Brain development · Adolescence
The adolescent cortex loses volume mainly because it thins, not because it shrinks in area
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Short answer
Cortical thinning drove most of the adolescent volume decline across four independent longitudinal samples, with correlations of 0.72 to 0.95 between thickness and volume change.
What happened
Tamnes and colleagues tracked annual change in cortical volume, surface area and thickness in participants aged 7 to 29 across four longitudinal datasets, processing all of them through the same pipeline and fitting non-linear trajectories. All three measures declined. Change in thickness correlated with change in volume at 0.72 to 0.95 across samples, far more tightly than surface area did. Spatially, area and thickness changes correlated positively in sulcal depths and negatively on gyral crowns.
Why it matters
Volume is the measure most developmental studies report, and it conflates two things that change for different reasons. Separating them says the adolescent signal lives in thickness — which is what any cellular account, whether pruning or myelination, has to explain.
Evidence
- Study type
- Four independent accelerated longitudinal MRI samples processed through identical pipelines
- Sample
- 388 participants aged 7–29, 854 scans (CPB 30, Pittsburgh 73, NCD 76, BrainTime 209)
- Journal
- The Journal of Neuroscience · peer reviewed
- Replication
- The thinning-drives-volume pattern held in all four samples and converges with a separate four-sample analysis in this corpus
- Limitations
- MRI cannot identify the cellular processes behind thinning, whether synaptic pruning or myelination. The accelerated design combines cohorts rather than following individuals across the full range, and the samples are typically developing.
What this connects to
Sources
The 2 studies this explanation is built from, by the role each plays. Every source links to PaperFren’s explanation of it and to the original paper.
Primary study
- 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.
What it does not showLimitations
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.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Supporting evidence
- How does the human brain structurally change as we grow up?
Brain structures change significantly from childhood to adulthood, but the way researchers adjust for overall brain size can dramatically alter their conclusions about these developmental patterns.
What it does not showLimitations
This study cannot determine the biological cause behind these volume changes, such as whether grey matter reduction reflects synaptic pruning or changes in myelination. Furthermore, the results are sensitive to the specific age ranges of the cohorts; for instance, the site studying a narrow range of 10 to 16 years showed linear trajectories rather than the non-linear patterns found in broader cohorts. Additionally, because the study focused on gross cortical measures, it does not show how these size-correction methods impact smaller subcortical structures.
PaperFren explanationStudy with cards and a quizOriginal paper (DOI)cc by
Before
Adolescent cortical development was largely reported as volume loss, leaving it ambiguous whether the cortex was thinning, shrinking in area, or both.
Now
Thickness is the dominant component, and it replicates across four samples and three countries. MRI cannot show the cellular processes behind it, and the accelerated design combines age cohorts rather than following individuals from 7 to 29.