Humans are motivated to actively seek information to reduce uncertainty, which has been shown to alter episodic memory (Shen et al., 2022). Specifically, we found that uncertainty during hypothesis testing was both linearly and quadratically related to episodic memory. Yet, little is known about the neural mechanisms underlying how hypothesis testing relates to subsequent memory. In the current fMRI study, 40 participants were presented with three multi-dimension keys. They were instructed to figure out the target feature of a key to open a treasure chest. Reinforcement learning model was used to capture decision uncertainty around different features of the keys. We replicated our prior findings and showed that a reinforcement learning model captured hypothesis-testing behavior and there was a quadratic relationship between decision uncertainty and memory, such that memory was enhanced at the intermediate level of decision uncertainty. In terms of neural results, we found that the quadratic term of decision uncertainty was coded in the ventral striatum. We also found that decreasing decision uncertainty was related to greater activation in the ventral striatum, anterior and posterior hippocampus, and ventromedial prefrontal cortex, while increasing decision uncertainty was related to greater activation in the ventral tegmental area. More importantly, we found that activation in the ventral striatum in response to the quadratic term of decision uncertainty correlated with the quadratic relationship between decision uncertainty and memory, such that participants with greater activation in the ventral striatum showed a more pronounced quadratic relationship between decision uncertainty and memory. Together, this work extends existing research on how uncertainty influences memory via changes in motivation in the framework of hypothesis testing. / Psychology