Most people have experienced the recollection of a vivid memory after encountering a trigger, such as a familiar place, object, or smell. In neuroscience, this process is called ‘pattern completion’. For years people have thought that a particular part of the brain, called the ‘trisynaptic circuit’ of the hippocampus, might be responsible for this process; however, we lacked the tools to test this idea in a fully satisfying way. Recently, other labs have developed the tools to do so: the ability to measure what is happening in a large part of the brain while simultaneously changing what is happening in a different part of the brain. This project will take advantage of these tools to test the idea that the trisynaptic circuit is responsible for pattern completion. We will measure what is happening in one part of the hippocampus while turning off the trisynaptic circuit. If our theories are correct, then the hippocampus should no longer recreate an entire memory from its parts when the trisynaptic circuit is turned off, and mice should no longer remember the location of a prior reward. If our theories are wrong or only partially right, then the hippocampus should still recreate the entire memory or most of the memory from its parts, and mice should remember the location of a prior reward. The outcome of these experiments will allow us to come up with more specific theories about this important feature of memory and help us better understand cases in which memory does not function correctly.