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The When and Where of Memory

How the brain encodes time and place.

Each memory has three key elements: what, when, and where. Now neuroscientists at MIT have identified a circuit that may handle the when and where.

As explained in the video above, the newly discovered circuit connects the hippocampus and the entorhinal cortex. Neuroscientists know the hippocampus is key to memory formation, but the role of the entorhinal cortex is news, and it’s fascinating.

The entorhinal cortex splits each memory into two streams of information: one for location and one for timing. The hippocampus used to be where researchers thought these streams originated, but it appears that isn’t the case.

Inside the entorhinal cortex are two clusters of neurons researchers have dubbed “island cells” and “ocean cells.” Island cells link pairs of events that happen in rapid succession. Ocean cells create a representation of where the events happen.

These cells may also do more than make memories. When researchers used optogenetics to control island cells’ behavior with light, they discovered the speed at which they fire depends on how quickly the brain’s owner is moving. This suggests to neuroscientists that island cells may help people navigate through space, while ocean cells may help them keep track of where they are.