Our memory as we remember a particular experience has three main elements what, when and where. In a new study which was published in a recent issue of Neuron, the scientists at the MIT have identified a brain circuit which they say processes the when' and where' elements of the memory. The work of this circuit, which links the hippocampus and a region of the brain called entorhinal cortex, is to separate location and timing into two streams of information. Two populations of neurons were identified in the entorhinal cortex by the researchers which convey this information, and they named them ocean cells' and island cells.'
Hippocampus is a critical part of the brain known for its function of memory formation. Previous studies had suggested that it is the part of the brain that separates timing and context information. But, this new study says that this information is split well before it reaches the hippocampus.
Chen Sun, an MIT graduate student in brain and cognitive sciences and one of the lead authors of the paper said that their study suggest that there is a dichotomy of function upstream of the hippocampus. One pathway conveys temporal information into the hippocampus, and another which gives contextual representations to the hippocampus.
This study is a collaborative work of various authors which includes MIT postdoc Takashi Kitamura, senior author is Susumu Tonegawa, the Picower Professor of Biology and Neuroscience and director of the RIKEN-MIT Center for Neural Circuit Genetics at MIT’s Picower Institute for Learning and Memory. Also there is Jared Martin, Stanford University graduate student Lacey Kitch, and Mark Schnitzer, an associate professor of biology and applied physics at Stanford.
The entorhinal cortex is located just outside the hippocampus, and it relays sensory information from other cortical areas to the hippocampus. A few years back Tonegawa and colleagues had identified island and ocean cells, and since then, they are working to discover their functions. Last year, Tonegawa's lab found 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 present study, the team has discovered that ocean cells are needed to note a location where an event took place.
Sun said that ocean cells are important for contextual representations. For every different setting – in the library, on the street, on the subway, etc., you have different memories associated with each different context. For the study, the researchers labeled the two cell populations with a fluorescent molecule that lights up when it binds to calciuma signal that the neuron is firing. This let the team determine which cells were active during tasks requiring mice to link two events in time.
The team made use of a technique called optogenetics, which let them control neuron activity using light. They used it to investigate the changes in mice’s behavior when either island cells or ocean cells were silenced.
The firing rates of island cells depend on how fast the animal is moving which suggested that island cells help the animal navigate their way through space. On the other hand, ocean cells help the animal in recognizing where it is at a given time. It was also found that these two streams of information flow from the entorhinal cortex to different regions of the hippocampus – ocean cells send their contextual information to the CA3 and dentate gyrus regions, while island cells project to CA1 cells.
The team of researchers is now working to find how the entorhinal cortex and other parts of the brain represent time and place. They also aim to find how information on timing and location is processed in the brain to create the full memory of an event.