The Cerebral Cortex

What the cortex is

The cerebral cortex is the outermost layer of the cerebrum, a continuous sheet of grey matter draped over each hemisphere. In cross-section it is strikingly thin, between about 2 and 4 millimetres, yet it is densely packed with neurons: tens of billions of cell bodies arranged in orderly layers. Because it forms the surface of the brain, the cortex is what gives the organ its familiar wrinkled appearance.

The word cortex comes from the Latin for bark, and the image is apt: like bark on a tree, it is a thin rind over a larger core. That core is white matter, the mass of fibres connecting one part of the cortex to another and to deeper structures. The cortex is not uniform. Most of it is neocortex, the six-layered tissue that dominates the human brain, but older regions with fewer layers, such as parts of the limbic border, follow a different plan.

Why the cortex is folded

The most obvious feature of the human cortex is that it is not smooth. Its surface is thrown into a pattern of ridges and grooves, and this folding is not decorative: it is a solution to a packing problem. A large, thin sheet cannot fit inside a compact skull unless it is crumpled, much as a large piece of paper must be folded to fit a small box.

The ridges are called gyri and the grooves between them sulci (a particularly deep sulcus is called a fissure). By folding the sheet this way, the brain fits far more cortex into the skull than a smooth surface would allow, roughly tripling the usable area. In humans, about two-thirds of the cortical surface is hidden from view, tucked inside the folds. Larger and deeper folds are a hallmark of brains with more cortex to accommodate.

Grey matter and white matter

The cortex is grey matter, and understanding the distinction between grey and white matter is central to understanding how it works. Grey matter is tissue dominated by neuron cell bodies, along with their dendrites and the synapses between them. This is where signals are received, combined, and processed: grey matter is where the computing happens.

Directly beneath the cortex lies white matter. This is made of axons, the long output fibres of neurons, most of them wrapped in a pale myelin sheath that gives the tissue its colour. White matter is the wiring: it carries signals from one patch of cortex to another, between the hemispheres through the corpus callosum, and down to deeper structures and the spinal cord. The cortex, then, is a thin processing sheet sitting on top of a much larger volume of connecting cable.

The six layers

Look closely at the neocortex and it resolves into six horizontal layers, or laminae, running parallel to the surface. Each layer holds a characteristic mix of cell types and connections, and the layered plan is remarkably consistent across the whole neocortex, which is one reason the cortex can be described as a general-purpose sheet applied to many different tasks.

Numbered from the outer surface inward, the layers can be summarised in general terms. Layer I, the molecular layer, is sparse in cell bodies and rich in connecting fibres. Layers II and III, the outer granular and pyramidal layers, are heavy with the neurons that link one cortical area to another. Layer IV, the inner granular layer, is the main receiving station for incoming sensory signals arriving from the thalamus, and it is especially thick in sensory areas. Layers V and VI, the deep pyramidal and multiform layers, contain the large output neurons that send commands down to the brainstem, spinal cord, and other subcortical targets.

This vertical arrangement matters as much as the horizontal one. Neurons in a column running through all six layers tend to respond to the same feature and work together as a unit, so the cortex can be pictured as an array of such columns, each a small processor, tiled across the folded sheet.

The thickness of individual layers varies from region to region, and this variation is itself informative. Sensory areas, which take in a great deal of incoming information, have a thick layer IV; motor areas, which mainly send commands out, have a thin layer IV and prominent output layers V and VI. Early anatomists mapped these differences in layering across the whole cortex and used them to divide it into dozens of distinct areas, a scheme still referred to today when neuroscientists speak of numbered cortical areas. The lesson is that the six-layered plan is a theme with many variations, each tuned to the work a region does.

Sensory, motor, and association areas

Although the cortex is built from the same basic layered tissue everywhere, different regions specialise in different jobs. Broadly, cortical areas fall into three functional classes, and this division is a useful spine for understanding what the cortex does.

The balance of these areas is telling. In the human brain, the sensory and motor areas together occupy only a modest fraction of the cortex; the great majority is association cortex. It is this expansion of integrative tissue, more than any single specialised area, that most distinguishes the human cortex and underlies its capacity for abstract thought and language.

Association areas are not a single uniform region but several. The prefrontal association cortex at the front supports planning, judgement, and self-control; the parietal-temporal-occipital association cortex, where those three lobes meet, binds together vision, hearing, and touch into a unified perception of the world; and the association cortex of the temporal lobe helps with recognising objects and faces and with attaching meaning to what is perceived. Between them these regions turn raw sensation and simple movement into the rich, flexible behaviour of a thinking animal.

Cortical maps and the homunculus

One of the most elegant findings in neuroscience is that the sensory and motor areas are organised as orderly maps of the body. In the primary somatosensory cortex, which handles touch, and the primary motor cortex, which drives movement, the body is laid out in a continuous strip: adjacent parts of the body are represented by adjacent patches of cortex, so the map preserves the layout of the body it serves.

These maps are not drawn to scale. The amount of cortex devoted to a body part reflects not its size but its importance for fine sensation or control. The hands, lips, and face take up disproportionately large areas, while the trunk and legs take up relatively little. If the body were redrawn in the proportions the cortex assigns it, the result would be a distorted figure with huge hands and lips and a small torso, an image known as the cortical homunculus, meaning "little man".

Hemispheres and lateralisation

The cortex is split into two hemispheres, left and right, joined by the corpus callosum. In general each hemisphere handles sensation and movement for the opposite side of the body. Beyond this crossing, some functions are handled more by one hemisphere than the other, a property called lateralisation. In most people, for example, the main language areas lie in the left hemisphere, while aspects of spatial attention and the processing of faces lean toward the right.

Lateralisation is genuine, but it is routinely misunderstood. The popular notion that individuals are logical left-brained or creative right-brained personalities has no scientific basis. Lateralisation describes where certain processing tends to be concentrated across the population; it is not a claim that a person uses one hemisphere more than the other, and it does not divide personality between the sides.

Understood correctly, the two hemispheres are best thought of as a single cooperative system with a modest division of labour, not as two rival minds. The steady traffic across the corpus callosum keeps them working as one.

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