How the Brain Encodes Time and Place

When you keep in mind a specific experience, that memory has three important elements — what, when, and where. MIT neuro-scientists have now discovered a brain circuit that processes the “when” and “where” elements of memory.

This circuit, which joins the hippocampus and an area of the cortex well-known as entorhinal cortex, distinguishes location and timing into two sources of information. The investigators also discovered two populations of neurons in the entorhinal cortex that express this information, known as “ocean cells” and “island cells.”

Earlier models of memory had recommended that the hippocampus, a brain part crucial for memory development, distinguishes timing and context information. On the other hand, the new research shows that this data is broken up before it reaches the hippocampus.

Lead author of the study Chen Sun states that

“It indicates that there is a dichotomy of function upstream of the hippocampus, there is one path that feeds temporal information into the hippocampus, and one more that feeds contextual representations to the hippocampus.”

The study’s other lead author is Takashi Kitamura. The senior author is Susumu Tonegawa. Other authors are Jared Martin, Lacey Kitch, and Mark Schnitzer.

When and where

Situated just outside the hippocampus, the entorhinal cortex relays sensory details from other cortical places to the hippocampus, where memories are established. Tonegawa and co-workers recognized island and ocean cells a few years ago, and have been working since then to find out their functions.

Last year, Tonegawa’s lab claimed that island cells, which form small clusters bounded by ocean cells, are required for the brain to develop memories connecting two events that happen in rapid succession. In the new Neuron research, the team identified that ocean cells are needed to create representations of a place where an event happened.

Commenting on their study, lead author Chen Sun said,

Ocean cells are essential for contextual representations, when you are in the library, when you are crossing the road, when you are on the subway, you have various memories related with each of these contexts.”

To discover these characteristics, the scientists marked the two cell populations with a fluorescent molecule that lights up when it holds to calcium — a sign that the neuron is firing. This permitted them

to figure out which cells were active while in tasks requiring mice to differentiate between two different conditions, or to link two events in time.

The investigators also used an approach called optogenetics, which enables them to manage neuron activity using light, to examine how the mice’s behavior modified when either island cells or ocean cells were silenced.

When they obstructed ocean cell action, the animals were no more capable to associate a specific environment with fear after getting a foot shock there. Adjusting the island cells, at the same time, permitted the investigators to prolong or reduce the time gap between events that might be connected in the mice’s memory.

Information flow

Earlier, Tonegawa’s lab identified that the firing rates of island cells rely on how quick the animal is moving, leading the scientists to think that island cells assist the animal navigate their way through space. Ocean cells, at the same time, assist the animal to identify where it is at a given time.

The scientists also identified that these two streams of data flow from the entorhinal cortex to various parts of the hippocampus: Ocean cells deliver their contextual details to the CA3 and dentate gyrus regions, while island cells project to CA1 cells.

Tonegawa’s lab is now going after additional research of how the entorhinal cortex and other areas of the brain signify time and place. The scientists are also analyzing how information on timing and location are even more processed in the brain to create a entire memory of an event.

Another lead author Takashi Kitamura said, “To form an episodic memory, each element has to be recombined together, this is the next concern.”