Alterations in information processing (learning and decision making) play a role in the development of addiction and psychosis. In addiction, based on preliminary work, the following are postulated: (1) an increased reward response to substance-related stimuli, (2) a reduced reward response to non-substance-related rewards (such as food), (3) reduced cognitive control over reward processes, and (4) a reduced response to confrontation with the negative consequences of substance use. In psychosis, an altered weighting of prior beliefs and new information is postulated, drawing on concepts from Bayesian statistics. The exact nature of these alterations appears to vary depending on the clinical stage and the type of prior belief, and the evidence available to date is inconclusive. In both disorders, the described alterations in information processing are associated with specific pathologies of the dopamine system. In addiction, the role of dopamine in reward processes (which are altered in favor of the substance) is particularly important, while in psychosis, the function of dopamine as a learning signal appears to be disrupted. In our own work on addiction, we were able to show that the dopaminergic reward system of smokers reacts more strongly to substance-related stimuli and less strongly to alternative reward stimuli, and that it is more strongly activated in heavily drinking individuals when they make decisions to consume alcoholic beverages. Furthermore, we were able to show that the reactivity to threatening depictions of physical consequences of smoking does not differ significantly between smokers and non-smokers, but that in smokers, the preceding presentation of these depictions evokes control processes during the subsequent presentation of substance-related stimuli. Finally, and contrary to our initial hypothesis and some preliminary findings, we found no reduced activity in areas associated with cognitive control during the regulation of craving in smokers and during decisions to consume alcohol as the drinking severity increased in subjects with abusive alcohol consumption. In individuals with a higher (subclinical) psychosis proneness, we found a relative predominance of new information (a reduced influence of prior beliefs) in perceptual, but not in probabilistic decision making. Furthermore, we found an increased influence of prior beliefs regarding socially significant stimuli (faces and direct eye contact). Finally, we found an altered learning from new information in the sense of reduced suppression of unlikely / surprising information related to higher psychosis proneness. A limitation of the presented work is the small sample size. Furthermore, the results, especially of the studies on psychosis, were partly contradictory, even when taking into account previous work, with regard to the influence of prior beliefs in different tasks. Thus, the hope that the diversity of psychotic symptoms can be traced back to a few core pathologies does not seem to be fulfilled and more complex models have to be developed. Despite these limitations, the works represent important building blocks for the important endeavor of developing a common cognitive-neurobiological understanding of psychiatric disorders. The decisive validation step for the tests and markers derived from the theories (both neurophysiological fMRI markers and markers from computational modelling of behavior) is their translation into clinical practice. Future studies will have to show whether these markers are suitable for predicting diagnosis, prognosis and response to specific therapies, and thus for making a real improvement to the lives of the affected patients.
