We have all been there – we come back home and place something not in its usual place and then cannot find it sometime later. Sometimes it is just keys and sometimes it is wallet of phone. One of the ways to remember the location of the misplaced possession is to track down our previous route.
Science says that it is because environmental contexts are strongly linked to memory formation. Now researchers at the RIKEN-MIT Center for Neural Circuit Genetics have conducted a research to figure out how does this mechanism work.
This new research shows that contexts are first differentiated by a type of neuron in the temporal lobe of the brain called Ocean cells. They encode surrounding contexts and are crucially important for the formation of memories that make contextual learning possible.
Scientists think that memories of events, called episodic memories, are established in part through a neural circuit between two neighbouring brain regions—the hippocampus and the entorhinal cortex. Up until now scientists did not manage to pinpoint the exact location where contexts are first represented in the brain. Many scientists for a long time thought that it is the hippocampus that generates context-specific information, but this new research shows that this information is already formed in the entorhinal cortex and only reaches hippocampus later.
As usually is the case, research process in this study was rather complex and difficult to explain to common public. Also, as usually is the case, scientists used mouse models for their experiments. At first they used live calcium imaging to visualize brain activity in active mice. Calcium imaging helps to observe brain activity as high levels of calcium indicate that neurons are working.
Researchers placed markers in two types of entorhinal cells—Ocean cells and Island cells. This allowed them to observe the response of these cells to different contexts. Then team placed mice in two different rooms and found that Island cells always showed the same amount of activity regardless of the room the mouse is put in. However, Ocean neurons were more active in either one or another room. This, according to scientists, means that Ocean cells differentiate environmental contexts.
In the next step researchers looked at entorhinal cells and how they affected context-related neural activity in the hippocampus. Researchers found that the activity of hippocampal neurons also discriminated between the two rooms and for an experiment inhibited Ocean cells optogenetically with green light while the mice were in the rooms. Scientists found that this cancelled the ability of hippocampal neurons to discriminate between the two rooms. Inhibiting Island cells had no effect. Scientists say that this proves that identifying a context is function of the Ocean cells and after doing so they send this information to the hippocampus.
Our ability to make associations with and later recognize our surroundings shapes our contextual learning. In other words, past experiences are associated with contexts we are placed in. For example, it takes only one bad visit to the dentists for us to feel anxious and uncomfortable next time waiting in the dentist’s office. This is called fear conditioning and is actually a pretty useful type of contextual learning, when scientists are trying to study learning and memory in animals.
When during this research scientists inhibited Ocean cells in mice during fear conditioning, they found that the characteristic freezing response was greatly reduced when mice were later placed in the same room. This means that the mice never associated the room’s context with the fearful experience in this experiment.
Takashi Kitamura, lead author of the study, said: “we have now discovered a role for Ocean cells in episodic memory that compliments that of Island cells. Ocean cells contribute the contextual components of an experience, while Island cells contribute the temporal information. The next step is to understand how the two components are integrated in the brain to form memories”.
Entorhinal cortex is one of the first regions to be affected by Alzheimer’s disease – losses in this brain region can already be observed in very early stages of the disease. This means that by enhancing knowledge about how Ocean and Island cells contribute to specific types of memory formation, scientists can think of new ways to develop markers to improve early-phase diagnosis of Alzheimer’s disease. The earlier the diagnosis, the more help there is for the patient to slow down the progress of the disease, which means that innovative diagnostic techniques cannot come soon enough.