Whether it’s meeting a friend for dinner at a new restaurant or hiking through a forest for the first time, new experiences lead to new memories. In order to create these memories, our brains do a lot of work behind the scenes. A region of the brain called the hippocampus ‘records’ experiences as they happen. It then replays these recordings later on, both while we are awake and once we have fallen asleep. During active behaviour, dopamine cells in a different part of the brain, called the ventral tegmental area (or VTA), recognize unexpected rewards, known as “reward prediction errors”, to guide learning. For example, was the chocolate mousse or conversation unusually good, or was there an unexpected and spectacular waterfall? In order to reap rewards for our future selves, we need to remember such unexpected rewards in the context in which they occurred. However, despite the importance of this ability, research has not yet established how the brain combines our experiences as we navigate the world around us with information about reward prediction errors in our memories. Gomperts et al. have now investigated this process by giving rats a memory task. In the first phase of the task, the rats were placed in an arena and directed to turn either left or right to receive food. In the second phase, the rats were given a choice as to whether to turn left or right. If they turned in the opposite direction to that required in the first phase, they received food rewards. The rats performed multiple trials of this task each day, and over a number of days the rats learnt the layout of the arena and the rule to locate the food. Gomperts et al. recorded the rats’ brain activity across the multiple trials of the task and after the task as the rats slept. As expected, the findings showed that the hippocampus replayed the rats’ movements through the arena, both between the trials and after the task during sleep. Between trials, the activity of VTA neurons that encoded reward-related information coordinated with the hippocampus’ memories of paths through the arena and indicated where in the arena the rats found food rewards. Unexpectedly however, coordination of these VTA neurons with the hippocampus was greatly reduced when the rats were asleep. This observation suggests that waking memories and sleep-associated memories may serve distinct purposes, for example, to aid learning and planning or to stabilize the memories. A key challenge now is to determine these distinct roles.
展开▼
