The fragile memory
Do you and your brother have different recollections of who knocked over the Christmas tree when the cat ran away and the Christmas porridge got burnt? Perhaps you are both wrong. That is the claim of researchers who study how we remember our lives.
One of the most famous patients in brain research is a man referred to as Patient HM in the medical literature. He suffered from epilepsy which eventually gave him around ten seizures per day. The doctors knew where the disease originated from and in an attempt to suppress the seizures, a neurosurgeon removed large parts of the hippocampus as well as adjacent areas from both sides of the brain.
When HM woke up from the operation, he did have fewer epileptic seizures, but his memory ability was also severely impaired. Large chunks of the years leading up to the operation were gone. However, his greatest loss was that he could no longer create new long-term memories of events that happened after the operation. Yet, HM was able to remember his loved ones, his own name och recognise himself in the mirror. He was also able to remember skills he had acquired earlier in life. But if he met a new person, who then left the room briefly, he could not remember meeting them, even though it had just happened some minutes before.
When his mother passed away he was unable to remember her passing, and he was upset every time it came up. He was also unable to recognise his aging appearance, and became startled by his reflection in the mirror. At the same time, his childhood memories were intact. HM willingly took part in different studies, which provided researchers with the opportunity of understanding basic things about our memory.
He was, among other things, asked to draw a figure by looking in a mirror, not on the actual paper. The exercise was repeated many times, and each time HM was asked if he had drawn the figure before. He always replied that he had not. At the same time he learned how to draw the figure more and more flawlessly. The researchers concluded that the ability to remember events is not stored in the same part of the brain as the ability to learn motor skills.
Stored in the hippocampus
They also concluded that new memories of events are stored in the hippocampus while older memories are stored somewhere else. The conclusions still hold up according to modern memory research, see fact box. What we in colloquial terms refer to as “memories” are usually events that we are able to retell. They are called episodic memories and are initially stored in the hippocampus. After some time, the memory of the event is either forgotten or transferred to the cerebral cortex – the wrinkly grey layer that makes up the outer layer of the brain.
But perhaps it is more complicated than it sounds. Exactly where in the brain our memories are stored is still “one of the great mysteries of memory research”, says Lars Olson, Professor at the Department of Neuroscience.
It is not the case that some neurons store certain memories, while other neurons store others. Instead, our long-term memories are spread out across different parts of the cerebral cortex, says Lars Olson:
“Imagine a hologram depicted on a piece of glass which you then smash into pieces. It is said that you are then able to see an almost complete image in every individual shard, although at a lower resolution. That is sort of how our memories appear to be stored in the cerebral cortex."
Some estimate that the brain consists of up to 100 billion neurons, and most of them are tightly packed together in the cerebral cortex. It is comparable to the number of stars in a normal-sized galaxy, which is also said to be 100 billion. Every neuron is connected to roughly 10,000 other neurons through synapses, i.e. contact points used for communication between neurons. In this incredibly large network, our long-term memories are stored and then backed up over and over.
“If you were to remove small parts of the cerebral cortex on a person almost no memories would be erased. Since the information is stored in so many places, it is difficult to lose it,” says Lars Olson.
Incredibly large memory capacity
Some people are born with an incredibly large memory capacity. One such individual is Kim Peek, on which the main character in the movie Rain Man is based on. He was an incredibly fast reader and could memorise the contents of books word for word. At the time of his death, he was able to recite thousands of books and large quantities of detailed information about city maps, military history, music, and much more.
“He was like a human search engine. A vast amount of information was stored in his brain. It had no maximum storage limit,” says Lars Olson.
He is of the opinion that people like Kim Peek, who are called highly functioning savants, are of great interest for those who want to understand how memory works. Do individuals like this just have more of what people with normal memory function have or does their memory work differently? Or do regular people actually have the same memory capacity, only something is holding back the storing?
“It is exciting to imagine, right? The truth is we do not really know. What we do know is that many, but not all, so called savants suffer from some sort of brain damage. Although it is hard to draw any conclusions about these special abilities because people with this type of extreme memory capacity are so rare. At the most, there are a handful of them around the world,” says Lars Olson.
But in those of us with regular memory capacity, where only some events and facts actually stick, what is the selection criteria? What determines whether something is worth remembering or not? According to Lars Olson, the emotional connection we have to a certain event plays a large role in whether we will remember it or not. If we become very scared, happy or angry about something, it is more likely that the memory will be consolidated, that is encoded deeper into the cerebral cortex via more synapse connections.
The rest is part of the mystery of our memory; it is unclear how the selection is done and which memories we keep for the future. But every time we access a memory and think about it, it is stored once more, deeper and more easily accessible than before. This process is called reconsolidation. Constantly reconsolidating our memories makes them frail and unreliable.
When we recall an event, we are likely to polish it up a bit, fill in the blanks we cannot remember very well or let associations we make during our flashback rub off on the story. It is then this touched-up image that we commit to memory. And that is why both you and your brother might be wrong about that one Christmas when you were little, even though both of you feel confident about what really happened.
