option for a larger reward. The researchers used this to measure how much physical effort a person was willing to put in to get a better payoff.

The easy task involved pressing a specific key on a keyboard 30 times with the index finger of the dominant hand within seven seconds. Successfully completing this task always resulted in a small reward of one dollar. This served as a baseline for the minimum amount of effort a person was willing to expend for a guaranteed but small gain.

The hard task required participants to press a different key 100 times using the pinky finger of their non-dominant hand within 21 seconds. The rewards for this more difficult task varied from about one dollar and 24 cents to over four dollars. This task was designed to be physically taxing and required a higher level of commitment to complete.

Before making their choice on each trial, participants were informed of the probability that they would actually receive the money if they finished the task. These probabilities were set at 12 percent, 50 percent, or 88 percent. This added a layer of risk to the decision, as a person might work hard for a reward but still receive nothing if the odds were not in their favor.

Throughout the four-hour sessions, the researchers measured the participants’ personal feelings and physical reactions at regular intervals. They used standardized questionnaires to track how much the participants liked the effects of the drug and how much euphoria they felt. They also monitored physical signs such as heart rate and blood pressure to ensure the safety of the volunteers.

Before the main sessions, the participants completed the task during an orientation to establish their natural effort levels. The researchers then divided the group in half based on these baseline scores. This allowed the team to compare people who were naturally inclined to work hard against those who were naturally less likely to choose the difficult task.

The results showed that methamphetamine increased the frequency with which people chose the hard task over the easy one across the whole group. This effect was most visible when the chances of winning the reward were in the low to medium range. The drug seemed to give participants a boost in motivation when the outcome was somewhat uncertain.

The data provides evidence that the drug had a much stronger impact on people who were naturally less motivated. Participants in the low baseline group showed a significantly larger increase in their willingness to choose the hard task compared to those in the high baseline group. For people who were already high achievers, the drug did not seem to provide much of an additional motivational boost.

To understand why the drug changed behavior, the researchers used a mathematical model to analyze the decision-making process. This model helped the team separate how much a person cares about the difficulty of a task from how much they value the reward itself. It provided a more detailed look at the internal trade-offs people make when deciding to work.

The model showed that methamphetamine specifically reduced a person’s sensitivity to the physical cost of effort. This suggests that the drug makes hard work feel less unpleasant or demanding than it normally would. Instead of making the reward seem more exciting, the drug appears to make the work itself feel less like a burden.

This change in effort sensitivity was primarily found in the participants who started with low motivation levels. For these individuals, the drug appeared to lower the mental or physical barriers that usually made them avoid the difficult option. In contrast, the drug did not significantly change the effort sensitivity of those who were already highly motivated.

Methamphetamine did not change how sensitive people were to the probability of winning the reward. This indicates that the drug affects the drive to work rather than changing how people calculate risks or perceive the odds of success. The volunteers still understood the chances of winning, but they were more willing to try anyway despite the difficulty.

As the researchers expected, the drug increased feelings of happiness and euphoria in the participants. It also caused the usual physical changes associated with stimulants, such as an increase in heart rate and blood pressure. Most participants reported that they liked the effects of the drug while they were performing the tasks.

A major finding of the study is that the boost in mood was not related to the boost in productivity. The participants who felt the highest levels of euphoria were not the same people who showed the greatest increase in hard task choices. “This suggests that different receptor actions of amphetamine mediate willingness to exert effort and feelings of wellbeing,” de Wit explained.

There was no statistical correlation between how much a person liked the drug and how much more effort they were willing to exert. This provides evidence that the brain processes that create pleasure from stimulants are distinct from those that drive motivated behavior. A person can experience the motivational benefits of a stimulant without necessarily feeling the intense pleasure that often leads to drug misuse.

The findings highlight that “drugs have numerous behavioral and cognitive actions, which may be mediated by different neurotransmitter actions,” de Wit told PsyPost. “The purpose of research in this area is to disentangle which effects are relevant to misuse or dependence liability, and which might have clinical benefits, and what brain processes underlie the effects.”

The results also highlight the importance of considering a person’s starting point when predicting how they will respond to a medication. Because the drug helped the least motivated people the most, it suggests that these treatments might be most effective for those with a clear deficit in drive.

The study, like all research, has some limitations. The participants were all healthy young adults, so it is not clear if the results would be the same for older people or those with existing health conditions. A more diverse group of volunteers would be needed to see if these patterns apply to the general population.

The study only tested a single 20-milligram dose of methamphetamine given by mouth. It is possible that different doses or different ways of taking the drug might change the relationship between mood and behavior. Using a range of doses in future studies would help researchers see if there is a point where the mood and effort effects begin to overlap.

Another limitation is that the researchers did not directly look at the chemical changes inside the participants’ brains. While they believe dopamine is involved, they did not use brain imaging technology to confirm this directly. Future research could use specialized scans to see exactly which brain regions are active when these changes in motivation occur.

“The results open the door to further studies to determine what brain mechanisms underlie the two behavioral effects,” de Wit said.

The study, “Effects of methamphetamine on human effort task performance are unrelated to its subjective effects,” was authored by Evan C. Hahn, Hanna Molla, Jessica A. Cooper, Joseph DeBrosse, and Harriet de Wit.