How does the brain's outer layer change during adolescence?
During adolescence, the brain's outer layer shrinks in volume primarily because it becomes thinner, not because its surface area decreases.
Source
Development of the Cerebral Cortex across Adolescence: A Multisample Study of Inter-Related Longitudinal Changes in Cortical Volume, Surface Area, and Thickness
What they did
The researchers tracked brain structural changes in 388 healthy participants aged 7 to 29 across four independent longitudinal study groups, analyzing a total of 854 MRI scans. They calculated year-over-year changes in three cortical measurements: total volume, outer surface area, and overall thickness. By processing all datasets with identical software and using non-linear curve-fitting models, they mapped and compared developmental trajectories across the different samples.
What they found
The study revealed that cortical volume, thickness, and surface area all decrease continuously from late childhood through early adulthood. Cortical thinning was identified as the primary driver of the overall volume reduction, displaying robust positive correlations with volume changes ranging from 0.72 to 0.95. In addition, they mapped a complex spatial relationship between surface area and thickness changes, finding positive correlations in deep cortical folds but negative correlations on the outer ridges.
The limits
What it doesn't show
Because this study relied on MRI scans, it cannot directly prove the underlying cellular processes, such as synaptic pruning or cellular myelination, that cause these structural changes. Additionally, the study used an accelerated longitudinal design that combined multiple age cohorts rather than tracking the exact same individuals all the way from age 7 to age 29. The findings are also limited to healthy, typically developing individuals, meaning they may not represent neurodivergent or clinical developmental pathways.
Key terms
- Cortical Volume
- The total space occupied by the cerebral cortex, which is mathematically determined by the combination of surface area and thickness.
- Cortical Thickness
- The physical depth or distance between the outer gray matter surface and the inner white matter boundary.
- Cortical Surface Area
- The total two-dimensional size of the boundary between the white matter and the gray matter of the cerebral cortex.
- Longitudinal Design
- A research methodology that involves tracking and scanning the same individual participants repeatedly over multiple points in time.
- Symmetrized Annual Percentage Change
- A standardized metric representing the linear rate at which a brain structure grows or shrinks each year relative to its average size across all time points.
- Generalized Additive Mixed Model
- A flexible statistical modeling technique used to map developmental trajectories without forcing the data into rigid linear or polynomial shapes.
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Quiz yourself
What were the two primary aims of this multisample longitudinal study?
Common questions
Why does the brain lose volume during adolescence if this is a time of cognitive growth?
Volume loss during this period is a normal part of healthy maturation. It is primarily driven by cortical thinning, which is thought to reflect processes like synaptic pruning (getting rid of unused connections to make the brain more efficient) and increased myelination (which wraps brain wires in insulation and makes the boundary look different on an MRI).
Why did the researchers use four separate datasets instead of putting them all into one big group?
By analyzing four independent datasets collected at different sites using different MRI scanners, the researchers could test whether their findings were truly universal or just an artifact of a specific scanner, location, or study population.
Do these changes happen at the same speed across the entire brain?
No, the rate of change varies by region. For example, the parietal lobe showed the most rapid decreases in volume, surface area, and thickness across the samples, while the occipital lobe generally showed the slowest rates of change.
Does a larger surface area always mean a thicker cortex?
Not necessarily. The study found a complex relationship: in some parts of the brain (like deep folds or sulci), shrinking surface area went along with thinning. In other parts (like outer ridges or gyri), shrinking surface area actually related to a relatively thicker cortex.
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