Thus, our memories are constantly changing. The ability to store memories also vary over a lifetime. Earlier in life, the cerebral cortex cannot quite perform its task, as the undeveloped brain contains vast amounts of synapses that are gradually being sifted out. This is the reason why we have few or no memories from our first years – the brain is not fully matured. Later in life, our memory ability is reduced again.
Out of all our memory systems (see fact box), the episodic memory and the working memory are most sensitive to aging. Other memories, like the ability to ride a bike or recognise a cat, are considerably less affected by the aging of the brain. Yet it becomes more difficult for us to add new chapters to our life story. It also becomes harder for us to access previously stored memories, especially those we have stored recently. This is true both for people who go through the normal aging process and those who suffer from dementia, says psychologist Lars Bäckman, Professor of Cognitive Neuroscience at the Department of Neurobiology, Care Sciences and Society at Karolinska Institutet.
“What happens is that neurons die and synapses disappear. This occurs in people diagnosed with dementia as well as in others. It is open to question whether it is a specific difference or a difference in degree between the two groups. I would argue that it is a difference in degree and I would like to use the term accelerated aging instead of dementia. During accelerated aging, the memory abilities give way quicker, but the process is the same as during healthy aging,” he explains.
More emotional weight
We generally tend to remember the years between our 20s and 30s the best. The question is why? Do memories from this point in our lives, when we experience a lot of things for the first time, tend to carry more emotional weight? Could it be that these memories have often been recalled, and thus reinforced? Or is it that the memory generally works better at this age? Lars Bäckman strongly dismisses the latter idea:
“Every textbook on aging claims that the memory ability starts to decline already at an early age, somewhere in our 20s or 30s. It is false. The thesis is based on cross-sectional studies where older people are compared with younger ones. It is misleading because the studies compare different generations who have lived under different conditions that affect important memory ability factors, such as level of education, physical activity or diet,” he says.
Other studies, where individuals are followed over many years and compared with themselves, instead show that memory capacity generally decreases around the age of 65. But the individual variation is large – and Lars Bäckman and his colleagues are trying to pinpoint the causes behind it. In this work, they are particularly interested in the substance dopamine, which works like a lubrication for the brain and keeps the synapses in tune.
The researchers want to find out to what extent measurable declines in dopamine levels are linked to memory loss later in life. They also want to examine what it is that characterises individuals who have high dopamine levels later in life – did they have higher starting levels or are there distinct protective measures that keep the levels from falling?
“No matter what the results are, we can establish that anyone who wants to look after their brain health and memory should engage in physical activity. Oxygenating the brain helps it along. This has been well established in lots of studies carried out on both humans and laboratory animals,” says Lars Bäckman.
Clear sex differences
In addition to the effect of age, there are clear sex differences when it comes to memory ability. Agneta Herlitz, Professor at the Department of Clinical Neuroscience at Karolinska Institutet, knows a lot about this. She explains that women generally have better episodic memory than men, and they find it easier to remember things that can be retold. Men, on the other hand, generally find it easier to create spatial memories, for example how to get from one point to another or navigate through a forest.
But if it is possible to create a story around how to get between the two points, for example “start by going to the supermarket and then continue on to the flower shop across the square”, then there are very few differences in terms of spatial memory between the sexes. The memory capacity of both men and women has gradually improved compared to previous generations. What is interesting is that women’s memory abilities have improved more than men’s.
According to several studies that Agneta Herlitz and her research team are working on, it appears that all cognitive skills are improving a bit more for women than for men between generations. This is largely due to increased labour participation, higher levels of education and improved gender equality. It appears that women derive particular advantage from improved standards of living – especially when it comes to the episodic memory.
This means that women’s advantage in their ability to remember events is slightly larger in countries with a better standard of living. But what does all of this mean? Does this affect the health and well-being of the individual? Do you fare better if you can recall that one Christmas Eve correctly?
“I would phrase it like this: it is good for society to have competent citizens. It is good for democracy and it contributes to a society being at the forefront of research and development. In a healthy society more people can be healthy,” says Agneta Herlitz.
Three ways to categorise memories
1. The working memory, or short-term memory, is where we temporarily store small amounts of information that we need for the task we are performing at the moment. We then forget this information. Repetition of the information contributes to it being stored in the long-term memory.
2. The long-term memory is divided into four parts: The procedural memory involves motor skills like riding a bike, whipping up a batter or playing an instrument. The perceptual memory helps us to navigate our surroundings and recognise things. It is a memory used for things that are hard to put into words, like the feeling of cold air in winter, the weight of a wet towel or the sound of someone’s voice. The episodic memory encodes memories of things that have happened to us, things that can be retold, like what you had for breakfast this morning or what happened on your holiday last summer. Deeply stored memories, for example factual information such as “Stockholm is the capital of Sweden”, are found in the semantic memory.
3. Procedural memories and perpetual memoriesare acquired unconsciously and used spontaneously. Episodic memories and semantic memories are acquired consciously and retrieved when we want to or try to recall them.
Text: Annika Lund, Originally published in Medicinsk Vetenskap nr 3/2018.