Reward learning in cannabis users – Preliminary data from an fMRI study

Introduction: Recently, there has been consider-able interest in possible abnormalities of decision-making processes in drug abusers, including chronic cannabis abusers. Most of these studies administered the Iowa Gambling Task, a behavioral task that affords participants to learn to discriminate between response options that differ with regard to their short- and long-term outcomes. Unfortunately, the complex structure of the task hampers the identification of the specific cognitive processes that are responsible for faulty performance: It has been argued that some of the problems observed in patient populations may be caused by compromised adaption to changes of stimulus-reward associations. This ability seems to depend on striatal as well as ventral and medial frontal regions, which often show functional abnormalities in drug abusers. Nevertheless, there are few data concerning cannabis abusing populations.

Method: We compared reward-based learning of cannabis abusers and controls in an fMRI experiment. Reward-based learning was assessed by means of a probabilistic reversal learning paradigm, which affords a feedback-based discrimination between a positive and negative choice alternative, as well as an updating of this discrimination due to infrequent alternations in reinforcement schedules.

Results: Current analyses include a sample of N=11 chronic cannabis abusers and N=15 control participants with limited cannabis experience. Cannabis abusers reported smoking cannabis M=4.91 days a week. Questionnaire data indicate higher levels of impulsivity in the cannabis abuser group. Concurrent with these findings, behavioural data from the probabilistic reversal learning paradigm show that cannabis abusers also were more likely to switch between choice alternatives spontaneously. With regard to the functional imaging data, analyses for the whole group data broadly replicate previous studies, showing dorsomedial prefrontal and ventral prefrontal activations associated with behavioural switches. Due to the limited sample size (especially of the cannabis abuser group), we cannot detect reliable activations differences between the two groups, but applying a liberal significance threshold (p<.005 uncorrected), there is some evidence for activation differences in the anteromedial and dorsolateral prefrontal lobe, as well as posterior cingulate.

Conclusions: There is some evidence that cannabis abusers show behavioural and neural differences in a reward-learning paradigm, although interpretations remain tentative due to the limited sample of cannabis abusers.