A new study has suggested that cocaine users are less able to predict loss than non-users, potentially explaining why the consequences of imprisonment or loss of relationships often aren’t enough to make them reconsider their use of the drug.
According to the 2013 National Survey on Drug Use and Health, around 1.6 percent of Americans age 12 and over had used cocaine at least once in the past year, and 0.6 percent had done so in the last month. Many of these will be struggling with addiction to the drug, and the new study goes some way to explaining how and why cocaine addiction persists through negative consequences of abuse.
‘Reward Prediction Error’
The study focused on what’s called a “reward prediction error,” a measure of the difference between the likely reward or loss associated with a behavior and the individual’s ability to predict the outcome. So if somebody assumes a positive outcome for a given behavior (for example, that putting a bet on a number at roulette will lead to a win) but then is confronted with a contradictory outcome (a different number comes up on the wheel), this is a reward prediction error. Most people will quickly learn from the error and change their behavior accordingly. Past research has shown that the prediction of reward or loss is closely tied to dopamine—one of the primary neurochemicals implicated in many addictions, including cocaine addiction—and that unexpected outcomes trigger changes in dopamine levels that encourage us to learn from the experiences.
The authors tested the hypothesis that reward prediction errors are impaired in those addicted to drugs and recruited 50 cocaine-addicted individuals and 25 healthy controls for the study. The researchers hooked the participants up to an EEG (an electroencephalograph), which measures electrical activity in the brain, and had them play gambling games where the odds of winning (25, 50 or 75 percent) were known. The task was to predict whether each trial would result in a win or a loss. Half of the cocaine users in the test had taken the drug within the last 72 hours, and half hadn’t, enabling additional testing of the impact of recent cocaine abuse.
Reward Prediction Error Signaling Issues
The control group behaved as theory would predict, showing changes in signaling in response to unexpected results, both positive and negative. The 50 cocaine-addicted individuals, however, didn’t show a change in signaling in response to an unexpected loss, thus indicating an impairment of the usual prediction-error-correcting mechanism of the brain.
One of the study authors, Muhammad Parvaz, PhD, explained, “This study shows that individuals with substance use disorder have difficulty computing the difference between expected versus unexpected outcomes, which is critical for learning and future decision making. This impairment might underlie disadvantageous decision-making in these individuals.”
However, there was an additional difference between the cocaine users who’d taken the drug within the last three days (who tested positive for the drug at the start of the study) and those who hadn’t. Those who’d taken cocaine recently responded to an unexpected win with an increase in activity—much like the healthy control group—whereas those who hadn’t used the drug recently had no such response. This shows that the issues with prediction error signaling are directly impacted by recent cocaine use. This was also taken as indicating that for addicted individuals, taking the drug is a form of “self-medication,” in this case meaning normalizing brain function in response to things like unexpected positive outcomes.
According to principal investigator Rita Goldstein, PhD, support for the self-medication hypothesis is interesting, but the true value of the finding lies in the potential for predicting susceptibility to addiction or relapse based on the impaired ability to predict loss and respond to unexpected negative outcomes. She even suggests that the findings may lead to targeted interventions.
Addicts Keep Using in Face of Negative Consequences
Bringing the results back into real-world terms, the study shows that those dependent on cocaine are less able to change their predicted outcomes for the same behavior in response to an unexpected negative outcome. When somebody takes too much cocaine and it leads to him or her missing work and consequently losing a job, or continued abuse leads to ruining a relationship with a partner, the study suggests that the ability to say, “If I continue to use cocaine, more negative consequences like this will likely occur” is damaged. This in turn makes it less likely that the user will alter his or her behavior in order to obtain a positive outcome. In short, this suggests that drug use continues because the brain isn’t sufficiently alerted in response to an error in prediction.
The finding concerns cocaine, but given that similar issues persist in people addicted to other drugs, it could well be that similar mechanisms exist for other substances, too. The practical value of the finding is limited until someone develops a way to correct this signaling, helping cocaine-addicted individuals understand the consequences of their abuse (like traditional rehab does) is the only thing that can be done. It does offer a greater understanding of what drives drug users to keep using, however, in both the inability to recognize when a behavior is likely to have a negative outcome and the desire to self-medicate to maintain some degree of ordinary brain functioning.