![]() ![]() The Science study both disproves those hypotheses and reconciles anomalies like the manatee with a simple physical law. These outliers seemed to suggest that different species possess different mechanisms to control cortical folding-that is, each species has its own way of growing a brain. Baboon and pig cortices display equivalent amounts of folding, yet baboons have 10 times as many neurons as pigs do. Human cortices have three times the number of neurons than elephant cortices, yet human brains are half the mass and far less folded. The alternative would be a cortex that did not fold but expanded like a balloon, an inefficient use of the cramped quarters of the head. The 19th-century neuroanatomist Franz Gall hypothesized that brains folded to allow a much larger cortical surface to fit inside the space of the skull. The second explanation posited that brains fold both as a result of an increasing number of neurons and also as a way to allow this neural growth. Dolphin brains, for example, are much more finely grooved than similarly sized human brains. Cetaceans also proved a problem their cortices were more convoluted than would make sense for the sizes of their brains. Although it has a brain the size of a baboon’s, the latter’s boasts a high degree of folding whereas a manatee’s is almost entirely smooth. But the idea failed to account for major outliers like the manatee. The first hypothesis held that the larger a brain is, the more folded its cortex will be. For a long time two explanations held sway but both were riddled with exceptions, according to neuroscientist and co-author Suzana Herculano-Houzel of the Federal University of Rio de Janeiro. Researchers knew that gyri and sulci arise in humans during the third trimester of gestation but could never pinpoint the forces behind their formation. Having a thinner, folded cortex means information transfer from one point in the brain to another covers less distance and can happen far more rapidly. There is a clear advantage to such cortical folding. The human brain cortex-its outer layer-is an intricately furrowed landscape with ridges called gyri and valleys called sulci. The research also casts light on the mechanisms behind certain structural disorders of the brain and could inform future efforts to ease those ailments. The “brilliant study” represents a significant advance in scientists’ understanding of how the brain develops, says neurologist Arnold Kriegstein of the University of California, San Francisco, who was not part of the study. As it turns out, the growing mammalian brain folds just like any sheet of office paper, governed by a single mathematical function. But if someone crumples the paper into a ball, they’re holding the solution to one of the most longstanding mysteries of brain development, according to a study published July 2 in Science. A brain and a crisp sheet of office paper may seem to have little in common. ![]()
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