How everyday activities can allow our brains to be flexible and adaptive across the lifespan
Often, people report that their cognitive abilities change as they age, including worsening memory. Like many others, I used to think of cognitive ageing as synonymous with cognitive decline. However, recently I learned that the ageing process is far more complex and nuanced.
The past academic year, I also delved into research on cognitive reserve for my undergraduate dissertation in Psychology. Put simply, cognitive reserve describes an individual’s ability to compensate for brain damage or degeneration. Through engaging with this topic, and by learning how the brain develops throughout the lifespan, my understanding of cognitive ageing has shifted.
It turns out that the mechanisms involved in our cognition — that is the processes involved in our learning, experiencing, and thinking about the world — age differently. Whilst some cognitive abilities do decline across the lifespan, others are preserved or indeed improved. Most importantly, I learned that engaging in everyday activities, such as socialising, working, and exercising, contributes to having a greater cognitive reserve. This, in turn, allows our brains to be more flexible and adaptive in the face of declining cognitive abilities, and brain damage.
As we age, it is normal for some brain areas to shrink in volume. In addition to this process, some of us will experience age-related disorders, such as Alzheimer’s disease (the most common type of dementia), which lead to further shrinkage or degeneration of brain tissue. Thus, knowing that there is such a thing as a cognitive reserve, and that everyday activities contribute to it, made me feel hopeful.
Indeed, discussing this topic with my research participants highlighted that many of us, regardless of our age, are focussed on — and discouraged by — the decline of cognitive abilities across the lifespan. In this blog, I would like to shift some of our attention to the abilities that are preserved or improved throughout the ageing process, so that we might acquire a more complete and comforting perspective. This seems particularly worthwhile, considering that we live in an ageing society. According to a 2019 report by the United Nations, roughly 25% of North Americans and Europeans will be aged at least 65 by 2050.
So, what actually is cognitive ageing?
Cognitive ageing — a complex process
Throughout normal cognitive ageing — an ageing process, which does not involve brain damage or age-related disorders — we undergo a decline in so-called fluid abilities, especially from the age of 60 onwards. The umbrella term ‘fluid abilities’ encompasses various cognitive functions allowing us to process information or knowledge quickly, and in a goal-directed manner. For example, we use fluid abilities to make plans, decisions, or to react to changing environments. If we want to do these things successfully, we have to select appropriate behaviours and relevant pieces of information, whilst suppressing their inappropriate counterparts.
For instance, when making plans to see a friend later on in the day, it is important to consider today’s weather forecast, workload, and transport options, but not so much where you first met your friend. In a different context, accessing the latter piece of information or other details of your friendship may be crucial.
To engage in these processes, we rely on general control mechanisms referred to as executive functions, such as the ability to selectively attend to a relevant piece of information.
Various brain areas are involved in executive functions. However, the frontal lobes, located right behind your forehead, and in particular an area known as the prefrontal cortex, are typically associated with executive functioning. As part of normal ageing, our prefrontal cortex shrinks in volume from around the age of 25 onwards. As such, it has been proposed that declines in executive abilities in later life are related to declines in prefrontal cortex volume.
In contrast to fluid abilities, crystallised abilities are preserved across normal ageing. These include our general or semantic knowledge of the world (e.g., facts, and concepts). As we age, we continue to grow our store of semantic knowledge, for example by encountering new people, and places, or by reading books. To acquire new semantic knowledge, we use a brain region referred to as the medial temporal lobes. This area is located at the centre of the brain, and, compared to the prefrontal cortex, it remains quite stable in volume throughout the ageing process.
Whilst this is only a brief overview of some of the processes involved in cognitive ageing, it shows that both declines and improvements in cognitive abilities accompany our development across the lifespan. Whereas younger adults are able to use their fluid abilities when engaging in cognitively demanding tasks, older adults may need to rely on their advantage in crystallised abilities.
So, how does cognitive reserve come into play?
