Past studies have suggested that the memory-forming hippocampus breaks down a memory into time and location. A new study by researchers from the Massachusetts Institute of Technology (MIT) proved that the brain circuit responsible for the recall of time and location breaks down a memory into two segments—when and where—before it even reaches the hippocampus.
The brain circuit connects the hippocampus to a cortex region called the entorhinal cortex where two populations of neurons have been identified—the "ocean shells" and the "island shells." These two neuron groups provide sensory data from other cortical areas to the hippocampus.
"There is one pathway that feeds temporal information into the hippocampus, and another that feeds contextual representations to the hippocampus", said Chen Sun, one of the study's lead authors. Their findings suggest a separation of function in the hippocampus.
Sun's team also found that the two streams of information reach two different recipients inside the hippocampus. "Ocean cells" provide logistics to the CA3 cells and dentate gyrus regions. "Island cells" provide time-based information to the CA1 cells.
In 2014, MIT researchers explained that in order to form two events that happened simultaneously, "island cells" will form clusters. The "ocean cells" will then surround the clusters of "island cells." This is needed for the brain to remember linked events.
The MIT researchers observed the activity of mice as they recall certain memories. By labelling the two neuron populations using a technique called optogenetics, the neuroscientists were able to distinguish the change in brain activity and behavior when a certain neuron population was blocked.
The mice received a foot shook in a particular environment. With the ocean cells blocked, the mice were unable to remember experiencing a foot shock—and a dose of fear—when they revisited the same environment. By tinkering with the "island cells," the researchers can manipulate how long the animals can recall two simultaneous events.
The team is gearing up for another in-depth study to see how the brain processes time and location to complete a memory. The research showed how a specific data is supplied by a specific brain part. The next step is to see how the brain recombines segregated data to form an episodic memory.