Learning and memory are the foundation of individual survival and development. Improving learning and memory is the focus of psychology and neuroscience. Recently, many studies have revealed that rewards facilitate declarative memory, and the influence of reward on declarative memory has become a hot research topic.
Rewards are related to the midbrain dopamine system, including areas such as the ventral tegmental area, the substantia nigra, and the ventral striatum, with dopamine as the relevant vital neurotransmitter. The hippocampus and adjacent cortices play an essential role in the encoding, consolidation, and retrieval of declarative memory. The midbrain reward system and the memory system (i.e., the ventral tegmental area and hippocampus) are connected both structurally and functionally. Rewards can act on memory encoding and consolidation, thus promoting memory performance. During the encoding and consolidation stages, rewards promote memory via the interaction of various brain systems (i.e., the reward system, the attention control system, and the memory system). The impact of rewards during these two stages involves different cognitive processes and neural mechanisms.
During the memory encoding stage, rewards affect both intentional and incidental memories. According to the intentional memory paradigm, participants are explicitly informed that a reward is contingent upon memory performance in a subsequent test when they encode the items. In this paradigm, this performance-dependent reward triggers the reward system and involves the attentional control system, and these two systems modulate the memory system, allocating more cognitive resources to reward-related items, thereby promoting memory with respect to these items. According to the incidental memory paradigm, rewards accompany some items during the encoding phase but are independent of memory performance regarding these items in the subsequent test. In this paradigm, participants are not aware of the subsequent memory test before they process the information; thus, the reward enhancement effect on memory can mainly be explained in terms of the interaction between the memory system and the reward system. However, even though participants do not intentionally allocate cognitive resources in this context, the rewarded items themselves automatically attract attention. Therefore, the influence of attention and the involvement of the attentional control system cannot be excluded entirely.
During the memory consolidation stage, the addition of a reward also affects memory performance, and the influence of attention can be excluded entirely at this stage; thus, the enhancement effect on memory consolidation can be explained in terms of pure reward. During the consolidation stage, the hippocampal memory system reactivates the encoded content. The reward facilitates dopamine release, modulates the hippocampal processing of reward-related items, and enhances the reactivation of reward-related items, thus directly affecting memory performance without the involvement of the attentional control system.
Future research should focus on the following three areas. First, rewards affect behavior not in terms of a simple and pure enhancement pattern but rather according to a complex pattern. The factors and mechanisms that impact the effect of rewards on memory must be clarified, and a more consummate model of the reward effect on memory should be developed to provide more accurate guidance for learning in real life (i.e., a model of when and how rewards should be applied in education). Second, only a few studies have investigated the effects of rewards during the memory consolidation and retrieval stages. More attention should be given to the effects of rewards during these two stages (i.e., the ways in which rewards affect consolidation during different states as well as memory retrieval and subsequent memory). Finally, most studies have investigated the effects of external rewards on memory, and future research should focus on the impacts of internal rewards on learning and memory. We should compare the behavioral patterns and neural mechanisms associated with the effects of internal and external rewards on memory and test the interaction effect of internal and external rewards on memory.