Abstract
When a stimulus is associated with an external reward, its chance of being consolidated into long-term memory is boosted via dopaminergic facilitation of long-term potentiation in the hippocampus. Given that higher temporal distance (TD) has been found to discount the subjective value of a reward, we hypothesized that memory performance associated with a more immediate reward will result in better memory performance. We tested this hypothesis by measuring both behavioral memory performance and brain activation using functional magnetic resonance imaging (fMRI) during memory encoding and retrieval tasks. Contrary to our hypothesis, both behavioral and fMRI results suggest that the TD of rewards might enhance the chance of the associated stimulus being remembered. The fMRI data demonstrate that the lateral prefrontal cortex, which shows encoding-related activation proportional to the TD, is reactivated when searching for regions that show activation proportional to the TD during retrieval. This is not surprising given that this region is not only activated to discriminate between future vs. immediate rewards, it is also a part of the retrieval-success network. These results provide support for the conclusion that the encoding-retrieval overlap provoked as the rewards are more delayed may lead to better memory performance of the items associated with the rewards.
Highlights
Episodic long-term memory is crucial to our daily lives given that our behavior and decisions are based on past experience
We ensured that the task-related performance during the encoding task was independent of possible differences in memory performance among conditions by performing one-way analyses of variance (ANOVA) on response time (RT) and classification accuracy
In further analyses, we claim that the task-related performance of each condition during the encoding session does not account for possible differences in memory performance of each condition
Summary
Episodic long-term memory is crucial to our daily lives given that our behavior and decisions are based on past experience. The core of episodic memory formation in the brain is thought to be the medial temporal lobe (MTL), which comprises the hippocampus (HC) and the surrounding gyri [1]. Various studies suggest that successful memory formation does not solely depend on MTL functions and relies on other cortical regions such as the prefrontal cortex (PFC), the premotor cortex, or the posterior parietal cortex (PPC) [2]. Not all events benefit from these regions to be transformed into long-term memory. To enhance efficiency in memory storage, salient stimuli–either rewarding or aversive– are better remembered than other neutral items [3]. Dopamine neurons in the substantia nigra and the ventral tegmental area (VTA), two regions
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