Gyri are the rounded ridges that give the human brain its characteristic wrinkled appearance. Each ridge is separated from its neighbors by a groove called a sulcus, and together these folds dramatically increase the surface area of the cerebral cortex, packing far more neural tissue into the skull than a smooth brain ever could. The folding also appears to serve a wiring purpose, bringing distant but functionally connected regions closer together in physical space. How gyri form, what they do, and what happens when they develop abnormally are questions that pull together mechanics, genetics, and neuroscience in ways researchers are still sorting out.
Why the Brain Folds at All
The most intuitive explanation for cortical folding is a space problem. The human cerebral cortex, if you could flatten it out, would cover roughly the area of a large pillowcase. Cramming that sheet into a roughly spherical skull requires compression, and folds are the result. But the mechanism is more specific than simple crumpling. The leading model treats folding as a mechanical instability: the outer layer of the brain (gray matter) grows faster in the tangential direction than the underlying white matter can accommodate. When that mismatch hits a tipping point, the surface buckles outward into gyri and inward into sulci, much like the skin of a drying fruit wrinkles as its surface shrinks unevenly.
Computational simulations have shown that this buckling instability alone can reproduce realistic-looking fold patterns without requiring any pre-programmed blueprint for where each fold should go.1PubMed Central. Gyrification from constrained cortical expansion That said, pure mechanics doesn’t explain everything. The timing and location of neuron birth and migration create regional differences in growth rate, and those differences bias where the first major folds appear. Researchers now favor a combined account: spatiotemporal patterns of cell proliferation set the stage by marking out the locations of primary folds, while differential growth of the cortical plate triggers the mechanical instability that propagates both primary and higher-order folds across the surface.2PubMed Central. Mechanics of cortical folding: stress, growth and stability
The Cellular Engine Behind Folding
At the cellular level, a particular type of stem cell called an outer radial glial cell plays a starring role in cortical expansion and, by extension, in folding. These cells sit in a zone beneath the developing cortex and produce large numbers of neurons that migrate outward. Species with smooth brains have very few of these cells; species with elaborately folded brains have vastly more of them. In gyrencephalic species (those with folded brains), the expansion of outer radial glia and the intermediate progenitor cells they give rise to has been a key evolutionary driver of both cortical size and cortical folding.3PubMed Central. Sonic hedgehog signaling: A conserved mechanism for the expansion of outer radial glia and intermediate progenitor cells and for the growth and folding of the neocortex
Interestingly, the story doesn’t end with neurons. Work in primates has shown that after neuron production wraps up, the same zone continues to expand and begins producing glial cells (astrocytes and oligodendrocytes). This glial production phase correlates tightly with the period of rapid brain enlargement and the emergence of convolutions. In other words, gliogenesis, the formation of cortical connections through the underlying white matter, the elaboration of dendrites, and the amplification of neuropil all contribute to the physical expansion that drives folding.4PubMed Central. Gliogenesis in the outer subventricular zone promotes enlargement and gyrification of the primate cerebrum The cortex doesn’t fold simply because it has a lot of neurons; it folds because of everything those neurons and their support cells do as they grow, connect, and mature.
When Folds Appear in the Womb
The human brain doesn’t start folded. Early in fetal development, the cerebral surface is smooth. Folding follows a predictable timetable. One detailed study of the insular cortex, the first part of the cortex to fold, identified five stages of gyral and sulcal development closely tied to gestational age: the first sulcus appears around 13 to 17 weeks, surrounding sulci develop at 18 to 19 weeks, central insular sulci form and the insula begins to be covered by neighboring cortex at 20 to 22 weeks, the posterior insula is covered by 24 to 26 weeks, and the Sylvian fissure closes by 27 to 28 weeks.5PubMed. Development of the human fetal insular cortex: study of the gyration from 13 to 28 gestational weeks
This sequence matters because the major folds (primary sulci and gyri) are the first to appear and are the most consistent from person to person. Secondary and tertiary folds arrive later in gestation and into early postnatal life, and they are progressively more variable, both between individuals and even between the two hemispheres of the same brain.6Brain Multiphysics. Consistency and variation in the placement of cortical folds: A perspective The predictability of early folds and the randomness of late ones reflect the shift from gene-driven patterning to mechanics-driven buckling as development proceeds.
