Being concerned about our memory is a common occurrence when we age. But which specific memory symptoms might indicate that we are developing dementia? And are there are other types of memory that are less affected by dementia?

Let’s find out.

‘Fifty shades of memory’

People are often surprised that there is more than one type of memory. When we usually talk about memory, we mean one specific type of memory – our memory for events in the past. Such event-based memory is referred to as episodic memory in science. “Remember that time, when we got caught in a thunderstorm on a cycling tour and had to shelter in a barn?”, would be a typical episodic memory. Episodes are clearly defined in time and space, such as the thunderstorm during the cycle ride. We might not remember the whole cycle ride but we definitely remember the thunderstorm and where we sheltered. This gives already another clue that episodic memory is highly selective as to what is remembered. And that is a good thing, just imaging we would remember every moment of every day, it would be simply overwhelming. So, it is our brain which ‘selects’ the episodes we remember.

How does the brain ‘select’ those episodes to remember?

A very good question but a very difficult one to answer, as we don’t exactly yet understand the mechanisms which lead to episodic memory encoding. Encoding means figuratively how these episodes are laid down in our brain. Still, we know already some of the factors which determine how well we remember episodes in our lives. One critical aspect is that we need to pay attention to what is happening as to remember it. As my exasperated wife will tell you, I am often not paying attention and hence have no recollection whatsoever of what my beloved wife has just told me.

However, if we pay attention we should remember very well events or episodes in our life. Of course, we all know that since our time at school, when teachers tried to get our attention to follow the material they wanted us to learn. And similarly, if we revise for an exam, we will do much better remember the information when we are paying attention to learning that material. Such focused learning is only one form of episodic memory encoding and is actually not common in everyday life. For example, we did not learn the episode when we sheltered in a barn from a thunderstorm when we were on a cycling ride. But we still remember that. How is that possible?

The other major factor for us remembering events is that we had a heightened emotional experience during that time. We can, of course, have positive or negative emotional experiences but regardless of their emotional valence (positive vs. negative), we will remember those events much better. Going back to our thunderstorm memory, it was clearly a heightened emotional experience when we got caught by the thunderstorm during the cycle. At first, it was quite a negative and frightful experience which was then followed by a positive and relieving experience of finding shelter. The perfect scenario for creating an episodic memory. If we think back through our lives it is such emotional memories that really seem to ‘stick’ in our memory. For many people, it will be their wedding day or another significant event, such as the birth of a child, a near-miss event or even an accident. Any event having a significant emotional experience will likely stick in our memory.

Why are emotional experiences so well remembered?

The short answer is we do not know. And often when scientists don’t have an explanation they speculate that it might be due to evolution. It’s the same case for emotion contributing significantly to our episodic memories. I guess this makes sense, as we want to remember on an evolutionary basis emotional events. Replace the thunderstorm with a lion and the barn with a tree and hey presto you have an evolutionary memory which is clearly important for our survival. Avoid lions in that place! But as I said, it is a neat explanation for which we do have only limited evidence to date. Still, even on a brain level, it makes sense that emotions are important for our memory as some of our emotional processing centres in the brain lie literally adjacent to brain areas critical for episodic memory. So, it makes sense that these brain regions work closely with each other, which is often the case in the brain – brain areas close to each other have similar or shared functions.

So, it’s all about episodic memory then?

Not quite, there are actually several types of memory we use every day but episodic memory is the one we usually talk about when talking about memory.

What are the other types of memory?

The first thing to realise is that our memory functions are split into explicit vs implicit memory types. With explicit, it means memories we have conscious access to, so that we can retrieve those memories at will if we want to. Implicit memories are a bit more mysterious as we use them every day but we have only limited or even no conscious access to them. For explicit/conscious memories there are two main types: episodic memory and semantic memory. Now, we know already episodic memory but many might have never heard of semantic memory, even though we use it every day.

