Brain development and aging are a single process
A network of brain structures has been discovered, the aging of which mirrors its development
Yulia Kondratenko, "Elements"
There has long been a beautiful theory that aging is a mirror image of the processes of development. Scientists from Norway have managed to identify a network of brain structures responsible for intellectual abilities and episodic memory, which really changes with age in a mirror way: later than other areas reaches the peak of development (by the age of forty) and begins to degrade earlier than others. Interestingly, the structures of this network are most susceptible to disorders in such mental disorders as Alzheimer's disease and schizophrenia.
To understand the state of which brain structures are associated with age, it is necessary to collect as many structural images of the brains of people of different ages as possible. Norwegian researchers took brain scans using magnetic resonance imaging, which allows you to see areas of gray matter (the body of neurons). The study involved 484 subjects aged 8 to 85 years. To analyze the resulting array of images, the so-called independent component method was used, which allows us to present the initial data as a set of features that differ most from different subjects.
Automatically detected signs do not necessarily have any obvious interpretation, and the task of researchers is to understand for which signs such interpretations are possible. Because in this study, scientists were interested in age-related changes, and they were looking for a link between these signs and the age of the subjects. It turned out that only two signs have a pronounced dependence on age. The first is the total volume of gray matter, which monotonously decreased with age, its variability depended on age by 90%. The second was the volume of a certain network of brain structures, which changed in a more interesting way: the graph had the shape of an inverted letter U. The structures of this network develop relatively late and begin to degrade faster than other components of the brain (Fig. 1).
Fig. 1. Two signs identified by analyzing the brain images of the subjects showed a clear dependence on age. The first sign is the total volume of gray matter, which decreased on average with age (left). The second is the volume of the network of certain brain structures, which developed later and began to degrade earlier than others (right). The development of this network was described by a symmetrical curve with a peak of about 40 years. Graphs from the discussed article in PNASWhat structures does the detected network consist of that develops "mirror-like" in time?
It includes areas of the cerebral cortex that are engaged in high–level information processing - that is, not limited to information of only one modality, for example, visual or auditory. Therefore, such structures are called heteromodal. These include the inferior temporal cortex, the inferior parietal cortex and the prefrontal cortex (not all of these areas were included in the network, but only certain parts of them). In addition, some limbic and paralimbic structures were included in the network – in particular, the posterior cingulate cortex, apparently associated with the formation of a "self model" (it is shown that this area is activated when a person thinks about himself, and turns off during meditation (see Garrison et al., 2013. Effortless awareness: using real time neurofeedback to investigate correlates of posterior cingulate cortex activity in meditators' self-report), as well as the medial temporal lobe, the role of which is associated with the formation of motivations and emotions, as well as self-control. Exactly which regions were included in the detected network can be seen in Fig. 2 (middle row, highlighted in orange).
Fig. 2. Comparison of the structures of the network that develops with age "mirror" (highlighted in orange), as well as the areas most susceptible to degradation in Alzheimer's disease (highlighted in blue), and areas characterized by the most severe developmental disorders in adolescent schizophrenia (highlighted in green). Figure from additional materials to the discussed article in PNASInterestingly, the structures of the detected network turned out to be similar to the areas most susceptible to brain diseases that are associated with developmental disorders (adolescent schizophrenia) and impaired normal aging (Alzheimer's disease).
They are also shown in Fig. 2. It turns out that an interesting connection between the processes of development and aging is revealed for the detected network: its structures are most susceptible to disorders in pathologies of both development and aging of the brain. Interestingly, it was possible to identify such a network of structures susceptible to brain diseases according to the data of healthy subjects.
The development of the detected network over time was slightly different in women and men: in women, the peak was more pronounced and appeared at a somewhat later age (41 years in women and 39 years in men; Fig. 3). Moreover, the symptoms of schizophrenia and Alzheimer's disease in women also appear on average somewhat later than in men, which indirectly can confirm the connection of the functioning of the identified network with the processes of development and aging.
Fig. 3. Comparison of the development of network structures that change with age "mirror" in women (graph on the left) and men (graph in the center). In the figure on the right, the graphs are combined for ease of comparison. Figure from additional materials to the discussed article in PNASThe network structures were associated with intellectual abilities and episodic memory.
Thus, the level of development of the network in the subjects correlated well with the indicators of tests for mobile intelligence (the ability to solve problems regardless of previous experience) and the test for memorizing words (California Verbal Learning Test). Scores for the crystallized intelligence test (the ability to use existing knowledge and skills) correlated well with the development of the network only in subjects up to the age of forty (that is, before reaching the peak of network development), and then reached an almost constant level.
Unfortunately, the method of magnetic resonance imaging used in the work does not allow us to determine which cellular processes are responsible for macroscopic changes, the peak of which is reached by the age of 40. Finding mechanisms that may link the processes of brain development and aging has yet to be done.
The revealed network of structures in the brain is thus associated with cognitive functions, violations of which are a symptom of pathologies of normal development and aging processes. Pathological processes characteristic of adolescent schizophrenia and Alzheimer's disease manifest themselves during the development or aging of the brain. Perhaps it is during these periods that it is easiest to influence the identified system of brain structures, the state of which most depends on age and in which the processes of development and aging reflect each other.
A source: Douaud et al. A common brain network links development, aging, and vulnerability to disease // PNAS, 2014.
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