What Folding Does for Wiring
Folding isn’t just about packing more cortex into the skull. It also reorganizes the brain’s internal wiring in a way that appears to save biological resources. When two areas that need to communicate frequently are located on adjacent walls of the same gyrus, their connecting fibers can take a short path through the white matter beneath that gyrus instead of traveling long distances across the brain. This reduces the total volume of white matter needed, and indeed cortical folding reduces the ratio of white matter to gray matter in mammalian brains.7Current Biology. Wiring Economy and Organization of the Nervous System Gyri, in a sense, serve as shortcuts for neural connections.
A Tour of Named Gyri and What They Do
Each major gyrus has a name, and many are associated with specific functions, though the brain is far more interconnected than a simple “one gyrus, one job” map would suggest. A few examples illustrate the range.
The precentral gyrus, just in front of the central sulcus, houses the primary motor cortex. Distinct zones along it control movements of different body parts, from the toes at the top of the brain down to the face near the bottom. Immediately behind the central sulcus, the postcentral gyrus contains the primary somatosensory cortex. Imaging studies have confirmed that these two gyri contain distinct finger-specific motor and tactile representations that integrate fine movement and touch.8PubMed Central. Morphology, Connectivity, and Encoding Features of Tactile and Motor Representations of the Fingers in the Human Precentral and Postcentral Gyrus Passive movements of the hand, elbow, and shoulder activate the precentral gyrus almost as much as active movements, while the postcentral gyrus is more sensitive to how you are touched than whether you are actively moving.9PubMed. Motor homunculus: passive mapping in healthy volunteers by using functional MR imaging–initial results
The left inferior frontal gyrus, often associated with Broca’s area, is central to language production and the processing of grammar. Comparative neuroscience suggests that language didn’t spring from a brand-new brain region; instead, this gyrus gradually shifted from motor control functions toward syntactic processing over evolutionary time, aided by the strengthening of its connections to temporal cortex through the arcuate fasciculus.10PubMed Central. Investigating the role of the left inferior frontal gyrus in language evolution: insights from comparative neuroscience The superior temporal gyrus, meanwhile, is reliably activated during language comprehension for both spoken and signed languages.11PubMed Central. Superior temporal activation as a function of linguistic knowledge: insights from deaf native signers who speechread
Deeper inside the brain, the cingulate gyrus straddles the line between emotion, action, and memory. Its front portion connects reward signals to actions and is heavily involved in emotion, while its rear portion sends outputs to the hippocampal system and plays a role in memory.12PubMed Central. The cingulate cortex and limbic systems for emotion, action, and memory The parahippocampal gyrus, tucked along the inner surface of the temporal lobe, is involved in distinguishing real memories from imagined events; it stores traces of things you actually saw, helping you separate what happened from what you only thought about.13PubMed. The role of the parahippocampal gyrus in source memory for external and internal events
These functional maps can be surprisingly distributed. Multivariate brain imaging has shown that information about body parts in the somatosensory cortex is not confined to a single zone; hand information, for instance, can be decoded even in the foot region of the postcentral gyrus, and vice versa.14Cell Reports. Widespread information content across the human primary somatosensory cortex The old textbook picture of strict one-to-one mapping along a single gyrus is giving way to a more distributed picture of how the cortex represents the world.
