The untouched water of hippocampus replay.
Oct 25, 2025
Concept of interest: Sharp Wave Ripples
In this post, we see that from a structural point of view, the hippocampus is a prominent structure. When we look at the activity of the hippocampus in time, the most prominent window is during so called Sharp Wave Ripples, which is when many neurons fire together, achieving a temporarily high co-activity. These events mostly happen when the animal is in a stationary state, such as sleeping, eating, or grooming. Although transient, 50 ms to 300 ms each and altogether less than 2% of time is devoted to those high firing moments, those bursts are essential for learning as removing some of these events are sufficient to induce learning deficit in rats and making these events longer helps rats learn faster.
Since these Sharp Wave Ripples (SWR) are so magical, the field has been trying to understand how these neuronal co-firing helps learning. One clue comes from the fact that during some of the SWRs, the order in which neurons fire recapitulate the same order in which they fire during running. Since neurons in the hippocampus tend to fire a particular locations during running, we can read the neuronal firing order to find which spatial trajectory the neurons likely represent or replay.
However, not all SWR events come with these clear trajectories. In fact, SWR events with more neuronal firing tend to be those without clear trajectories. However, these SWRs are largely untouched by the field. My main PhD project is on deciphering these events.
