Number and Complexity of Cortical Gyrii
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The gyrification index (GI) increases from prosimians to humans, with humans having the highest gyrification index of all observed extant primates. However, species from other orders are known to have greater GI than humans, with cetaceans and elephants serving as the primary example: human GI = 2.56, dolphin GI = 4.3, Pacific pilot whale GI = 5.55, elephant GI = 3.81. The ratio of cortical gray matter to body size is also highest in humans, though dolphins and porpoises are close.
However, the degree of gyrification is not greater relative to other species across the entire human brain. For instance, the largest difference in gyrification between humans and primates occurs in the prefrontal cortex. If gyrification is indeed caused by high tensile strength between densely connected neurons, then the observation that humans have a higher gyrification index is compatible with the observation that humans have a greater relative volume of white matter connections, especially in prefrontal cortex. It is debated whether the seemingly equivalent gyrification index values in the caudal cortex (visual areas) of great apes and humans reflects a true parity or a failure of the gyrification index to properly account for volumetric reorganization in humans.
Furthermore, the common preconception that humans have the largest brains, or at least largest frontal cortex relative to body size, is likely false. MRI data indicates human frontal cortex size is not disproportionately large in comparison to the great apes, although comparative cytoarchitectonic studies that would prove this assertion have not been done.
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