Semantic memory

Semantic memory describes our memory for our everyday knowledge. For example, we know that a cup is a cup or that Paris is the capital of France. It is knowledge, we have acquired over our lives and is not specific to a specific event. This differentiates it from episodic memory, which is highly specific to an event. By contrast, semantic memory has no specific relation to a time or place you might have experienced it. Instead, it is more of a generic knowledge we have of the world. As I said, we use this knowledge every day without thinking about it. But let’s step back for a minute and just consider how much knowledge we need every day to function normally. We not only know how most objects in our environment work, despite some of them have truly bizarre mechanisms if we think about it. If you have ever been baffled by how an unfamiliar can opener works, then you will have realised how important semantic memory is for our everyday life.

Despite us being little aware of this knowledge, we still have conscious access to it, as we can learn new semantic knowledge all the time. Indeed, the longer we live the more semantic knowledge we have since we have more time to accumulate knowledge. Semantic memory is therefore often the strongest in older people, whereas their episodic memory is less strong than younger people. It is therefore maybe not a surprise that older people are often fond of crosswords or games such as Scrabble since one can excel at those activities with large semantic knowledge. So, despite having poorer episodic memory when we age, we should console ourselves that our semantic memory is still improving.

The other important aspect of semantic memory is that it remains unaffected by most types of dementia at the beginning of the disease. The exception are certain types of frontotemporal dementia, such as semantic variant primary progressive aphasia (also aptly called semantic dementia) and behavioural variant frontotemporal dementia. In particular people with semantic dementia have problems with semantic memory right from the beginning of the disease. It often results in them having fairly good episodic memory but very poor semantic memory. For example, many people with semantic dementia struggle to name pictures of objects and animals or lose the ability to use objects correctly (those blasted can openers again!), while still being able to tell you in detail what they did the last weekend or holiday (episodic memory). Clinicians often ask therefore people with dementia to name picture of animals to check whether their semantic memory is intact or impaired. To most people with dementia these seem nearly infantile tests (when where you asked the last time to name a picture of a kangaroo – very likely at school) but those tests are to spot semantic memory problems. But just to re-iterate most people with dementia will have no problems with semantic memory – at least in the early stages of the disease. The same applies to the other category of memory – implicit memory.

Implicit memory

Implicit memories, as the name gives away, are implicit and therefore not under our conscious control. We have therefore little or no conscious access to them by using our will, instead, it seems like magic that our brain can rely on those memories and perform everyday actions. So, what are those mysterious implicit memories? There are several types of implicit memories but most relevant for dementia are skills and habits.

We all know what skills and habits are but many of us might not know that they are actually part of our unconscious memory. But if we think about it, it makes perfect sense, since we do not want to remember consciously many of our skills or habits every time we need to do them. Just imagine having to remember how to do a step forward with your foot, every time you take a step. It would be very time consuming and we might not move very fast. So, after we learn how to makes steps as a toddler this skill it becomes unconscious and we don’t think about it anymore and just use it every day unconsciously – unless we have an injury or accident and we have to ‘re-learn’ to walk. It makes therefore perfect sense for our brain to have such skills in our unconscious memory as it would be an enormous waste of time to remember each skill every time you need it. Even typing on the computer this sentence I am using my unconscious finger typing skills to get this sentence down onto the screen. Just imagine all the other skills we acquire over our lifetime, from reading to riding a bicycle to driving a car and so on, our lives are full of unconscious memories and the skills we need every day.

It is a similar story for habits, which are basically unconscious behaviours we all have. These can range from amusing habits (for example which side of the bed we prefer to sleep on) to tics to more serious obsessive-compulsive behaviours which can literally take people’s lives over. Habits are hard to break, as any smoker can attest, as they are deeply rooted in our unconscious memory and it takes a lot of conscious effort to change those unconscious memories. On average, it takes at least 3 months of conscious efforts to change a habit and even then there is no guarantee that the change will ‘stick’ to our unconscious memory. But if we make it we can truly change our lives, for better or worse. For worse, of course, is a lot about bad habits, such as smoking and drinking which have the additional negative aspect of being addictive. But we can also use it for good and for example change our lifestyle, by being more physically active or eating a more balanced diet. And we all know that this can make a significant difference to our risk for dementia in the future. So, habits can be a good thing if we can change our unconscious memory over time.

So, how are all these types of memories affected in dementia?

