The mantra for many decades in Alzheimer’s disease is that people who are at-risk or have the disease forget information quicker than people of the same age who do not have the disease.
This is certainly true for most people, however, a recent study (* Lim and colleagues, Neurology, 2020) actually suggests that it might not be so much about forgetting information, as learning new information.
What’s the difference you might ask?
It seems a small difference as to whether you more quickly forget or slower learn but it is actually quite important to understand how Alzheimer’s disease affects the ‘laying down’ of memories.
To understand how the ‘laying down of memories’ works, we need to take a step back and first of all understand that there are different types of memory, which work all slightly different in our brain. For Alzheimer’s disease, the memory type that is most affected is called ‘episodic memory’ and it refers to our memory for events and places in our lives. For example, remembering our wedding day or remembering to call your friend next week.
Episodic memory is reliant on many brain structures, but particularly the hippocampus (Latin for seahorse). The hippocampus is a critical part for our ‘laying down’ (encoding in scientific lingua), storing and retrieval of our episodic memories. In fact, the hippocampus is suggested to work as an index for memories. For each new memory of an event or place, the hippocampus creates an index where this memory can be found and retrieved from.
We know that Alzheimer’s disease affects the hippocampus and hence this indexing mechanism. This leads to newer memories having a faulty or no index created for them, meaning that people with Alzheimer’s disease cannot remember new events or places.
However, the indices in the hippocampus remain at the beginning of the disease relatively intact and therefore families often report that the person with Alzheimer’s disease ‘lives in the past’. With that, they mean that the person can still remember very well information from the distant past, say their wedding day, while recent events, say what they did on the last weekend, cannot be recalled. It is the faulty indices of the hippocampus which causes these memory problems.
Commonly people who are developing Alzheimer’s disease are therefore tested on memory task to see how quickly they forget information which was previously learnt. The rationale behind this approach is that we can check whether people who forget information quicker actually have a faulty index in the hippocampus. This faulty index can be a sign that this person has Alzheimer’s disease, as the disease commonly affects the hippocampus.
A recent study turned this mantra actually around and asked, is this really about forgetting information or do we not measure the intactness of this index better by the ‘laying down’ of new memories? The logic is that the laying down of new memories, such as by learning new information, is a much better test that the created index of the memory is faulty and potentially affected by Alzheimer’s disease.
Indeed, this is exactly what they found in their study, during which they required people either at higher or lower risk of developing Alzheimer’s disease. The researchers showed that people at higher risk of Alzheimer’s disease, people who have no apparent memory symptoms, are slower at learning new information than people who are at lower risk of Alzheimer’s disease.
Looking at learning might provide therefore a much better insight into the health of the hippocampal index. If Alzheimer’s disease already affects the hippocampus, it makes sense that we should be less efficient in creating new indices for episodic memories as the disease affects this mechanism.
For the future, episodic memory tests for detecting Alzheimer’s disease might therefore be more focused on how well we learn new information, instead of the traditional way of how quick we are forgetting it.
I certainly learnt some new information there…
*Original publication: Lim YY, Baker JE, Bruns L Jr, Mills A, Fowler C, Fripp J, Rainey-Smith SR, Ames D, Masters CL, Maruff P. Association of deficits in short-term learning and Aβ and hippoampal volume in cognitively normal adults. Neurology. 2020 Sep 4
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