Gyri Across Species
Not all mammals have folded brains. Mice and rats are lissencephalic, meaning their cortices are almost entirely smooth. Larger-brained mammals tend to be more folded, but size alone doesn’t determine the degree of gyrification. A study measuring gyrification across 25 mammalian species from four orders (primates, carnivores, ungulates, and rodents) found that each order follows its own scaling relationship: brain weight and gyrification increase together, but the slope of that relationship differs depending on the group. Ungulates turned out to be the most gyrencephalic mammals for their brain size, significantly more folded than primates, carnivores, or rodents of comparable weight.15PubMed. Order-specific quantitative patterns of cortical gyrification
Cetaceans (whales and dolphins) push folding to an extreme. A quantitative analysis found that cetaceans had an average gyrification index of about 5.43, making them the most gyrencephalic mammals studied. Unusually, their gyrification index did not scale with brain mass the way it does in most other mammals; species with brain masses ranging from under 600 grams to over 5,600 grams all showed similarly high gyrification indices, between roughly 5.2 and 5.7.16PubMed Central. Quantitative analysis of neocortical gyrencephaly in African elephants (Loxodonta africana) and six species of cetaceans: comparison with other mammals Why cetacean brains are so uniformly folded regardless of size is still debated, but the finding underscores that gyrification is not simply a passive consequence of having a big brain.
How Much Gyri Vary from Person to Person
If you compare the brains of two people, the major sulci and gyri will be in roughly the same places, but the finer details will differ. Twin studies have found that identical twins have significantly more similar gyral patterns than fraternal twins, and fraternal twins are no more alike in their folding patterns than unrelated people. Yet even in identical twins, the match is far from perfect. The heritability of the overall gyral pattern was described as low and ill-defined; the primary conclusion is that brain size is determined almost entirely by genetics, while cortical folding pattern, although influenced by genes, is shaped primarily by nongenetic factors.17Brain. Genetic variability of human brain size and cortical gyral patterns
The gyrification index itself, a quantitative measure of how folded the cortex is overall, does appear heritable. In one study, the estimated heritability was about 0.30 in humans and about 0.71 in baboons, suggesting that in humans, environmental and stochastic factors play a larger role in determining the degree of folding than they do in some other primates.18PubMed Central. On the genetic architecture of cortical folding and brain volume in primates The same study found a strong negative genetic correlation between brain volume and gyrification index, meaning that genes associated with larger brains tend to be associated with relatively less folding per unit of volume, a counterintuitive finding that may reflect how larger brains distribute their growth differently.
When Folding Goes Wrong
Disruptions to the migration of neurons during development can produce brains with too few folds, too many, or the wrong kind. The most dramatic example is lissencephaly, literally “smooth brain,” in which the cortex has few or no convolutions and its layered structure is severely disorganized. Lissencephaly results from failures or delays in the process by which newborn neurons travel from deep in the brain to their correct positions in the cortex.19PubMed Central. Cytoskeleton in action: lissencephaly, a neuronal migration disorder Several genes have been linked to this condition, and newer work has identified additional ones; for instance, pathogenic variants in a gene called CEP85L, which encodes a centrosome protein, cause a form of lissencephaly that predominantly affects the back of the brain.20PubMed Central. Pathogenic Variants in CEP85L Cause Sporadic and Familial Posterior Predominant Lissencephaly
At the other end of the spectrum is polymicrogyria, in which the cortex develops an excessive number of small, abnormally structured folds. Mutations in tubulin genes, which encode structural components of the cell’s internal skeleton (microtubules), have been associated with both polymicrogyria and pachygyria (abnormally broad, simplified folds).21PubMed Central. Symmetric polymicrogyria and pachygyria associated with TUBB2B gene mutations Some affected individuals show a distinctive pattern of smooth cortex in the back of the brain combined with excessive small folds in the parietal regions, and stereotyped clinical and imaging findings in families suggest these mixed malformations can be inherited rather than arising randomly.22PubMed. Posterior agyria-pachygyria with polymicrogyria: evidence for an inherited neuronal migration disorder People with these conditions often have epilepsy, intellectual disability, and motor impairments, reflecting the deep disruption to cortical organization.
