Difference between pathology and dementia
Dr David Ward
When we talk about the developments of dementia, there are two distinct events that are worth mentioning. The first is the development of the disease. So in the case of Alzheimer's that's the amyloid plaques and the neurofibrillary tangles, and these can cause some detrimental effects such as atrophy or shrinkage to the brain. Now the other event that occurs, is obviously the onset of dementia, so that's the cognitive impairment, and the deficits in memory and attention that are associated with Alzheimer's disease.
Interestingly we know that there isn't a direct relationship between the level of damage in someone's brain and their cognitive function. For instance, some people can have quite a high load of damage, a large presence of Alzheimer's pathology, yet they're still functioning normally, and they aren't suffering from any of these detrimental memory deficits.
Other people can have just a small level of damage to their brain, yet they start showing these deficits on a day-to-day basis, and they have more difficulty with memory function and planning. We aren't exactly sure why some people seem to be more resilient to Alzheimer's, while other people seem to be more susceptible to its effects, but one of the factors that may partially account for some of these individual differences in when dementia symptoms begin to emerge, is cognitive reserve.
Introduction to cognitive reserve
Dr David Ward
What we see is that the level of protection that someone has from the emergence of dementia is related to their cognitive reserve, and cognitive reserve seems to be built from a lifetime of exposure to cognitive stimulation. Simply put, if you live a life that is more cognitively engaged and you're more involved with these complex learning activities, then, on average, you'll have a lower risk for dementia later in life. For example, we might have two individuals, person A, and person B, and they both have exactly the same level of damage within their brain as a result of Alzheimer's disease. However, person A has greater cognitive reserve, and what that means is that they don't display any of these deficits, any of these dementia symptoms, and they're going about their business fairly normally. Person B however, they have less cognitive reserve, and although they only have the same level of damage as person A, what we see is that they might start showing some of these day-to-day difficulties in memory and attention, some of the earlier symptoms of Alzheimer's disease.
However, something worth mentioning is that cognitive reserve is a theoretical construct. Now a theoretical construct is something that we believe to exist, because it explains a number of associations, but we can't directly measure it or observe it. In the case of cognitive reserve, it provides a great explanation for why people with more years of education have a lower risk of dementia, but we can't directly measure it. We can't go into someone's brain and look at a specific structure that seems to expand when they're exposed to more years of education. Despite this, we do have a few ideas as to how cognitive reserve does look in the brain, and we can start to differentiate between brains that have high cognitive reserve and low cognitive reserve, based on their structure and function.
What cognitive reserve looks like in the brain
Dr David Ward
What we think happens, is when a brain is exposed to this prolonged engagement in challenging cognitive stimulation, this induces changes to the structure and the function of the brain, to make it more resilient and more flexible. There are two main components here at work. The first is neural resilience. Neural resilience refers to differences in the efficiency of the neural networks within a brain. Neural flexibility, on the other hand, refers to differences in the ability to recruit additional neural networks to process a task, when the original network is disrupted by pathology. A neural network is a collection of neurons and synapses within the brain that work together to help process a task. So, in one case it might be if you enter a supermarket and you forgot your shopping list, there might be a number of networks involved with trying to remember those items. If there is damage to these networks, or they can't function normally, then that's when you start to have difficulties remembering, and with other cognitive processes.
Neural resilience and flexibility
Dr David Ward
To better understand how these differences in neural resilience and neural flexibility may actually operate, a nice metaphor may be to use a delivery van driving along city streets. In this metaphor, the delivery van represents a neural impulse that's travelling across the brain to help process one of those tasks, such as remembering the items on the shopping list, and the city streets represents the neural networks within your brain, and the pathways. If you have high cognitive reserve, then you have greater neural resilience and greater efficiency within your neural networks. When the delivery van is driving along a street and it wants to make a delivery at the other side of town, if it's driving along a nice high quality street, then it's not going to face any difficulties at all. Even if a bit of damage starts to occur to this road, because of its pre-existing high quality, quite a large amount of damage needs to occur before that delivery van starts to have difficulties in actually making its delivery, and a very high amount of damage needs to occur before it can't make its delivery.
