Hippocampal replay

Hippocampal replay is a phenomenon observed in rats, mice, cats, rabbits, songbirds and monkeys. Although replay was first characterized in non-human animals, replay-like sequential reactivation has also been reported in humans using non-invasive methods such as simultaneous EEG–fMRI, where transient replay events are associated with hippocampal activity and coordinated changes across large-scale brain networks. During sleep or awake rest, replay refers to the re-occurrence of a sequence of cell activations that also occurred during activity, but the replay has a much faster time scale. It may be in the same order, or in reverse. Cases were also found where a sequence of activations occurs before the actual activity, but it is still the same sequence. This is called preplay.

The phenomenon has mostly been observed in the hippocampus, a brain region associated with memory and spatial navigation. Specifically, the cells that exhibit this behavior are place cells, characterized by reliably increasing their activity when the animal is in a certain location in space. During navigation, the place cells fire in a sequence according to the path of the animal. In a replay instance, the cells are activated as if in response to the same spatial path, but at a much faster rate than the animal actually moved in. Hippocampal replay has been proposed to support memory consolidation and the construction of internal "cognitive maps" of space and events. More recent computational and behavioral work suggests that replay may also contribute to planning and flexible decision making by allowing prospective sequences of states or actions to be evaluated before they are executed.