object can be used to recall the spatial context. Pattern separation Pattern separation is the ability to differentiate one memory from other stored memories. Pattern separation begins in the dentate gyrus, a section of the hippocampus involved in memory formation and retrieval. Granule cells in the dentate gyrus process sensory information using competitive learning, and relay a preliminary representation to form place fields. Place fields are extremely specific, as they are capable of remapping and adjusting firing rates in response to subtle sensory signal changes. This specificity is critical for pattern separation, as it distinguishes memories from one another. Reactivation, replay, and preplay Place cells often exhibit reactivation outside their place fields. This reactivation has a much faster time scale than the actual experience, and it occurs mostly in the same order in which it was originally experienced, or, more rarely, in reverse. Replay is believed to have a functional role in memory retrieval and memory consolidation. However, when replay is disturbed, it does not necessarily affect place coding, which means it is not essentially for consolidation in all circumstances. The same sequence of activity may occur before the actual experience. This phenomenon, termed preplay, may have a role in prediction and learning. Model animals Place cells were first discovered in rats, but place cells and place-like cells have since been found in a number of different animals, including rodents, bats and primates. Additionally, evidence for place cells in humans was found in 2003. Rodents Both rats and mice are often used as model animals for place cells research. Rats became especially popular after the development of multiarray electrodes, which allows for the simultaneous recording of a large number of cells. However, mice have the advantage that a larger range of genetic variants are available. Additionally mice can be headfixed, allowing for the use of microscopy techniques to look directly into the brain. Though rats and mice have similar place cells dynamics, mice have smaller place cells, and on the same size track have an increase in number of place fields per cell. Additionally, their replay is weaker compared to the replay in rats. In addition to rats and mice, place cells have also been found in chinchillas. Rats furthermore have social place cells, cells which encode the position of other rats. This finding was published in Science at the same time as the report of social place cells in