It’s unclear how the brain keeps track of the timing of events within a memory. One theory posits that, as memories are formed, temporal information about the experience is represented by gradual changes in activity in a particular population of neurons situated in the brain’s lateral entorhinal cortex (LEC, yellow region). These neurons, called temporal context cells, become active at the beginning of an experience—as a rat explores an arena, for example—and then relax gradually, at different rates. Other brain cells (not shown) may also become more active throughout an experience, or change their activity on a slower time scale, spanning multiple experiences. This information is fed into the hippocampus (pink region), which generates time cells. These cells become active sequentially at specific moments during an experience to mark the passage of time.
Some researchers hypothesize that, because the signal provided by the LEC is unique at any one time point, activity in this brain area could help timestamp memories themselves to allow temporal organization of individual episodes, in addition to marking time within experiences. Together, these records of time may help create the brain’s sense of when and in what order events happened, and could potentially aid the recall of memories later on by reinstating past patterns of activity.