Putting episodic memory in context Cellular mechanisms of environmental processing

When we are asked to think of a memory we will often describe an important personal event such as a graduation, birthday or the first day of a new job. In memory research these types of memories for specific events or episodes from our lives are called episodic memories. These memories influence the kind of people we are and the way that we think about ourselves. Many of us have had the misfortune to experience how the loss of this type of memory can affect an elderly relative. In some types of dementia, like Alzheimer’s disease, sufferers lose the ability to form new episodic memories. Patients with AD can remember detailed events from 20 years ago but have severe problems when asked to remember what happened to them earlier that day. One way to go about tackling the disease is to examine how the brain usually processes episodic memory as this will allow us to see which areas and mechanisms within the brain go wrong in the disease.
Episodic memories are formed by combining the features of an event. These include where we were, what were the features of the environment (e.g. was it inside or outside?) and what we were doing at the time. An area of the brain called the hippocampus is important for combining these pieces of information. People who have damage to their hippocampus have problems creating new episodic memories. However, we do not know how these different types of information reach the hippocampus. At the moment we know a lot about how the brain processes spatial locations. However, we do not know how the other parts of episodic memory like the details of the environment in which we were are processed. The experiments supported by this grant have examined which networks in the brain allow us to remember the details of episodic memory and how these details are combined. The grant has been running for approximately 18 months and so far the key findings include:
1. The input pathway to the hippocampus from the lateral entorhinal cortex (LEC) shows increased activity, as measured by immediate early gene expression, when rats remember objects within specific contexts.
2. The LEC is necessary to enable rats to remember an object they have seen in a specific context. This is the rat equivalent of remembering where you left your keys.
3. The LEC is necessary to enable rats to remember combinations of information needed to form episodic memory including object, place and context.
4. The LEC is not necessary for remembering objects, places or contexts by themselves. It is only when these pieces of information are combined that the LEC is needed.