Cerebral cortex

The cerebral cortex is the outermost sheet of neural tissue covering the cerebrum in humans and other mammals. It serves as the primary hub for neural integration within the central nervous system, handling essential functions such as attention, perception, awareness, thought, memory, language, and consciousness.

Roughly 90% of the cortex consists of the six-layered neocortex, while the remaining 10% is the allocortex. A deep groove called the longitudinal fissure splits the cortex into left and right hemispheres, which are connected underneath by the corpus callosum and other commissural fibers. In most mammals, the cortex is folded to increase its surface area within the limited space of the skull. This folding, known as gyrification, is not just about saving space—it also shapes brain circuitry and functional organization. In small-brained mammals, the cortex remains smooth and unfolded.

The raised ridges on the cortex are called gyri (singular: gyrus), and the grooves are called sulci (singular: sulcus). These folds develop during fetal growth and continue maturing after birth. In humans, most of the cortex lies hidden within the sulci rather than on the visible surface. Major sulci and gyri define the brain’s lobes: the frontal, parietal, occipital, and temporal lobes. Additional regions include the limbic lobe and the insular cortex, often referred to as the insular lobe.

The human cerebral cortex contains between 14 and 16 billion neurons. These cells are arranged horizontally into layers and radially into cortical columns and minicolumns. Different cortical areas handle specific jobs—for example, the motor cortex, located mainly in the precentral gyrus, controls movement, while the visual cortex in the occipital lobe processes sight.

**Structure**

The cortex covers the cerebral hemispheres and is folded into gyri and sulci. In humans, its thickness ranges from 2 to 3–4 mm and accounts for about 40% of the brain’s total mass. The neocortex makes up 90% of this tissue, with the allocortex comprising the other 10%. The 14 to 16 billion neurons are organized radially into columns and minicolumns, as well as horizontally into layers. Different regions of the neocortex are referred to as cortices—examples include the motor and visual cortices. Roughly two-thirds of the cortical surface is buried in sulci, and the insular cortex is entirely hidden. The cortex is thickest at the top of a gyrus and thinnest at the bottom of a sulcus.

**Folds**

Folding allows a large surface area of neural tissue to fit inside the skull. When unfolded, each human hemisphere’s cortex has a total surface area of about 0.12 square meters (1.3 square feet). Folding occurs inward from the brain’s surface and also appears on the medial side of each hemisphere within the longitudinal fissure. Most mammals have convoluted cortices with gyri and sulci, but some small mammals, such as certain rodents, have smooth, non-gyrified surfaces.

**Lobes**

The larger sulci and gyri divide the cerebrum into lobes. The four main lobes are the frontal, parietal, temporal, and occipital. The insular cortex is often considered a fifth lobe (the insular lobe), and the limbic lobe—a rim of cortex on the medial side of each hemisphere—is also frequently included. Three lobules are also described: the paracentral lobule, the superior parietal lobule, and the inferior parietal lobule.

**Thickness**

Across mammal species, larger brains (in absolute terms, not just relative to body size) tend to have thicker cortices. The smallest mammals, like shrews, have a neocortical thickness of about 0.5 mm, while those with the largest brains, such as humans and fin whales, reach 2–4 mm. There is an approximately logarithmic relationship between brain weight and cortical thickness. MRI scans allow measurement of human cortical thickness and its correlation with other factors. Sensory cortex is generally thinner than motor cortex. One study found a positive association between cortical thickness and intelligence. Another study reported that migraine patients have a thicker somatosensory cortex, though it remains unclear whether this is a cause, an effect, or linked to a shared cause. A later, larger study found no change in cortical thickness in migraine patients. A genetic disorder that reduces folding in certain areas can create a microgyrus—where the cortex has four layers instead of six—and this has been linked to dyslexia in some cases.

**Layers of Neocortex**

The neocortex consists of six layers, labeled I through VI, from the outermost (near the pia mater) to the innermost (near the white matter). Each layer has a distinct distribution of neuron types and connections to other cortical and subcortical regions. Cortical areas connect directly to one another and indirectly via the thalamus. A clear example of cortical layering is the line of Gennari in the primary visual cortex—a whitish band visible to the naked eye.