The speed with which certain areas of the brain transform in adolescence can predict the onset of mental disorders even years before symptoms appear.
The alteration in the speed of maturation of the different brain regions during adolescence could make one more vulnerable to mental disorders: this is the suggestive – but yet to be demonstrated – hypothesis put forward by a study in pre-print on the server biorXivaccording to which the timing of brain development is orchestrated by genes that are active even before birth.
According to the research, which for six years monitored almost 12,000 children in the United States, analyzing approximately 30,000 brain scans, the speed with which different areas of the brain mature in early adolescence can predict the onset of psychopathologies such as anxiety and depression even four years before the development of symptoms.
The idea is interesting because it challenges the hitherto prevalent hypothesis that mental disorders result from defects in specific brain circuits. Instead, it is suggested, they could be linked to an interruption of the physiological temporal development line of the brain from childhood to adolescence.
What the researchers found, however, is a statistical trend visible only when the data is averaged on a large number of people: that is, it emerges only in large populations like the one considered in the study, and is not applicable to the single individual.
By pruning you grow: what is pruning
During adolescence, the brain undergoes a natural process of selective pruning of some brain connections (synapses) formed during growth and no longer strictly necessary. This phenomenon, called pruning (“pruning”) serves to make the brain faster and more efficient: if, during childhood, the priority was to accumulate as much information as possible about the world, in this phase the brain focuses on optimizing knowledge and removing unused circuits.
It has long been thought that pruning problems may be at the root of some of the most common psychiatric disorders in adolescence, such as depression or anxiety. Often, in fact, psychiatric pathologies have their onset at the end of adolescence, when the process of thinning out is about to end.
A recognizable trajectory
The new study has the merit of having investigated this process on an enormous amount of data, taken from a project that monitored almost 12,000 children, who underwent brain scans from the age of 9, every two years, and for whom parents regularly reported information on mental health.
Thanks to this data, scientists at the University of Iowa discovered that, rather than thinning at a constant rate, the brain regions pruned according to a well-defined trajectory: the process began with an acceleration, then proceeded steadily for some time and finally decelerated as that region approached maturity.
By comparing the various regions, it was possible to observe a wave of maturation passing through the brain. Thinning of the cortex, the outermost, most advanced layer of the brain, began to slow in primary sensory areas and the back, sides and top of the brain around age 13. That of the frontal regions, which preside over functions such as planning and emotional regulation, occurred around the age of 16 (because the maturation of these areas is completed a little later).
In almost all regions, cortical thinning was faster in girls than in boys.
A question of maturity
Children who had thicker cortex in sensory regions of the brain at age 9 were more likely to experience anxiety or depression at age 13; those who had a thicker cortex in the prefrontal region at age 11 were more likely to experience mental disorders around age 13.
In practice, as brain regions approach a stage of maturity, a thicker cerebral cortex could reflect a delay in the pruning process and therefore in the ability of that brain to adapt to the external context. The researchers observed that these delays in pruning were attributable to networks of genes that were already active before birth.
The authors of the study point out that the effects found are small and not sufficiently robust to predict the mental health outcome of the brain maturation processes of individual individuals. However, the study offers a new key to understanding the origins of mental disorders in adolescence.
