6. The cerebral cortex is also organised in a hierarchical way with superimposed loops

My proposal that the nervous system is organised in a hierarchical way with superimposed neural loops, increasingly distant from the external world, also extends to the cerebral cortex.

6.1 Evolution and development of the cortex

The neocortex is a relatively recent evolutionary development well developed in mammals¹. The first components that appear in evolution are related to the specific topographical organisation of sensory inputs and motor outputs. A basic brain structure of a cortex and subcortical nuclear structures involved in olfaction, bodily sensing, and memory functions are shared between reptiles, birds, and mammals². 

As was well described by Weninger and Parlotta in 2023, “only a few hundred genes showed human-specific expression patterns, and these were disproportionately near genomic regions with signs of evolutionary selection in humans. These results suggest that the specific properties of the adult human cortex may derive from relatively few cellular and molecular changes”³. LeDoux clearly described the expansion of neocortex in evolution with early mammals having about twenty functional areas whereas early primates had forty or fifty areas. These changes in primates culminated with an expansion of the prefrontal lobes⁴. The frontal lobes have gone through the largest changes in evolution as was already noticed in the 1920s by Leonardo Bianchi, an Italian neurologist who coined the term social brain, referring to the human frontal lobes.

Gross differences are also observed in the human brain when it is compared to those of our evolutionarily closest relatives. For example, the human brain is triple the size of modern great ape brains, but motor and visual cortices are about the same absolute size⁵. The sensory and motor association areas became larger during human evolution and are larger than in other mammalian species. This observation suggests that expansion of the human cerebrum disproportionately involves secondary and association cortical areas beyond those subserving basic sensory and motor functions⁶.

These data have led to the proposal that during both evolution and development, the cortical plate expands from the more primitive primary cortices by a so-called “tethering process”. This tethering hypothesis would explain the expansion of the cortex beyond the primary cortex to form secondary and association cortices. According to this hypothesis, the developing cerebral cortex forms from a modest number of “core organising maps that act as anchors”, starting from the primary sensory and motor cortices which emerge early in development. “These anchors covered much of the ancestral mammalian cortex but now occupy little of the modern human cerebral mantle”⁷.

The cortex is folded inside the skull to form gyri (ridges) and sulci (grooves) which greatly increase the surface area of the cortex that can be packed inside the skull. When flattened, its surface area is about 960 -1016 cm², compared with only 120 cm² in the macaque, for example⁸. The cortex represents about 80% of the mass of the human brain and contains more than 16 billion neurons. Recently researchers catalogued more than 3,000 different types of neurons in the cortex, all of which are some kind of interneuron⁹. The number of synaptic connections in the cortex, including interconnections between different parts of the brain, is gigantic. Altogether, these neurons are connected via approximately 149,000 to 176,000 kilometres of myelinated axons and more than 164 trillion synapses¹⁰. 

Although there are significant differences across the cortex, the fundamental structure of the cortex in cellular layers is relatively constant. The main problem is that the cerebral cortex is continuous sheet of neurons and nerve fibres organised in relatively constant layers without clear discontinuities along its surface.

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1. W Singer, TJ Sejnowski, P Rakic (eds, 2019) The Neocortex. MIT Press. ↩︎
2. L Krubitzer & J Kass (2005) The evolution of the neocortex in mammals: how is phenotypic diversity generated? Current Opinion in Neurobiology 15, 444-453. ↩︎
3. A Weninger & P Arlotta (2023) A family portrait of human brain cells – A cell census provides information on the source of human brain specialization. Science 382, 168-169. ↩︎
4. J LeDoux (2019) The Deep History of Ourselves: The Four-Billion-Year Story of How We Got Conscious Brains. Viking. ↩︎
5. SM Blinkov & II Glezer (1968) The Human Brain in Figures and Tables: A Quantitative Handbook. Basic Books, Plenum.
   HD Frahm, H Stephan, M Stephan (1984) Comparison of brain structure volumes in Insectivora and Primates. I. Neocortex. Journal für Hirnforschung 23, 375-389 (this is the first of a series of papers on the relative size of brain regions). ↩︎
6. JH Kaas (2008) The evolution of the complex sensory and motor systems of the human brain. Brain Research Bulletin 75, 384–90.
   JH Kaas & S Herculano-Houzel (2017) What makes the human brain special: key features of brain and neocortex. In: Physics of the Mind and Brain Disorders (eds I Opris & MF Casanova), Springer, pp 3–22.
   L Krubitzer (2007) The magnificent compromise: cortical field evolution in mammals. Neuron 56, 201–208.
   PT Schoenemann (2006) Evolution of the size and functional areas of the human brain. Annual Review of Anthropology 35, 379–406.
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7. RL Buckner (2013) The evolution of distributed association networks in the human brain. Trends in Cognitive Sciences 17, P648-665. ↩︎
8. DC Van Essen & DL Dierker (2007) Surface-based and probabilistic atlases of primate cerebral cortex. Neuron 56, 209–225.
   JK Rilling & TR Insel (1999) The primate neocortex in comparative perspective using magnetic resonance imaging. Journal of Human Evolution 37, 191–223. ↩︎
9. See https://www.science.org/doi/10.1126/science.adl0913 ↩︎
10. W Singer, TJ Sejnowski, P Rakic (eds, 2019) The Neocortex. MIT Press. ↩︎

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