The Wills and Cacucci labs are excited to announce the publication of their latest paper, out now in Current Biology.
The hippocampus is a brain region that is important for the formation and consolidation of episodic memories (memories of events, as well as where and when they happened). In rodents the hippocampus contains neurons that encode an animal’s current position in space (place cells). Different place cells are active in different discrete locations of an environment forming a neural map of space, necessary for animals to create spatial memories (for example learning the location of a reward in a maze). The consolidation of experiences is known to involve the ‘reactivation’ of place cell waking activity during sleep. We were interested when ‘reactivation’ emerges in early development. Episodic memory is mostly absent in young altricial mammals (including humans): we hypothesised that this may be due to a lack of reactivation in developing place cells.
We recorded the activity of large ensembles of place cells from the hippocampus in young rats (2-5 weeks old) while these explored an environment and also recorded the same neuronal ensembles in subsequent sleep. Surprisingly, we found that reactivation of place cells’ waking activity is present even in the youngest animals. However, reactivated networks represented only single locations in the environment the rat had just visited. When adults sleep, instead, place cells ‘replay’ sequences of place cell firing corresponding to extended trajectories through the environment the animal had previously explored. This ‘replay’ of place cell sequences, which allows the hippocampus to stitch together different locations into ordered sequences, emerges gradually during the following few weeks, a time span that correlates well with the period when spatial memory emerges in rats.
Our results might therefore suggest a reason why young children do not form episodic memories of experiences: the developing hippocampus cannot yet link different elements of an event to a cohesive memory, but only stores individual fragments of it.
You can find the full paper here