The most neocortical neurons belong to a dolphin, not to us
The neocortex, a folded sheet only 2 to 4 mm thick, handles perception, planning and language and makes up about three-quarters of the human brain. Yet among mammals studied so far, the species with the most neurons in this layer is not Homo sapiens but the long-finned pilot whale, an oceanic dolphin.
The name joins the Latin for bark or rind with the Greek for new, because it is the most recently evolved part of the cerebral cortex. In humans it occupies 90% of the cortex and 76% of the whole brain, with the older allocortex making up the rest. Its six layers, numbered I to VI from the surface inward, are found in every mammal and in no other animal, though birds show clearly cortex-like thinking using a differently built pallium that many researchers consider broadly equivalent.
The layers divide the labour. Layer IV receives most incoming signals, largely relayed from the thalamus, and spreads them to its neighbours. Pyramidal cells in layers II and III talk to other cortical areas, while those in V and VI send output down to the brainstem and spinal cord. About 80% of neurons excite their targets and about 20% inhibit them. Researchers often describe the tissue as built from repeating columns about 0.5 mm across, but definitions clash so badly that there is no agreement the columns are real functional units.
Small mammals such as rodents have a smooth neocortex; elephants, dolphins and primates have deep grooves and ridges that multiply the surface area, and every human brain shares the same broad folding pattern. How the folds form in the embryo is still disputed, with tension in nerve fibres, mechanical buckling and uneven cell growth all proposed. Size appears tied to social life: the ratio of neocortical grey matter to the medulla is about 30 to 1 in chimpanzees and 60 to 1 in humans.
Damage reveals its roles. Injury to the front of the temporal lobe erases factual knowledge, producing semantic dementia, while memories of personal events survive. One recent review argues that the neocortex builds predictive models of the world, and that transformer-based AI has independently arrived at similar principles.
Source: Neocortex
