INTEGRATING REINFORCEMENT LEARNING AND CHOICE FREQUENCY TO INVESTIGATE HABITUAL BEHAVIOR

Habits are integral to daily life, yet their development remains poorly understood. We studied the role of behavioral repetition in habit formation. Using a computational model in an instrumental-learning binary-choice task, we investigated whether choice frequency affects behavior in addition to and independently from reinforcement. In this pre-registered fMRI study (https://osf.io/wbs8n/), 60 participants completed a binary-choice instrumental-learning task, learning stimulus-response-outcome associations under time pressure. We compared several computational models formalizing learning from reinforcement and/or from behavioral frequency. A model combining effects of reward and frequency best predicted choices in a test phase in nominal extinction, indicating that past choice frequency, independent of rewards, influences behavior. Generalized linear mixed-effects models provided converging evidence, showing participants preferred stimuli chosen more frequently before, regardless of reinforcement value. At the neural level, BOLD responses in the ventral striatum correlated with reinforcement values (t=4.19, p<0.05 family-wise-error small-volume corrected) and those in the dorsolateral striatum with choice frequency (t=4.23, p<0.05 family-wise-error small-volume corrected). However, not all preregistered effects were detected. This pattern suggests that reinforcement learning and choice-frequency mechanisms may rely on partially distinct neural circuits that jointly contribute to decision making, particularly at the level of action selection. Speculatively, frequency-based habits may arise from interactions between dorsolateral striatal and motor-related circuits that bias behavior through repeated action tendencies. In summary, by combining preregistered computational modeling with hypothesis-driven fMRI analyses, this study provides evidence that habitual behavior reflects the interplay of reward learning and action repetition.