Rebecca Bodeker, Teaching Assistant in Behavioural Psychology, University of Canterbury, Randolph Grace, Professor of Psychology, University of Canterbury
Methamphetamine is New Zealand’s most harmful illicit drug and wastewater testing shows its use and availability are on the rise.
Much of the harm results from reckless and impulsive behaviour – including ram raids and gang violence – some people show when under methamphetamine’s influence.
Methamphetamine impairs decision making because it increases the likelihood of users acting impulsively, without regard to risk or long-term consequences.
Impulsivity is a maladaptive pattern of choice behaviour linked to crime, violence and drug use. Research shows higher levels of impulsivity are associated with a higher risk of initiating drug use, increase the amounts used, drop out of rehabilitation programmes and relapse.
Our new research investigates how methamphetamine affects impulsivity in rats. We argue that our findings are applicable to people and could improve treatments to reduce the high relapse rate of about 77% of methamphetamine users.
Stimulant drugs and impulsive behaviour
There has been substantial research and several tests have been developed to measure and define impulsivity. However, the effect of stimulant drugs such as methamphetamine on impulsivity remains unclear.
Some studies report amphetamines reduce impulsivity whereas others have found methamphetamine increases it. A probable cause of these conflicting results is the multi-dimensional nature of impulsivity.
Although often reported as a singular concept, impulsivity comprises several distinct but related components that must be assessed individually.
Laboratory research can help us better understand impulsivity. Specially designed behavioural experiments present animals with choices that provide an equivalent of those humans might experience.
The results can help us unravel the complex nature of impulsivity which we can then translate to human experience and inform treatment programmes. In our research we used rats to study two situations related to impulsivity.
The first is a choice between a smaller reward given sooner or a larger reward received later, known as “delay discounting”. The other choice is between a smaller but certain reward and a larger uncertain reward, known as “probability discounting”. We also examined how the overall magnitude of the rewards affected choice.
How we consider choices
In human studies, people are often asked to make hypothetical choices about money.
In delay discounting, opting for the smaller/sooner reward is an impulsive choice. For example, imagine you are given a choice between $400 now and $500 in one year, and choose the $400 now.
However, if you were asked instead to choose between $40,000 now and $50,000 in one year, you may select the delayed option. When the rewards are larger, we are less likely to choose impulsively.
In probability discounting, choice of the larger/uncertain reward is impulsive. Imagine you are given a choice between $50 for sure or a 50% chance at $100. You might be willing to gamble on the larger amount.
But what if your choice was between $5,000,000 for sure and a 50% chance at $10,000,000? You would be more likely to choose the certain reward because we tend to be less impulsive when the possible loss is greater.
Complex nature of impulsivity
In our research, rats could choose between two alternatives that resulted in food rewards by pressing levers in an experimental chamber.
Some rats completed delay discounting sessions in which they chose between smaller/sooner and larger/later food outcomes. Other rats completed probability discounting sessions and chose between smaller/certain and larger/uncertain outcomes.
We also varied the overall amounts of food to confirm rats were less likely to choose impulsively with larger amounts. We measured the rats’ sensitivity to differences between delay, probability and magnitude.
Results were similar to studies with humans in that the rats’ choices reflected trade-offs between delay, probability and the amount of food. Impulsive choices were reduced with larger amounts.
We then gave the rats gradually increasing doses of methamphetamine and observed how their choices changed. Our results reflected the complex nature of impulsivity. Increasing methamphetamine doses resulted in decreasing sensitivity to the most salient difference between the two choices the rats experienced.
On methamphetamine, more rats chose the larger delayed reward. This means a decrease in impulsive choice because sensitivity to delay was reduced and the smaller/sooner option was less attractive.
However, we found the opposite in probability discounting. Here, methamphetamine increased preference for the larger/uncertain reward, indicating an increase in impulsivity because sensitivity to risk decreased.
Sensitivity to magnitude also decreased, meaning rats were more likely to choose impulsively even when the reward was large.
What this means for people
People are obviously cognitively more complex, but methamphetamine users also demonstrate decreased sensitivity to risk in response to tasks similar to those we used with rats.
Therefore our findings are applicable to human methamphetamine users and highlight that long-term changes to impulsivity should be taken into account in treatment programmes. This is especially important because effects on decision making can persist long after drug use during periods of abstinence.
Psycho-education on impulsivity could be incorporated into existing treatment programmes. This would mean educating methamphetamine users about their increased risks related to decision making and how that may affect treatment outcomes.
The Salvation Army’s Bridge Programme, a well-known drug rehabilitation programme with 20 centres throughout the country, is a good example. It uses a community reinforcement approach as part of their treatment, which involves participants building skills to cultivate rewarding experiences outside of drugs or alcohol use.
Rehabilitation processes could implement a psycho-education component focused on the increased vulnerability to risky choices, regardless of amount, of current and former methamphetamine users. This could raise the personal agency of participants by making them more aware of the increased risk factors for relapse and other negative decision making.
Psycho-education could help release people from the grip of this pervasive and increasingly prolific drug.
The authors do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.
