Gyri and Sulci of the Brain

The brain’s outer surface isn’t smooth. It’s full of folds and grooves that give it a wrinkled appearance. These folds aren’t random. They’re made up of gyri (ridges) and sulci (grooves), and they play a crucial role in how our brains are organized and function.

Understanding these structures helps us map out different brain regions and link them to specific functions like movement, memory, and language.

What Are Gyri and Sulci?

• Gyri (singular: gyrus) are the raised ridges or folds on the surface of the brain.
• Sulci (singular: sulcus) are the grooves or furrows that separate the gyri.

This pattern of folds increases the brain’s surface area, packing in more neurons without increasing the brain’s size. The more surface area, the greater the brain’s ability to process complex information.

Why Is the Brain Folded?

The folding of the brain allows a large cortex to fit into the limited space of the skull. This is vital for higher-level functions like reasoning, memory, and language.

Two theories compete to explain how the folds actually form. The differential growth theory says the outer grey matter expands sideways faster than the white matter beneath it. The sheet then buckles under its own growth, the way a drying fruit’s skin wrinkles.

A rival theory exists. It credits mechanical tension pulling along nerve fibers in the white matter (Van Essen, 1997). Strongly linked regions pull together into a gyrus; weakly linked ones drift into a sulcus. Growth looks like the main driver now.

This reframed folding as a problem in soft-matter physics, not a gene-by-gene blueprint, and gave differential growth a concrete, testable demonstration.

The layout of these folds isn’t exactly the same in every person, but key gyri and sulci appear consistently across human brains. These consistent features help scientists navigate and study the brain.

Gyri

Gyri are made up of grey matter: clusters of nerve cell bodies and dendrites that handle thinking, sensing, and processing.

By increasing surface area, gyri allow more neural connections and higher cognitive capacity. Each gyrus is linked to a particular function.

Types of Gyri

1. Cingulate gyrus

The cingulate gyrus is part of the limbic system. It consists of a curved fold covering the corpus callosum, a bundle of nerve fibers connecting the right and left cerebral hemispheres.

The anterior portion has a significant role in processing emotions, enabling emotional vocalization, and facilitating bonding between caregiver and child.

The posterior portion handles spatial memory and coordinates movement, orientation, and navigation through its connections with the parietal and temporal lobes.

2. Precentral gyrus

The precentral gyrus is located in the posterior position of the frontal lobe and contains the primary motor cortex.

This structure creates and organizes the homunculus (‘little man’) map of the body. It controls motor movements on the body’s opposite side to where it is located within the brain.

3. Superior temporal gyrus

The superior temporal gyrus houses the auditory cortex, which processes sounds through precisely mapped sound frequencies.

Within this gyrus lies Wernicke’s area, which is crucial for language comprehension and the ability to produce spoken words.

4. Postcentral gyrus

The postcentral gyrus sits directly behind the central sulcus, at the front of the parietal lobe, and contains the primary somatosensory cortex. It handles touch. This is the part of the brain that receives pressure, temperature, and body-position information too.

Like its neighbor across the central sulcus, it is mapped as a body chart called the sensory homunculus. Fingertips and lips take up the largest share of cortex, reflecting how sensitive they are.

Disorders Related to Gyri

When gyri don’t form properly, it can lead to brain malformations:

• Schizophrenia has been linked to structural abnormalities in the superior temporal gyrus, especially in patients who experience auditory hallucinations (Barta et al., 1990).
• Lissencephaly: A condition where the brain appears smooth due to a lack of gyri.
• Pachygyria: Abnormally large gyri.
• Polymicrogyria: Excessively small and numerous gyri with shallow sulci, often leading to developmental delays, seizures, and speech or motor difficulties.

Sulci

Sulci are the grooves that divide the brain’s ridges. The deeper grooves are called fissures.

These structures:

• Increase the surface area of the brain.
• Divide the brain into lobes and functional areas.

Some sulci appear early in fetal development (primary sulci), while others emerge later in life (secondary and tertiary sulci), often shaped by experience and brain growth.

Types of Sulci

1. Longitudinal fissure

The longitudinal fissure is a deep furrow that serves as the primary division between the left and right hemispheres of the brain.

Within this fissure lies the corpus callosum, which connects the hemispheres and enables the transfer of visual, auditory, and somatosensory information between them.

2. Central sulcus

The central sulcus, also known as the sulcus of Rolando, creates an important separation between the parietal and frontal lobes.

It defines the boundary between the primary motor cortex and primary somatosensory cortex.

