The brain’s hidden folds have been overlooked for decades — and they reveal how minds differ
The big folds of the human brain have been mapped for over a century. But there is a second layer of smaller, shallower folds that most imaging studies have simply ignored —…
The major folds of the cerebral cortex — the gyri that bulge outward and the sulci that dip inward — are largely consistent across healthy adults and have been catalogued exhaustively. But the cortex contains a second tier of structure: smaller, shallower folds that don’t appear reliably across individuals and that have remained largely invisible in standard MRI scans due to their modest scale.
A study published in eLife focuses on precisely these so-called tertiary folds, sometimes called cryptic sulci. Researchers analysed high-resolution brain scans from multiple individuals and found that the position and presence of these small folds vary substantially — far more than the large, familiar folds of the brain’s outer surface. That variation was not random: it correlated with functional and anatomical properties of the cortex.
What folds reveal about the brain’s internal map
The cerebral cortex is divided into regions with distinct functions, but the boundaries between those regions are not visible on the brain’s surface in any obvious way. The researchers found that hidden folds frequently coincide with functional boundaries in the cortex — locations where the nature of information processing shifts. That makes them potentially useful as anatomical markers for individual brain organisation.
This finding has implications for how neuroscience research is conducted. When brain scans are averaged across groups of people, individual variation in cortical organisation can obscure meaningful signals. If that variation is larger than previously assumed — and concentrated at these sub-surface structural boundaries — then group-level analyses may be systematically missing relevant information about how individual brains are put together.
A window onto brain aging
The connection to aging research is indirect but plausible. Cortical thinning — the gradual reduction in the thickness of the brain’s outer layer with age — is an established marker of brain aging that proceeds unevenly across regions. If hidden folds mark functionally significant boundaries, their structure or positioning might also shift with age or in the early stages of neurodegenerative disease, potentially serving as early indicators of change.
For now, the study is primarily descriptive: it establishes that these folds carry more biological meaning than previously recognised. Longitudinal research — scanning the same individuals repeatedly over years — would be required to determine how hidden cortical structure changes with time, and what those changes predict about brain health.