Families and carers of people with dementia often report that the person with dementia seems to be increasingly ‘living in the past’. It seems a strange symptom, as people with dementia can often recall detailed events from long ago but have difficulties remembering what just happened a few days ago. How is this symptom caused by dementia? And can it tell us something about how our memory system in the brain is affected by dementia?

Let’s find out.

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Families or carers of people with dementia often report that the person with dementia seems to be ‘living in the past’. What they mean by that is that person with dementia is often re-telling stories from the past and can remember events from the past in great detail, whereas their memory for recent events is only ‘sketchy’. It seems an odd occurrence.

How can a person with memory problems remember those long-ago event but not recent events? Shouldn’t the whole memory be affected by the disease?

Indeed, this can be a very puzzling phenomenon. But once we understand a bit more how our memory works and it can be differently affected by dementia, it should start to make sense why we see such a difference between recent and remote memories in dementia. To start understanding this better we need to first understand that there are different types of memory.

Memory types

Many people assume that memory is ‘one thing’, we just remember whatever we want to recall and our brain delivers it but that is in fact not true. There are different types of memory, which can be differentially affected by disease such as dementia. At the top-most level, there is a distinction between memory types that we can access consciously or not. For example, many of our habits or learned skills are mostly unconsciously remembered. This makes perfect sense for our everyday behaviour. Imagine you would have to remember how to pick up a glass, to write a sentence or to ride a bicycle, every time you would have to do that activity. It would be extremely time-consuming. Instead, our brain transfers those habits and skills to our unconscious mind, so we can do them without even thinking about them. The beauty of unconscious (scientifically called implicit) memories is that we recall them without even thinking about them and can retrieve them even after a very long time. You might not have cycled a bicycle for many years but if you learnt it you could get on a bike and instantly ride off. If we think about it, it is quite miraculous how our brain remembers this. Of course, there is also a dark side to this, as anyone can confirm who tried to ‘kick a habit’. That is in fact very difficult to do, which is of course epitomised by the saying that one ‘cannot teach an old dog new tricks’. This is actually not correct, as you can teach an old dog a new trick/habit but it will take a long time and a lot of patience. Just remind yourself of that if you are trying to change one of your habits. The good news is that implicit memory remains intact for most people with dementia until the very late stages of the disease, so we need not be too concerned about them. By contrast, conscious or explicit memories are far more affected by dementia.

There are – roughly – two types of explicit memory: episodic memory and semantic memory. Let’s explore semantic memory first. Semantic memory is our memory for knowledge or facts. For example, we know (or should know) that Paris the capital of France, that a cup is a vessel to drink from and that most mammals have four legs (with the odd exception). In essence, semantic memory is the memory for facts and rules in the world. We often do not think about semantic memory, since we learn most of our facts during childhood and early adulthood. But through our life we accumulate more and more facts, meaning that often older people have the best semantic memory of all age groups (finally a cognitive skill which is better in older age!). It is maybe therefore not surprising that participants in knowledge-based quiz shows are mostly middle-aged or older since knowledge-based quizzes tap exactly into semantic memory by asking for facts. It also explains why older people are often keener on crosswords, whereas they can be anathema for younger people. Not only is semantic memory often better in older people than younger people, but is also little affected in the beginning stages of dementia. Most people with dementia will remember facts and their knowledge very well at the beginning of the disease (the only exception is the aptly named semantic dementia, now called semantic variant primary progressive aphasia – svPPA). As an older person, you might therefore easily beat your children or grandchildren in board games such as Trivial Pursuit or Scrabble (which rely heavily on semantic memory) but you would really struggle to beat them in board games such as Memory. Memory requires you to remember pairs of objects and taps into the other type of explicit memory – episodic memory.

Episodic memory

Episodic memory, as the name gives away, is our memory for episodes or events in our life. It is really this memory that is affected across most dementias, but particularly Alzheimer’s disease. When we talk about what we did last year for our holiday or last weekend, then we are using episodic memory. It is the memory we mostly associate with when we talk about ‘our memory’. Those event memories range across our whole lifetime, with most of us having their first episodic memories when we are at pre-school age – with very few people having reliable episodic memories before the age of 3. In fact, having episodic memory allows us to have continuity throughout our lives and makes us who we are (as already the philosopher John Locke realised in the 17th century). What do I mean by that? Just imagine, we cannot remember what we did a while ago but people told us that were there. It would be quite disturbing to realise that we might not remember this event at all. In fact, we might doubt whether they might mean another person as we have clearly no recollection of this event. If this happens repeatedly, it might create doubts in us as to who we actually are as a person, as we do not have a continuity of our remembered experience.

This imagined experience gives somewhat of a glimpse of how it must feel to have dementia since we cannot remember many events but people tell us we were there! We literally cannot ‘trust’ our memory or even ourselves anymore, since we do not remember whether we are the same person which attended those events or not. It should come, therefore, not as a surprise that our self-confidence hits rock bottom, as we doubt ourselves constantly, and explain why many people with dementia become so hesitant or even withdrawn as they cannot be sure anymore what they remember.

However, long ago memories remain still at the disposal of people with dementia and hence they often retrieve those memories. It makes them feel safer as they clearly remember those events and who they were then. One could see these long-ago memories as a comfort zone, people with dementia retreat to. But to the outside world, it can appear as if they are ‘living in the past’ mostly now, with a constant retelling of old stories and events, the family has likely heard many times over. This can be clearly annoying to the families and particularly if the person with dementia gets people from now confused with those from the past they remember clearly. But we should remind ourselves, that this retreat to older memories that are intact, is good for the person with dementia, as it allows them to be more confident and no doubts all the time what they remember.