Interestingly, its size relates to handedness, being larger in the left hemisphere for right-handed individuals and vice versa.

3. Parieto-occipital sulcus

The parieto-occipital sulcus forms a deep groove that creates the division between the parietal and occipital lobes.

Unlike primary sulci, this structure forms after birth as a secondary sulcus.

4. Lateral sulcus

The lateral sulcus, also known as the Sylvian sulcus, creates a deep groove that separates the parietal and temporal lobes, with the insular cortex nestled deep within it.

Disorders Related to Sulci

Changes in sulci can reflect or contribute to neurological issues:

• Perisylvian Syndrome: A rare condition involving the lateral sulcus, often resulting in speech and language impairments.
• Abnormal Central Sulcus Development: Can affect motor control and movement planning.

These structural variations can have long-term impacts on brain function and behavior.

Sulci matter for treatment too, not just disease. Locating the central sulcus on a brain scan tells a surgeon exactly where the motor and sensory strips lie and must be spared during an operation.

The lateral and parieto-occipital sulci serve the same purpose for imaging more broadly. Because these landmarks sit in roughly the same place in every healthy brain, they make it possible to compare scans and report findings using one shared anatomical language.

Table: Major Gyri and Sulci of the Brain

Structure	Type	Location	Function
Precentral Gyrus	Gyrus	Frontal lobe, anterior to central sulcus	Primary motor cortex – controls voluntary movement
Postcentral Gyrus	Gyrus	Parietal lobe, posterior to central sulcus	Primary somatosensory cortex – processes touch and proprioception
Cingulate Gyrus	Gyrus	Medial surface above corpus callosum	Emotion processing, bonding, spatial memory
Superior Temporal Gyrus	Gyrus	Temporal lobe, beneath lateral sulcus	Auditory processing, language comprehension (Wernicke’s area)
Central Sulcus	Sulcus	Between frontal and parietal lobes	Separates motor and sensory cortices
Lateral Sulcus (Sylvian)	Sulcus	Between frontal/parietal and temporal lobes	Houses auditory cortex and insula; involved in language
Longitudinal Fissure	Sulcus	Midline, between hemispheres	Divides brain into left and right hemispheres
Parieto-occipital Sulcus	Sulcus	Between parietal and occipital lobes	Separates visual processing from spatial/motor integration areas

Brain Folding and Development

The brain starts forming folds during prenatal development. These folds reflect both genetics and environmental influences.

The timing follows a strict schedule. Early in pregnancy the cortex is smooth, and the first deep sulci appear around the midpoint of gestation.

Folding then speeds up dramatically in the third trimester, transforming a nearly smooth surface into the deeply convoluted brain present at birth. The finer secondary and tertiary sulci keep deepening well into childhood, long after the major folds are set.

• Primary sulci appear early and are mostly hardwired.
• Secondary and tertiary sulci develop later and may be influenced by experience.
• Regions involved in complex functions like language tend to have more intricate folds.

Evolutionary Perspective

Cortical folding does track brain size, but it isn’t a simple stand-in for intelligence. Larger brains fold more because a bigger sheet of cortex is mechanically prone to buckling, not because folding itself sharpens thinking (Zilles et al., 1989).

Folding is not intelligence. Highly folded brains, such as those of dolphins and elephants, don’t necessarily out-think less-folded ones. It is best read as a marker of an expanded cortex, not a direct score of cognitive ability.

The gyrification index, which measures the degree of folding, is much higher in humans than in animals with smooth brains (like rodents). This folding allows for:

• More advanced problem-solving, language, and social skills
• More cortical surface area
• Denser neural networks

Critical Evaluation

Before looking at each concern in detail, here are the main criticisms raised about how gyri and sulci are studied and interpreted:

1. More Folds, Not More Intelligence: cortical folding scales with brain size for mechanical reasons, not because extra folds sharpen thinking.
2. Individual Variability Limits Mapping: the fine, later-forming sulci vary so much between people that they cannot be trusted as precise landmarks.
3. Folding-Disorder Links Stay Correlational: structural changes in specific gyri track conditions like schizophrenia, but that link does not prove folding causes the disorder.

More Folds, Not More Intelligence

That’s a tempting shortcut. A more folded brain is not automatically a smarter one. Folding scales tightly with brain size for a mechanical reason. A bigger sheet of cortex is simply harder to keep flat, so it buckles more (Zilles et al., 1989; Tallinen et al., 2014).

