New paper published in eLife

Many events in our lives resemble experiences we have had before, without being identical to them. Whenever you attend a party, for example, you may well take along a gift, such as a bottle of wine or a box of chocolates, but the gift will differ on each occasion. Psychologists believe that as our memories for such events become older, the incidental details unique to each event (such as the identity of the gift) are mostly forgotten. However, the common underlying patterns (what parties are like in general) are retained. This allows us to accumulate knowledge to guide our behavior in similar situations in the future.

Studies in rodents and people have shown that a region of the brain called the medial prefrontal cortex stores long-term memories about experiences. But to what extent do neurons in this region represent abstract generalized knowledge as opposed to the specific incidental details? To find out, Morrissey et al. used hair-thin electrodes to record the activity of hundreds of cells in the medial prefrontal cortex as rats performed a learning and memory task. The rats learned that either a tone or a light signaled the delivery of a mild electric shock. Initially, cells in the medial prefrontal cortex responded differently to the tone and to the light. However, after three weeks, the cells began to show similar responses to both stimuli.

The medial prefrontal cortex activity had thus transitioned from representing incidental details (tone versus light) to representing abstract relationships (stimulus predicts shock). This may relate to how the brain extracts commonality across experiences. A lingering question is how cells in the medial prefrontal cortex become selective for abstract relationships. We know that memories are reactivated during sleep. Therefore, one possibility is that combined reactivation of different experiences selectively strengthens memories for any features common to those experiences.