Awake and asleep, brain is busy making memories

Hippocampal activity during waking hours plays a key role in new memory consolidation and retrieval, as shown by a research team led by Margaret Carr and Shantanu Jadhav at the University of California, San Francisco.

The hippocampus is commonly associated with the process of converting short-term memories into long-term ones. However, it is uncertain when this occurs, and how long the process lasts. Evidence suggests several properties of the hippocampus are associated with memory formation.

Consolidation of memories is primarily associated with sleep, and retrieval of memories, with wakefulness. This study furthers the idea that the same process underlies both functions of the hippocampus.

Lesions in the hippocampus lead to severe impediment in the retrieval of recently stored memories. This indicates that the hippocampus is crucial in memory retrieval for at least a short period after initial exposure.

Memory replay occurs on a much shorter time scale relative to the original experience itself, and is able to occur in the absence of any physical reenactment of the behavior.

It was found that sharp wave ripples (SWRs), an activity pattern demonstrating reactivation of stored experiences, are essential to memory storage. Although SWRs were first discovered during sleep, recent studies show evidence of such activity during the waking state as well, indicating that memory consolidation occurs during both waking and sleeping states.

Furthermore, memory replay was found to follow specific sequential patterns in experiments where animals ran a course for reward. SWR activity occured in the forward direction directly prior to the beginning of a run, while reverse replay occurs after the animal reaches its reward.

Interestingly, reverse replay doesn’t occur during sleep, which suggests that hippocampal reactivation of past experiences is controlled to serve different functions. While forward replay allows consolidation of memories, reverse replay promotes learning by linking current activity to previous knowledge.

The results of this study also showed that sensory information can trigger replays of stored representations. Reactivation usually begins at experience that reflects the present location, and moves in either a forward or reverse direction.

Sensory cues thus allow stored memories to guide behavior and anticipate future encounters.

Many questions call for future experimentation, including the method that experiences are stored by to allow both forward and reverse replay, the length of time hippocampal replay requires and whether or not it is present for the entire process of consolidation and the exact relationship between the hippocampus and neocortical regions at which long-term memories are ultimately stored.