Change comes in many flavors; if we move to a new country, city or house it is easy to recognize the novelty of the environment, but if we come home to find the furniture rearranged or a new piece of art on the wall, this recognition may be much slower. Scientists believe this is because memory formation requires comparing current information with previous experience and the larger the overlap, the more difficult the distinction. It has long been known that the hippocampus is a region of the brain crucial for this type of memory, however the identification of neurons responsible for this comparison has remained elusive.
In this study Marie Wintzer, Roman Boehringer, Denis Polygalov and Thomas McHugh used genetically modified mice and advanced cell imaging techniques to demonstrate that while the entire hippocampus is capable of detecting large changes in context, the small and often overlooked CA2 region is exquisitely sensitive to small changes.
Mice were familiarized with one context and then placed either in a much different context or back in the original with small alterations, such as several new small objects. By detecting the expression of activity induced genes Wintzer and colleagues were able to demonstrate that just a few new objects in the otherwise unchanged context completely altered the pattern of active cells specifically in CA2. Mice that had been genetically engineered to lack this CA2 response explored the new context much less than their normal siblings.