So, how do the changes in the brain explain this ‘living in the past’ and remembering long ago but not recent memories?

The critical memory role of the ‘seahorse’

Episodic memory is processed in different brain regions, however, one brain region is particularly critical for episodic memory – the hippocampus. The hippocampus (Latin for seahorse) is a region that is critical for our processing of sensory (in particular visual), place and time information (as an aside, it is therefore also an important region for spatial navigation and explain the spatial and temporal disorientation seen in Alzheimer’s disease). The hippocampus integrates all this information together and is, therefore, the perfect place for the generation of episodic memory. If we think about it, events are the experience of sensory information at a particular time and place. Sounds too abstract? Let’s take then an example. Let’s imagine we went with our family to a restaurant for Sunday lunch. In the best case, we can relive some of that restaurant visit in front of our ‘inner eye’, by seeing where everybody sat, what they wore, what they ate, how the food smelled or who we burned our tongue on the too-hot soup. This is a highly specific episodic memory as it allows us to draw on highly detailed sensory information (what people wore, smells, burning sensation) at a specific place (restaurant) at a specific time (Sunday lunchtime). Now, the hippocampus is the ideal place to create such memories as it integrates all the sensory, place and time information.

Does this mean that the hippocampus is the brain region where our episodic memories are stored?

This is in fact still controversially discussed in the scientific literature, however, most scientists would agree that the hippocampus is not the place where our memories are stored. Instead, the hippocampus is a place where the links or indices for our episodic memories are stored. So, instead of thinking of the hippocampus as the storage area of episodic memories, it should be thought of as an indexing mechanism for our episodic memories. We can compare it to an internet search engine. The internet search engine does not store the actual websites you are searching for, instead, it has the link/index of that website we are searching for. Entering the right search terms will deliver the correct website. It is a very similar system with the hippocampus who has links/indices for episodic memories which are stored in different parts of the brain. Of course, the indexing system of the hippocampus is more complex than an internet search engine as it has not only one link/index but multiple links/ indices for each memory, maybe one for sensory experiences, one for the place, one for the time and so on. If we want to remember a certain episodic memory (that Sunday restaurant lunch with our family) we just have to activate or trigger those links/indices and voila we will have the memory in front of our ‘inner eye’. We can literally relive that experience.

What happens then in dementia?

‘Living in the past’

We know already that the hippocampus is affected in many dementias, but particularly Alzheimer’s disease. This means that the dementia disease processes start to affect the indexing system in the hippocampus. As a consequence, the indexing mechanism is becoming faulty and the indices for memories are not arranged properly anymore or only in a very impoverished way. It results in people with dementia having only gist memory for recent events as the indices are not as rich anymore or not properly arranged. For example, they might still remember going for lunch on Sunday but will have difficulty remembering who was there or what they ate. For some events, the index might not get arranged at all anymore in the hippocampus and people will, therefore, have no recollection whatever of those events.

How come then that older memories seem to be much more intact than more recent memories?

For that, we need to understand that episodic memory indices in the hippocampus are much stronger for older memories than newer memories in general. The strength of the indices increases the more often we retrieve that information. Since we retrieve memories from long ago more often during our lifetime, those will become stronger whereas newer memory indices are more ‘fragile’. It should become, hopefully, clear why people with dementia often seem to be ‘living in the past’, since they can still retrieve the memories from long ago – as the stronger indices are more resilient against the dementia changes affecting the hippocampus. But for newer memories, there is much less chance that the indices are still arranged or if so, then only in an impoverished way. It is, therefore, pointless to convince a person with dementia that a recent event happened, as they literally have no index for that memory in their brain and hence the event never seem to have happened to them.

The indexing of memories via the hippocampus raises another interesting concept in that the memory problems in the early stages of dementia are often more ‘access problems’. The memories might be still stored in the brain but cannot be accessed anymore. Imagine you would have lost the keyword to finding a website via the internet search engine. The website still exists somewhere but you cannot find it anymore. It is very similar in dementia with the hippocampus memory indexing. However, for certain memories we might trigger the remnants of an index and people with dementia might still remember this information. This can be quite astonishing to observe, in particular when people have moderate dementia and seem from the outside to have very little memory at all anymore. For example, music has been shown to ‘trigger’ such long-ago memories even in the more advanced stages of dementia (please also have a look at my recent article on music and dementia, explaining in more detail what happens in the brain). Similarly, reminiscence therapy which elicits memories from long ago is also often highly enjoyable for people with dementia. And no wonder, since the episodic memory indices from long ago are likely more intact for them and they can actually remember those events and reminiscene on them. It is a pleasurable experience for them to talk about those old memories and has been shown to clearly affect their mood and confidence.

Summary

In summary, the ‘living in the past’ memory symptom in dementia and particularly Alzheimer’s disease can be explained by the changes to a critical episodic memory region in the brain the hippocampus. The hippocampus is a brain region that creates memory links/indices for our episodic memories. The dementia disease process affects the hippocampus, which in turn affects the creation of indices for new memories. The indices for newer memories are not all or only in an impoverished way created, resulting in people with dementia not being able to recall recent events at all or only in a gist format. However, the indices for older memories are usually much stronger and hence people with dementia can still recall them in the early stages of the disease. Since they rely more on those older memories, it appears as if they are ‘living in the past’, since newer memory indices are not or only partly created. Providing, therefore, a space in which people with dementia can talk about their old memories is a good thing and will not only allow the families to connect with the person with dementia and will also improve their confidence and well-being.

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Links

• https://www.alz.org/alzheimers-dementia/10_signs
• https://www.alzheimers.org.uk/about-dementia/symptoms-and-diagnosis/symptoms/memory-loss

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