Psychopharmacology (2012) 219:981–990 occasional users of cannabis (see also Van Hell et al. 2010). It may also be possible that the effect of THC on striatal activity is domain specific, and limited for anticipatory reward activity. There have not been any previous studies examining effects of THC on striatal reward processing activity to compare our results to, but previous studies using different cognitive paradigms have shown attenuation of ventrostriatal activity after THC during retrieval of memory (Bhattacharyya et al. 2009) and an increase in brain activity in the caudate during response inhibition (Borgwardt et al. 2008), which would be in line with the hypothesis that THC effects may be domain specific. The results from the current study should be interpreted with due care. Our sample size was small for an fMRI study which compares data between sessions. A larger sample could have revealed more effects of THC, for instance, during task anticipation in striatal areas. In addition, although the study was designed to be double blind, THC induced behavioral effects that were identified by most subjects, possibly causing expectancy effects across sessions. The influence of expec- tancy was minimized by using a randomized cross-over design, thus balancing expectancy effects across sessions. Still, it cannot be excluded that expectancy effects may have affected our results to some extent. In conclusion, this study provides new arguments for eCB involvement in the reward system in humans. Findings suggest that THC affects appreciation of obtaining a monetary reward. The involvement of the eCB system in feedback processing may be relevant for disorders in which apprecia- tion of natural rewards may be affected such as addiction. Acknowledgements The PhICS study is performed within the framework of Top Institute Pharma, project number T5-107. 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