This study has several limitations First despite the

This study has several limitations. First, despite the relatively large number of participants for a pharmacological neuroimaging study, sample sizes for examining the modulation of THC effects by COMT CCT007093 were small. Therefore, these particular findings should be interpreted with caution and require replication, as we cannot exclude the possibility that these results are false positives or that additional findings were undetected due to limited statistical power. Second, inclusion of incidental cannabis users, as opposed to non-users, may affect interpretation of results as previous cannabis use may have affected the endocannabinoid system. The choice for incidental cannabis users was based on ethical grounds (van Hell et al., 2011b). Third, although the study was double blind, THC induced behavioural effects that could be identified by most subjects, causing expectancy effects across participants. We tried to limit these effects by randomising the order of drug administration. Finally, we acquired a relatively short resting state fMRI scan of four minutes, whereas a duration of at least eight minutes is commonly used (van den Heuvel et al., 2008).
Conflict of interest
Contributors
Role of funding source Funding for this study was provided by Top Institute Pharma, project number T5-107. Dr Bossong was supported by a Veni fellowship from the Netherlands Organisation for Scientific Research (grant number 016.166.038). The funding sources had no further role in study design; in the collection, analysis and interpretation of data; in the writing of the report; and in the decision to submit the paper for publication.
Acknowledgment <br> Introduction The interest in psychological issues related to sports has been on the rise over the last two decades. Many authors believe that the results achieved by athletes depend equally on their physical, technical-tactical and psychological preparation. Recognising temperament and personality traits of athletes constitutes a basic knowledge to be used both by trainers and athletes. By knowing whether an athlete is even-tempered, irritable, withdrawn or anxious, one may precisely develop suitable forms and methods of training (Parzelski & Mieńkowska, 2007). Numerous studies have investigated personality and temperamental profile of athletes striving to answer the question whether success in sport may be linked to a specific set of traits or not. The results obtained thus far do not allow unequivocal determination of traits which distinguish athletes from non-athletes or features which are characteristic of winners and losers (Singer, Hausenblas, & Janelle, 2001). In recent years, numerous authors have investigated the role of genes in personality and temperament. The results of these studies indicate that genetic factors may significantly contribute to the development of personality and temperamental traits (Strelau, 1998). Some temperamental features are related to emotions and contribute to the emergence of emotional reactions, acting as modifiers of stress states by increasing or lowering emotional reactions to stressors (Strelau, 1996). The role of temperamental traits is most visible when an individual functions in stressful conditions and has to perform a task which requires exceptional mobilisation of the body (Strelau, 1996). This is the case of athletes, who spend a great number of hours on training, sparring and taking part in competitions, thus being exposed to stress and immense physical and mental pressure. Jan Strelau, the author of Regulative Theory of Temperament (RTT), defines temperament as biologically determined personality traits. RTT does not determine neurophysical and biochemical mechanisms specific to a given trait, but assumes the existence of the so-called neurohormonal individuality, which encompasses complex, genetically determined biological mechanisms of temperamental traits.