24th November 2023

Gyrification

Gyrification describes developmental folding of the cortex that results in characteristic gyri (peaks) and sulci (troughs). Gyrification is best studied in vivo by sMRI. The degree of cortical folding in a given brain is described by the gyrification index: a ratio of the lengths of the long, folded cortical contour and the shorter, smooth brain surface. The greater the gyrification index, the higher the degree of folding, and vice versa.

Using foetal MRI, cortical folding patterns have been identified from 20 weeks gestation. Several large sMRI studies have examined gyrification cross-sectionally (that is, imaging many individuals of different ages at one point in time) or longitudinally (i.e. repeatedly imaging (typically fewer) individuals at numerous time points). Both cross-sectional and longitudinal sMRI studies find that the gyrification index increases after birth, peaks at around 2 years of age, and then decreases throughout adulthood (at a rate of approximately 1% per year). Infantile increases in gyrification are faster in the first than the second year of life and are faster in association areas than in visual, auditory and sensorimotor cortex. Subsequent reductions in folding are most pronounced in frontal and occipital cortices and might be slightly slower in males than in females.