In a new publication out in Current Biology, we describe the impact of reward size on memory formation and how memories coupled to a large reward are formed and stabilized faster.
Our ability to remember certain experiences in our lives depends in part on how we perceive their value. Generally, if an experience is perceived to have a large value, we are more likely to remember details of the experience, such as the location where it occurred.
“Rewards can not only enhance the consolidation of a memory, they also promote memory encoding and accelerate learning during the experience itself,” explains Fabian Kloosterman. His lab has now dissected the neural mechanisms underlying this value-based memory by comparing the brain signaling at spatial locations coupled to high and low rewards.
A dynamic place map in the brain
Memories related to spatial location are primarily formed and consolidated in a brain structure called the hippocampus, which interacts with several other brain structures to create a memory of where you are at the time of an event. Mechanisms of hippocampus-dependent memory formation have been studied most extensively in rodent models.
“In rodents, during active exploration, individual neurons in the hippocampus are preferentially active when the animal crosses a particular location in the environment. Collectively, the activity of these place cells forms a map-like representation of space in the brain. But the formation of this cognitive map is also dependent on experience and sensitive to change, a process referred to as ‘remapping’,” Kloosterman explains. Similar mechanisms are at play in humans.
Larger payoff, stronger memory
Studies in rodent models typically include a reward to motivate the animal to explore and complete a task. While rewards affect the dynamics of place map formation, the effect of reward size was not clear. The Kloosterman Lab addressed this question by measuring the dynamics of the hippocampal map when rodents navigated between two different locations each containing a reward of either low value (one food pellet) or high value (nine food pellets).
“We saw a quick reorganization of hippocampal place-cell activity over the course of a few trials, and this process was accelerated if animals received a large reward,” explains Frédéric Michon, first-author on the study. “The reorganization was driven by both rate changes and the appearance and disappearance of place fields.”
The occurrence of hippocampal replay—reactivation of experience-related neural patterns—largely followed these dynamics. The number of reverse replays of the path to the large reward gradually increased, which may have contributed to a faster reorganization of the place code.
Kloosterman explains that “high reward value likely enhances memory retention by accelerating both the formation and stabilization of the hippocampal cognitive map and by selectively enhancing its reactivation during learning.”