However, if you compare that to an individual with low cognitive reserve and low neural resilience, then the delivery van is driving along a road that's already pretty poor quality. There might be a few potholes and it's pretty clear that the council's been neglecting it over the last few years. The delivery van can still make its delivery, but if any further damage occurs to that road, then it's probably going to be un-driveable pretty quickly. Neural resilience refers to the differences in the quality of the pathways, the differences in the quality of the neural networks and their efficiency, whereas neural flexibility refers to the differences in the ability to recruit additional roads to make your delivery if necessary. So let's go back to the delivery van, and what we might see is that the van is driving along, and so much damage has occurred to that road that it can no longer pass and it's making its delivery more difficult. If someone has high neural flexibility, they'll be able to back up the van and they'll be able to think, “Well, if I take a left here, and then I take a right, then I'll still get to my destination, even though I'm not travelling along the road I'm intending to travel along.” We compare that to someone with lower neural flexibility, and what we might see is the van backs up, but because there aren't so many options within the street, the van has no way to get to its destination other than travelling along that damaged road, which it can't do. To sum up, neural resilience refers to the differences in the quality of the roads, whereas neural flexibility refers to the differences in the number of options.
How can you increase your cognitive reserve?
Dr David Ward
The question of how can we increase our cognitive reserve is really important. If there were a way to lower your risk of dementia, and many people in the world, or just in Australia, undertook that activity, then we would have a much lower prevalence of dementia. Typically, if an individual wants to increase their cognitive reserve, the best activities are those activities that involve prolonged engagement with cognitively challenging activities, that preferably involve new learning, learning of new knowledge or skills.
A great example of an activity that involves prolonged engagement in a cognitively challenging task, is learning a second language. When you learn a second language you're taking in new vocabulary, you're learning how to structure your sentences properly, and it takes quite a long time. It's quite difficult, but it's also achievable, and that's a really key point. There's no point throwing yourself into a task that is so difficult that you can't actually see yourself getting better at it.
As a result of this activity, your brain has to change, it has to restructure and reorganise itself, and that's what we think increase in cognitive reserve looks like in a brain. Although it's helpful to give a few examples of certain activities that might be good for increasing cognitive reserve, typically in the research and in the literature, we talk about more broad categories of lifetime exposures. What we find is that people who have more years of education have a lower risk of dementia later in life. It's not just limited to education however, and people who are involved in occupations that involve more complexity, with people or things, have a lower risk of dementia as well. A great example of these occupations are those positions that involve management of people, because that's a difficult and challenging thing to accomplish. An individual's engagement in cognitively stimulating leisure activities also seems to be associated with a reduced risk of dementia. We think that people who read more books or visit museums more regularly have a higher cognitive reserve than those who do these activities less frequently. The take home message, however, if you want to increase your cognitive reserve, is that you need to be challenging yourself with new learning on a regular basis.
Brain reserve vs. cognitive reserve
Dr David Ward
To make things a bit more complicated, you can split this up, these protective effects, into cognitive reserve and brain reserve, and that's quite a useful distinction to make. To understand the differences between cognitive reserve and brain reserve better, we can use a hardware and software metaphor. Brain reserve refers to the hardware, so it's what you have in your brain. It's how big your brain is, it's how many neurons you have, and it's how many synapses you have, among other things. Interestingly, if you have a larger head circumference, that is associated with a reduced risk of dementia. It's quite a crude measure, but what we think happens there is that the more brain you have, the more brain you have to lose, before you start showing some of these clinical deficits associated with Alzheimer's disease. Cognitive reserve, on the other hand, refers to the more software side of things, so that refers to the differences in how you use what you have. As discussed, there are differences in neural efficiency and neural flexibility, and these can lead to a brain that's big that's not very efficient, or it can lead to a brain that's small that's very efficient.