Gyrification, Intelligence, and Working Memory
Given that folding increases cortical surface area and tightens wiring, it’s reasonable to ask whether more folding means better cognition. The relationship exists, but it’s subtle. A study of two independent samples found that general cognitive ability was associated with greater gyrification across a network of regions including the prefrontal cortex, inferior parietal lobule, temporoparietal junction, insula, cingulate cortex, and fusiform gyrus. The pattern was nearly identical in both samples.23PubMed Central. Regional Variations in Brain Gyrification Are Associated with General Cognitive Ability in Humans In older adults, higher gyrification has been linked to better performance on a general cognitive factor, particularly in regions like the superior temporal gyrus, insular cortex, and orbitofrontal cortex, and these associations held even after accounting for cortical surface area.24PubMed. Cortical gyrification in relation to age and cognition in older adults
The link appears especially clear for working memory, the ability to hold and manipulate information in real time. One study found that greater parieto-frontal gyrification predicted better working memory performance independently of cortical surface area, but the association did not extend to more “crystallized” cognitive skills like vocabulary knowledge.25PubMed. Parieto-frontal gyrification and working memory in healthy adults So the advantage of extra folding seems to be most visible for cognitively demanding, fluid tasks rather than well-practiced ones.
What Happens to Gyri as You Age
Aging and neurodegenerative disease both leave visible marks on the brain’s folds. In healthy aging, the cortex thins, gray and white matter volumes shrink, ventricles expand, and the brain gradually loses some of its gyrification.26PubMed Central. Brain Shape Changes Associated With Cerebral Atrophy in Healthy Aging and Alzheimer’s Disease In Alzheimer’s disease, these changes are accelerated and regionally concentrated. Sulci become wider and shallower, and the effect is especially pronounced in the temporal lobe, where sulcal widening in Alzheimer’s patients shows roughly a 14 percent decrease from healthy controls. Sulcal widening in mild cognitive impairment falls in between, at about a 5 percent decrease, making it a potentially useful early marker. These shape changes track primarily with thinning of the cortex and shrinkage of the white matter running through gyri.27PubMed. Sulcal morphology changes and their relationship with cortical thickness and gyral white matter volume in mild cognitive impairment and Alzheimer’s disease
Prenatal Alcohol Exposure and Altered Folding
Gyral patterns are not only shaped by genes and mechanical forces; the prenatal environment matters too. Children with prenatal alcohol exposure show significantly lower local gyrification across large regions of the cortex compared to unexposed children. Lower gyrification in these children correlated with lower IQ scores, suggesting that the flattened folding pattern reflects genuine disruption to cortical development rather than a benign variant.28PubMed Central. Cortical gyrification is abnormal in children with prenatal alcohol exposure
When researchers looked specifically at children with both ADHD and prenatal alcohol exposure, they found reduced gyrification in overlapping prefrontal, parietal, and temporo-occipital regions compared to typically developing children. But the group with both ADHD and alcohol exposure had a unique deficit in the left mid-dorsolateral prefrontal cortex that wasn’t seen in children with ADHD alone. That additional reduction was tied to a broader profile including a flatter philtrum (the groove between nose and upper lip, a hallmark of fetal alcohol effects), lower IQ, poorer behavioral regulation, and greater hyperactivity.29PubMed Central. Cortical gyrification in children with attention deficit-hyperactivity disorder and prenatal alcohol exposure These findings highlight that gyrification measures can serve as a biological fingerprint of prenatal insult, potentially distinguishing overlapping neurodevelopmental conditions.
Gyri as Surgical Landmarks
One surprisingly practical consequence of cortical folding is that gyri and sulci serve as reliable anatomical landmarks during brain surgery. Standard surgical navigation systems register the brain’s position against preoperative scans, but once the skull is opened and tissue is removed, the brain shifts inside the skull, which can throw off the navigation map. Gyri and sulci shift with the brain tissue itself, so their relationships to one another remain stable even as the brain moves. Surgeons have used this property to guide tumor removal by mapping the sulci and gyri surrounding a tumor on preoperative imaging, confirming them with magnetic stimulation, and then using those cortical landmarks to define surgical boundaries that stay accurate throughout the operation regardless of brain shift.30PubMed Central. Sulci and gyri are topological cerebral landmarks in individual subjects: a study of brain navigation during tumour resection In effect, the brain’s own wrinkles become the map that replaces the external coordinate system when that system drifts out of alignment.