How the brain encodes time and place

When you remember a particular experience, that memory has three critical elements: what, when and where. MIT researchers have now identified a brain circuit that processes the “when” and “where” components of memory. This circuit, which connects the hippocampus, a brain structure known to be critical for memory formation, and a region of the cortex known as the entorhinal cortex, separates location and timing into two streams of information. Previous models of memory had suggested that the hippocampus separates timing and context information, however, this new study shows that this information splits even before it reaches the hippocampus. The researchers identified two populations of neurons within the entorhinal cortex that convey this information to the hippocampus, which they have dubbed “ocean cells” and “island cells.” According to the neuroscientists, island cells help the brain to form memories linking two events that occur in rapid succession, and ocean cells are required to create representations of a location where an event took place. By using optogenetics, which allows for control of neuron activity using light, the researchers found that the firing rates of island cells depends on how fast the subject is moving, leading the researchers to believe that island cells help the subject navigate their way through space. Ocean cells, meanwhile, help the subject to recognize where it is a given time. Next the researchers plan to investigate how timing and location information are further processed in the brain to create a complete memory.

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