Impaired flexible reward learning is associated with blunted reinforcement sensitivity and attenuated learning and choice signals in ventral striatum and parietal cortex of ADHD patients

Reward-based learning and decision-making are prime candidates to understand symptoms of attention deficit hyperactivity disorder (ADHD). However, only limited evidence is available regarding the neurocomputational underpinnings of the alterations seen in ADHD. This particularly concerns the flexible behavioral adaption in dynamically changing environments, which is challenging for individuals with ADHD. One previous study points to elevated choice switching in adolescent ADHD, which was accompanied by disrupted learning signals in medial prefrontal cortex. In the present study, we investigated young adults with ADHD (n=17, 18-32 years) and age and sex matched controls (n=17, 18-30 years) using a probabilistic reversal learning experiment during functional magnetic resonance imaging (fMRI). The task requires continuous learning to guide flexible behavioral adaptation to changing reward contingencies. To disentangle the neurocomputational underpinnings of the behavioral data, we used detailed reinforcement learning (RL) models, which informed the analysis of fMRI data. ADHD patients performed worse than controls particularly in trials before reversals, i.e., when reward contingencies were stable. This pattern resulted from noisy choice switching regardless of previous feedback. RL modelling showed decreased reinforcement sensitivity and enhanced learning rates for negative feedback in ADHD patients. At the neural level, this was reflected in diminished representation of choice probability in the left posterior parietal cortex in ADHD. Moreover, modelling showed a marginal reduction of learning about the unchosen option, which was paralleled by an equally marginal reduction in learning signals incorporating the unchosen option in the left ventral striatum. Taken together, we show that flexible behavioral adaptation in the context of dynamically changing reward contingencies is impaired in ADHD. This is due to excessive choice switching ( hyper-flexibility), which can be detrimental or beneficial depending on the learning environment. Computationally, this results from blunted sensitivity to reinforcement. We detected neural correlates of this blunted sensitivity to reinforcement in the attention-control network, specifically in the parietal cortex. These neurocomputational findings are promising but remain preliminary due to the relatively small sample size.