By Cristina Cabrera Pescador, Psychologist, Specialist in Neuropsychology
and Teacher of the Master in Applied Neuroscience.
The relationship between brain plasticity and cognitive reserve is that both are predictive measures of the subject’s modifiability in relation to classical intelligence measures.
Brain plasticity is the ability of the nervous system to change its structure and functioning throughout a person’s life, as a reaction to the environment. It would be the physical changes that occur at different levels in the nervous system. On the other hand, cognitive reserve would be the functional improvements associated with intellectual, social and physical factors that occur in people’s lives throughout their lives.
Thus, the concept of cognitive reserve in Neuroscience is closely related and encompassed to that of brain plasticity. In general, in daily life we use only a fraction of the cognitive resources available: the cognitive reserve hypothesis states that each individual has a series of congenital and environmental variable factors that provide quantitative and qualitative mechanisms that make them more resistant to brain pathological processes.
The brain changes with use, the brain circuits are modified depending on the activity and, of course, the synapses are restructured as a result of the experience. Therefore, the efficiency of the synaptic connections can increase or decrease in a more or less lasting way depending on their activity.
Thus, plasticity is a distinctive feature of the Nervous System and refers to how it changes from its interaction with the environment, since each person perceives the world and acts on it in a different way.
There are two types of synaptic plasticity:
The knowledge of the mechanisms on which these changes operate is currently one of the most sought-after topics in the field of neuroscience.
Neuroplasticity or brain plasticity is based, as we have already mentioned, in the way in which the neurons of our Nervous System are connected to each other. As Santiago Ramón y Cajal discovered, the brain is not composed of a tangle of compacted cells that form a single structure, but they are microscopic bodies with autonomy and separated from each other, and they send information to each other without ever uniting. When a group of neurons are activated at the same time, they have to send information to each other.
If this pattern of activation is repeated frequently, these neurons not only send information, but tend to seek a stronger union with the others that are activated at the same time, becoming more predisposed to send information to each other. This increased chance activating together is expressed physically in the creation of more stable neural branches that unite these nerve cells and make them physically closer, which modifies the microstructure of the nervous system. Even if we do not notice it, we are constantly experiencing experiences that occur almost at the same time, and that makes some neurons reinforce their ties and others weaken theirs more. This occurs with both sensations and the evocation of memories and abstract ideas.
The brain circuits are modified depending on the activity and experience
Brain plasticity makes our ability to adapt to changing situations very high, since we can deal with many of the new problems before which evolution has not had time to generate a mechanism of adaptation through natural selection. In the face of a natural catastrophe, for example, it is not necessary to wait for the environmental pressures to make some individuals reproduce more than the rest, making thousands of years later the entire population has an appropriate genetic inheritance to deal with the problem, simply, individuals of a few later generations learn to create technological and social solutions that have never been conceived before.
The theory of cognitive or brain reserve focuses on the ability of the brain to tolerate brain changes due to physiological aging or neuropathic processes, minimising the clinical manifestations of them. The reserve, although initially conceptualised as a static feature, referring to the anatomical potential of the brain (size, number of neurons and density of synapses), has been discussed in a broader way, also encompassing brain functionality (efficacy and brain capacity, use of compensatory mechanisms in the elderly, etc.).
This new way of understanding the reserve, in which the dynamic nature of the reserve is emphasized, is of special interest since it provides the idea of the malleability of the brain throughout life. If it is possible to vary the reserve throughout life and, therefore, increase the tolerance of the brain to physiological or pathological changes, the pertinent question would be: What are the variables that allow to acquire this reserve? In addition to genetic factors, there are environmental factors, such as education and participation levels in social or physical activities. It is the exposure to stimulating environments that really corresponds to the dynamic view of the concept, since this perspective considers the possibility of varying the reserve capacity throughout life. From here we can guess the importance of cognitive stimulation programmes in adulthood.
Following this binary classification of the characteristics of the reserve (static or dynamic), there are two theoretical models:
The first one (static) translates the reserve in the brain into number of neurons, brain size or synaptic density. An example of this model would be that a larger aged brain will be able to tolerate more changes before age-related deficits are manifested, because it will have a larger base substrate that will be used as a means to maintain normal functioning. This first model is known as the passive model and is associated with the term brain reserve.
The second one (dynamic), known as the active model and associated with the term cognitive reserve, understands that the reserve translates into the use of cognitive processes or pre-existing or alternative neural networks (compensatory) to perform an optimal task. An example would be the use of alternative brain mechanisms or networks in old age to compensate for the physiological changes of aging.
Both models indicate that there are interindividual differences and, thus, a greater or lesser tolerance, in terms of protection, to the changes produced by aging or diseases. While the first model emphasises the characteristics of the brain itself, the active model refers to the skills or resources acquired throughout life and which are the fruit of experience, such as education or occupation, that can contribute to vary both the brain and the cognitive reserves.
There are many results in favour of the theory of brain or cognitive reserve, which have been forged over the last few years. Various evidences have been collected in a great variety of studies, mainly from the contribution of anatomopathological studies and, in a remarkable way, through the application of structural and functional neuroimaging techniques, which have been used to study passive/static or active/dynamic factors of the reserve, respectively.
The methodology used to investigate the theory of brain or cognitive reserve is based mainly on the research of associations between reserve and brain measures, in order to understand the modulating role of this reserve. This methodology therefore requires some sort of indirect measure of reserve capacity, a measure that can be achieved, for example, through clinical interviews or questionnaires.
In this regard, various measures have been proposed: brain reserve measures, referring to brain characteristics (e.g., cranial perimeter, reflecting brain volume) and cognitive reserve measures, which refer to life-long experience, such as education or professional career. The results of the studies that have used this type of measures, either in healthy aging individuals or in patients with Alzheimer’s disease, generally support the idea that a larger reserve allows more tolerance to pathological processes.
That is, those individuals with more reserve can tolerate more changes associated with aging and/or dementia, delaying the appearance of clinical symptoms and, therefore, the diagnosis. Thus, it is important to highlight the implications of reserve theory in diagnosis and prognosis. Since the diagnosis is posterior in individuals with high reserve (as compared to individuals with low reserve), because the clinical symptoms occur when the pathological process is more advanced, it has been suggested that the progression will also be faster, once the diagnosis has been made.
This article is an extract from the module “Developmental Neuroscience”, part of the Master in Applied Neuroscience.