Cognitive reserve and its protective outcomes
The term cognitive reserve describes a hypothesis developed to explain why two people experiencing the same brain pathology (e.g., brain damage associated with an injury or with disease), do not necessarily also experience the same symptoms. For example, cognitive reserve is proposed to account for the finding that a sizeable proportion of older adults (roughly 25%-67% across different studies) who pass away without having experienced cognitive impairments, did actually suffer from Alzheimer’s disease or other types of dementia.
One of the most famous research projects on this topic is known as the Nun Study. Here, 678 U.S. American School Sisters of Notre Dame took part in cognitive assessments throughout their lifetime and agreed to have their brains studied after death. When analysing the nuns’ brains, researchers found that some of the nuns who were deemed cognitively ‘normal’ before their death, actually presented with levels of brain damage sufficient for a diagnosis of dementia. Thus, the Nun Study exemplified that older adults can suffer from brain pathologies meeting clinical criteria of dementia, whilst showing none of the typical symptoms.
First discussed in the context of Alzheimer’s disease, cognitive reserve has since been associated with protective outcomes across various clinical and healthy populations — but what exactly is it?
People who benefit from higher cognitive reserve tend to score higher on standardised assessments of intelligence (i.e., IQ tests), have higher occupational and educational attainments, and engage frequently in cognitively stimulating leisure activities. In particular, taking part in physically demanding, and complex activities, with high levels of interpersonal skills, seems to be crucial in giving rise to these protective outcomes.
Since we cannot measure cognitive reserve directly, researchers use IQ and lifestyle components of cognitive reserve as proxy measures. Interestingly, childhood intelligence is a strong predictor of many later life outcomes, including education, occupation, and a healthy lifestyle. Nevertheless, Opdebeeck and colleagues have reported that each lifestyle proxy uniquely contributes to the protective outcomes of cognitive reserve. This is good news as it illustrates that engaging in stimulating physical or social activities may still be associated with protective outcomes, even if you did not benefit from access to high-quality education growing up.
How does cognitive reserve work at the level of the brain?
The exact mechanisms underlying cognitive reserve remain to be uncovered. However, it seems likely that a combination of active and passive pathways are responsible for the flexibility and efficiency with which people with higher cognitive reserve complete tasks. For instance, a passive factor contributing to cognitive reserve would be individual differences in brain size. Put simply, a person with a larger brain will be able to tolerate slightly more damage without showing symptoms, than a person with a smaller brain.
Active pathways tend to be separated into two mechanisms: neural reserve and neural compensation. Having a greater neural reserve involves being able to use neural networks efficiently and flexibly, whilst engaging in neural compensation describes the recruitment of alternative neural networks. Neural compensation would be crucial when the networks typically used for a specific task are damaged or unavailable.
In the context of normal ageing, this can be observed when older adults successfully engage in tasks putting high demands on their fluid abilities. Our brain is separated into two connected halves or hemispheres, each of which contains a prefrontal cortex — the right and the left. As I mentioned earlier, our prefrontal cortex volume shrinks as we age. Since younger adults benefit from having a larger prefrontal cortex, they are able to complete fluid tasks by using either the right or left prefrontal cortex. In contrast, older adults activate both hemispheres to complete the same task, illustrating the recruitment of an extra area to compensate for the normal decline in prefrontal cortex volume.
Looking into the future
The reason I enjoy studying cognitive reserve is that it gives me hope for and comfort about my own future — and that of my family and friends. The types of activities seemingly contributing to greater cognitive reserve, are the ones we typically engage in on an everyday basis — being part of social networks, carrying out physically demanding tasks, such as exercising or putting an IKEA shelf together, looking after loved ones, or managing one’s finances and household.
Although cognitive reserve will not prevent you from getting Alzheimer’s disease, or any other brain pathology for that matter, it may well contribute to your brain’s ability to compensate for the brain damage caused for a little longer. This insight has changed how I view some of the (tedious) everyday activities I engage in, and hopefully, it will do the same for you.