Effects of subclinical depression on prefrontal-striatal model-based and model-free learning

Depression is characterized by deficits in the reinforcement learning (RL) process. Although many computational and neural studies have extended our knowledge of the impact of depression on RL, most focus on habitual control (model-free RL), yielding a relatively poor understanding of goal-directed control (model-based RL) and arbitration control to find a balance between the two. We investigated the effects of subclinical depression on model-based and model-free learning in the prefrontal-striatal circuitry. First, we found that subclinical depression is associated with the attenuated state and reward prediction error representation in the insula and caudate. Critically, we found that it accompanies the disrupted arbitration control between model-based and model-free learning in the predominantly inferior lateral prefrontal cortex and frontopolar cortex. We also found that depression undermines the ability to exploit viable options, called exploitation sensitivity. These findings characterize how subclinical depression influences different levels of the decision-making hierarchy, advancing previous conflicting views that depression simply influences either habitual or goal-directed control. Our study creates possibilities for various clinical applications, such as early diagnosis and behavioral therapy design. Author summary Human decision making is known to be driven by at least two distinct processes, goal-directed and habitual learning. Previous studies argued that these systems and their interaction are disrupted in depression. However, we have limited understanding of the integration of the two systems and how this case extends to early or mild depression. We used a computational model and fMRI to address this issue. We found that depression-related changes were observed in the different levels of the decision making process. Notably, we found that depressive individuals have higher sensitivity of the habitual learning process, indicating the impairment of the proper integration of the two. Our findings raise the hope about developing clinical applications for the early diagnosis of this disorder, as well as using behavioral/cognitive therapy or brain stimulus techniques for restoring the balance between goal-directed and habitual learning in individuals with subclinical depression.