Washington: Researchers at MIT have identified a brain circuit that processes the location and timing of our memory. This circuit, which connects the hippocampus and a region of the cortex known as entorhinal cortex, separates location and timing into two streams of information.
The researchers also identified two populations of neurons in the entorhinal cortex that convey this information, dubbed “ocean cells” and “island cells.” Previous models of memory had suggested that the hippocampus, a brain structure critical for memory formation, separates timing and context information. However, the new study shows that this information is split even before it reaches the hippocampus.
“It suggests that there is a dichotomy of function upstream of the hippocampus,” said one of the lead authors Chen Sun, from the Massachusetts Institute of Technology. “There is one pathway that feeds temporal information into the hippocampus, and another that feeds contextual representations to the hippocampus,” Sun said.
Located just outside the hippocampus, the entorhinal cortex relays sensory information from other cortical areas to the hippocampus, where memories are formed. In 2014, the researchers reported that island cells, which form small clusters surrounded by ocean cells, are needed for the brain to form memories linking two events that occur in rapid succession. In the new study, the team found that ocean cells are required to create representations of a location where an event took place.
For the study, researchers labelled the two cell populations with a fluorescent molecule that lights up when it binds to calcium – an indication that the neuron is firing. This allowed them to determine which cells were active during tasks requiring mice to discriminate between two different environments, or to link two events in time.
The researchers also used a technique called optogenetics, which allows them to control neuron activity using light, to study how the mice’s behaviour changed when either island cells or ocean cells were silenced. When they blocked ocean cell activity, the animals were no longer able to associate a certain environment with fear after receiving a foot shock there.
Manipulating the island cells, meanwhile, allowed the researchers to lengthen or shorten the time gap between events that could be linked in the mice’s memory. Previously, researchers found that the firing rates of island cells depend on how fast the animal is moving, leading the researchers to believe that island cells help the animal navigate their way through space.
The researchers also found that these two streams of information flow from the entorhinal cortex to different parts of the hippocampus: Ocean cells send their contextual information to the CA3 and dentate gyrus regions, while island cells project to CA1 cells. The study was published in the journal Neuron.
