gyri), a ridge on the surface of the cortex, creating the characteristic folded appearance of the brain in humans and most other mammals.

[1] In the human brain, two-thirds of the folded cortex is buried within the sulci, if the division of the hemispheres by the longitudinal fissure is taken into account.

Consequently the inner sides of almost all sulci are separated only by the pia mater and the subarachnoid space, in which the cerebrospinal fluid circulates.

[2] The sulcal pattern varies between human individuals, but the sulci and gyri do have a generalised arrangement, making a common nomenclature possible.

It is explained by a number of hypotheses including mechanical buckling, and axonal tension factors.

[9] The hypotheses are not mutually exclusive and can include their combined effects, with that of another mechanism of tangential expansion.

[10][11] In humans, cerebral convolutions appear at about five months and take at least into the first year after birth to fully develop.

Malformations of the cortical folds have been linked to the intellectual disabilities associated with epilepsy, schizophrenia, and autism.

[17] Anomalies in gyrification can affect the width or depth of sulci that are associated with many neurological or neuropsychiatric disorders.

[6] It has been found that the width of cortical sulci increases not only with age,[18] but also with cognitive decline in the elderly.