Addiction
How does meth dependence change learning and choices?
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People with methamphetamine dependence rely on simpler, short-term decision strategies and show lower expectations of non-drug rewards compared to healthy individuals.
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How does meth dependence change learning and choices?
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16 sober individuals with methamphetamine dependence
Sixteen (40% female; mean age = 35.4) sober MDI were recruited from a 28-day inpatient Alcohol and Drug Treatment Program at the Veterans Affairs San Diego Healthcare System and Scripps Green Hospital (La Jolla, CA).
16 matched healthy controls
In addition, 16 healthy comparison subjects (HCS; 33% female; mean age = 37.1) were recruited via flyers, internet ads (e.g., Craigslist), and local university newspapers.
Key findings
While both groups earned similar overall scores, 50% of the methamphetamine-dependent participants relied on a simple win-stay/lose-shift strategy rather than a more complex learning strategy. Those who did use a learning strategy showed a lower reward maximization bias (1.58 compared to 6.99 for controls) and started with a lower prior expectation of reward (0.11 compared to 0.40 for controls). Additionally, reduced gray matter volume in the thalamus was linked to lower reward maximization, which fully explained the difference between the groups.
Methodology
The researchers compared 16 sober individuals with methamphetamine dependence and 16 matched healthy controls as they played 20 rounds of a two-armed bandit game. During the 16 trials of each round, participants chose between two options with fixed but unknown reward rates to earn points for cash. Researchers analyzed their choices using computational modeling to separate learning biases from decision strategies, and examined brain structure using magnetic resonance imaging.
Limitations
The study utilized a small sample size of only 16 participants per group, which limits the generalizability of the findings. Because brain structural differences were measured at a single time point, the study cannot determine if lower thalamic volume caused the decision-making deficits or resulted from prolonged drug use. Finally, because the participants were currently enrolled in a 28-day treatment program, these findings might not apply to active users or those with other psychiatric conditions.
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