Pattern of Gyrification

Gyrification (or cortical folding) is the the process by which the brain’s surface forms ridges and valleys (e.g. gyri and sulci). This occurs during neuronal migration over the course of development. As the layers of cortex are established, interconnections grow between neurons. Gyri are thought to be formed by the tensile strength of these neuronal connections. Thus, areas that are densely interconnected will pull together to form gyri whereas areas loosely connected will drift apart and form sulci. Bringing densely interconnected regions closer together also functions to promote efficient processing, as there is less distance to travel between closely related neurons. In humans, the greatest development of gyrification occurs during the third trimester of pregnancy, when the relatively smooth (lissencephalic) brain starts to contort into its adult, wrinkled morphology. The overall degree of cortical folding can be quantified by the gyrification index.
Increased gyrification presents an alternative way to increase the amount of neurons packed into the brain without increasing the overall volume. That is, gyrification allows more cortical surface area to be packed into the same amount of space than is possible with a lissencephalic brain. In fact, the human cortical sheet is estimated to have about three times larger area as the inner surface of the skull. The greater amount of cells and efficiency in processing is hypothesized to allow for greater cognitive abilities. For instance, the degree of folding in temporo-occipital cortex is positively correlated with intelligence scores in humans, and many mental disorders are associated with lissencephaly and the absence of normal gyri and sulci. However, the relationship between gyrification and overall neuron number is complex - greater overall neuron count does not necessarily equal greater cognitive capacity. Indeed, some mental disorders such as autism are associated with greater cell number and gyrification due to insufficient neuronal pruning. This is further complicated by the occurrence of autistic savants, individuals with severe deficits in some cognitive domains (typically including social deficits) at the same time as incredible abilities in other cognitive domains (typically in mathematics and declarative memory).