Dolphins and elephants carry heavily folded cortices. Yet neither reliably outperforms humans on standard measures of cognition. Within humans, the gyrification index does not track age, brain weight, or overall cortical volume in any simple way (Zilles et al., 1988).

That is not intelligence. Folding is best read as a by-product of an expanded association cortex, not a direct score of how well that cortex thinks.

Size is not the whole story either. Primates fold more than similarly sized non-primates, and the human increase is concentrated over the frontal and association cortex rather than spread evenly (Zilles et al., 1988, 1989).

Individual Variability Limits Landmark Mapping

This is a mapping problem, not a folding one.

Anatomists rely on named folds to navigate the cortex. That only works where the folds sit in the same place from brain to brain. Only the deep, primary sulci really are that reliable.

The central, lateral and parieto-occipital sulci form early in fetal life and are largely hardwired. So they show up in almost the same spot in every normal brain. Secondary and tertiary sulci form later. They are shaped by growth and, plausibly, by experience.

The fine folds are simply too variable to trust.

A fold present in one brain may be smaller, doubled or missing in the next. It can even differ between a person’s two hemispheres. Precise claims about where a function sits are safest when anchored to the major landmarks, not the finer folds.

Folding-Disorder Links Stay Correlational

Much of this evidence is observational, not experimental. Reduced volume of the superior temporal gyrus, for instance, is linked to the severity of auditory hallucinations in schizophrenia (Barta et al., 1990).

That is a genuine limitation.

Correlation is not causation. That kind of finding cannot show that the folding difference causes the disorder. It could just as easily be a consequence of the illness, or the product of some third factor affecting both. Findings like these also vary from one study to the next.

That is the key caveat. A folding abnormality is best treated as one anatomically coherent marker within a wider disorder. It is a clue, not a stand-alone explanation. Strong causal claims run ahead of what correlational anatomy can support.

This caution applies most to psychiatric claims about folding.

Contemporary Research

The clearest recent test of how folding forms compared real brains, physical models and computer simulations side by side.

This caps a decade of converging work. Physical gel models first showed the same two-parameter buckling pattern in 2014 (Tallinen et al., 2014).

A parallel synthesis mapped the same mechanics onto human brain development the following year (Budday et al., 2015). The older tension-based account (Van Essen, 1997) remains a secondary influence on where folds settle, not the primary driver.

Summary

Gyri and sulci are more than just wrinkles on the brain. They’re the foundation for how the brain is organized, how it grows, and how it functions. From emotion and memory to movement and speech, these folds help make us who we are.

References

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Barta, P. E., Pearlson, G. D., Powers, R. E., Richards, S. S., & Tune, L. E. (1990). Auditory hallucinations and smaller superior temporal gyral volume in schizophrenia. The American Journal of Psychiatry.

Budday, S., Steinmann, P., & Kuhl, E. (2015). Physical biology of human brain development. Frontiers in Cellular Neuroscience, 9, 257. https://doi.org/10.3389/fncel.2015.00257

Haines, D. E., & Mihailoff, G. A. (2017). Fundamental Neuroscience for Basic and Clinical Applications E-Book . Elsevier Health Sciences.

Spreafico, R., & Tassi, L. (2012). Cortical malformations. Handbook of clinical neurology, 108, 535-557.

Tallinen, T., Chung, J. Y., Biggins, J. S., & Mahadevan, L. (2014). Gyrification from constrained cortical expansion. Proceedings of the National Academy of Sciences, 111(35), 12667–12672. https://doi.org/10.1073/pnas.1406015111

Van Essen, D. C. (1997). A tension-based theory of morphogenesis and compact wiring in the central nervous system. Nature, 385(6614), 313–318. https://doi.org/10.1038/385313a0

Yin, S., Liu, C., Choi, G. P. T., Jung, Y., Heuer, K., Toro, R., & Mahadevan, L. (2025). Morphogenesis and morphometry of brain folding patterns across species. eLife, 14, e107138. https://doi.org/10.7554/eLife.107138

Zilles, K., Armstrong, E., Schleicher, A., & Kretschmann, H. J. (1988). The human pattern of gyrification in the cerebral cortex. Anatomy and Embryology, 179(2), 173–179. https://doi.org/10.1007/BF00304699

Zilles, K., Armstrong, E., Moser, K. H., Schleicher, A., & Stephan, H. (1989). Gyrification in the cerebral cortex of primates. Brain, Behavior and Evolution, 34(3), 143–150. https://doi.org/10.1159/000116500