Relevance for dementia

It might not come as a surprise that episodic memory is by far the best known and most common memory symptom in dementia, and particularly Alzheimer’s disease (see also my entry on the ‘living in the past’ symptom’ in dementia, here). In fact, most diagnostic tests for dementia are heavily tended towards episodic memory problems, by requiring people to learn a name and an address or to try to remember words, pictures or more abstract objects. Performance on such tests provides clinical people with an insight into our episodic memory symptoms and allows a diagnosis of Mild Cognitive Impairment or even dementia. One problem with episodic memory is that it declines ‘naturally’ with age. This means that we have poorer episodic memory when we are older than when we are younger. If you have ever tried playing such games as the apt named Memory with your children or grandchildren you will know how hard it is to beat them, as hard as you may try. This decline with ageing means that episodic memory symptoms might not be as specific for dementia when we are older. For example, your episodic memory might not be as good anymore when you are in your 90s but maybe that is not a sign of impending dementia but more a sign that episodic memory has naturally declined. By contrast, if your episodic memory is quite affected in your 60s it might be more of a warning sign that you have potentially the onset of dementia. Of course, I am talking here about averages and people differ for their episodic memory performance over age but on average episodic memory gets worse the older we get, which is a fact. So, relying solely on episodic memory changes for a dementia diagnosis requires caution and of course, clinicians know that and check other symptoms as well to corroborate their diagnosis.

How about the other types of memory? Are they relevant for dementia?

Good question. In general, the other memory types are less relevant for the diagnosis of dementia but become more relevant when the disease progresses or for specific dementia subtypes. We already covered that certain forms of frontotemporal dementia can have quite significant semantic memory problems right from the onset of the disease. For other dementias, such as Alzheimer’s disease, semantic memory deficits are more subtle at the beginning of the disease and often overshadowed by the more significant episodic memory symptoms. But when the disease progresses in people with Alzheimer’s disease their semantic memory deficits can get worse. This can affect in particular their everyday functioning as we need our semantic knowledge to perform most of these functions. So, even if someone with Alzheimer’s disease can remember to do something (episodic memory) they might struggle on how to do it (semantic memory). Knowing this distinction clearly helps us to support people with dementia to stay longer independent. Specifically, it is not sufficient to remind people what or when to do something but they might also help on how to do it, without taking over the activity for them. The focus on episodic memory has therefore not really helped to support people with dementia as best as possible to address their memory problems, which encompasses episodic and semantic memory deficits, albeit the latter is more subtle.

Similarly, implicit memories are often not recognised by clinicians or professionals carers in people with dementia. One reason is that these implicit memories are much harder to measure and detect. But the flipside is that when the explicit memories (episodic & semantic memory) are affected by the disease, people with dementia often rely more on implicit memories as they remain intacts until quite late in the disease. This reliance on implicit memory results in people reverting back mostly to habits and skills which have been acquired for a long time and people with dementia can still perform these skills for a long time. Focusing therefore activities or managements of symptoms on the intact skills or habits or people with dementia makes perfect sense but has been, in my opinion, little explored on a systematic level.

Overall, different types of memories are affected by dementia. Those types of memories are not only differently affected by different types of dementia but also over the disease. The latter has been so far little recognised for memory symptom management in dementia.

Summary

Taken together, there is not only one type of memory but several. The most commonly known is episodic memory which is our memory for events or episodes in our lives. Episodic memory is commonly affected in dementia and hence a significant part of dementia diagnostics. But there are also other types of memory. Along with episodic memory, semantic memory is an explicit memory which is our knowledge of the world not associated with a specific time and place. Semantic memory is more subtly affected at the start of the disease, except for specific types of frontotemporal dementia, and can be therefore overshadowed by episodic memory problems. However, semantic memory problems become more significant once the disease progresses and can affect the everyday functioning of people with dementia, in addition to their episodic memory problems. Finally, implicit memories are the least explored in dementia, as they are more difficult to measure and not affected in the disease until very later. However, using implicit memory to help people with dementia to stay independent might provide an avenue to support them better but has so far been little explored. This is clearly a topic that requires further investigation as it might make a significant difference to people with dementia and their families and focuses on what they can still do instead of a deficit-focused approach.

Let’s remember that!

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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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