The Cerebrum

What the cerebrum is

The cerebrum is the uppermost and largest part of the brain, the structure that develops from the embryonic telencephalon and grows to overhang everything beneath it. In an adult human it accounts for the great bulk of brain volume, so much so that when you look at a brain from above or from the side, the cerebrum is almost all you can see; the cerebellum peeps out at the back and the brainstem disappears underneath.

It is not a single lump. Anatomically it consists of three components stacked from the outside in. On the surface is the cerebral cortex, a thin folded sheet of grey matter. Beneath the cortex lies a much larger volume of white matter, the bundled axons that connect one part of the cerebrum to another and to the rest of the nervous system. And buried within that white matter are the deep grey-matter nuclei: the basal ganglia, the amygdala, and the hippocampus. The whole assembly is duplicated on the left and right and separated by the deep longitudinal fissure.

Whether the diencephalon counts as part of the cerebrum is a matter of convention rather than fact. Most textbooks treat the cerebrum as the telencephalon alone, with the thalamus and hypothalamus described separately as the diencephalon, and that is the convention used throughout this reference.

Cerebrum is not the same as cortex

These two words are used interchangeably in casual writing, and doing so quietly destroys a distinction that matters. The cerebrum is a three-dimensional mass; the cortex is a two-dimensional sheet wrapped around it. Almost every anatomical confusion about the brain traces back to blurring the two.

The practical consequence is clinical. A lesion that destroys cortex produces one kind of deficit, typically a loss of a specific function such as vision in part of the visual field, or the ability to produce fluent speech. A lesion that spares the cortex but cuts through the white matter beneath it can produce a superficially similar deficit by disconnecting intact regions from one another. And a lesion in the deep nuclei, sparing both cortex and its main pathways, produces something different again: the movement disorders of Parkinson's or Huntington's disease. All three are lesions "in the cerebrum". Only the anatomical precision tells you what to expect.

The two hemispheres

The cerebrum is split almost entirely in two by the longitudinal fissure, a deep cleft running front to back down the midline into which a fold of dura mater, the falx cerebri, descends. The result is a left and a right cerebral hemisphere, near mirror images in gross shape and, in the great majority of their structures, in what they contain.

The near-symmetry conceals two important asymmetries. The first is functional: as discussed below, some capacities are handled more by one side. The second is anatomical, and it maps onto the first. The planum temporale, a region of the temporal lobe involved in language, is typically larger on the left, and the Sylvian fissure runs a different course on the two sides. These asymmetries are present at birth, which suggests they are laid down developmentally rather than produced by learning to speak.

Each hemisphere deals principally with the opposite side of the body. Motor commands from the left hemisphere cross the midline in the brainstem and reach the right arm and leg; sensation from the right hand arrives in the left parietal lobe. Vision follows a subtler rule: it is not each eye but each visual field that crosses, so the left half of what you are looking at, seen by both eyes, is processed on the right. This crossing, called decussation, is one of the oldest and most reliable facts in neuroanatomy, and it is the reason a stroke in the left hemisphere weakens the right side of the body.

The four lobes

Each hemisphere is divided by two prominent grooves into four lobes, named after the skull bones that cover them. The central sulcus runs down across the top of the hemisphere, separating the frontal lobe in front from the parietal lobe behind; the lateral (Sylvian) fissure runs back from the front along the side, with the temporal lobe below it.

Two further regions are conventionally added. The insula, a patch of cortex buried deep inside the lateral fissure and invisible from outside, handles taste, visceral sensation, and awareness of internal bodily states. The limbic lobe is a ring of cortex on the medial surface, including the cingulate gyrus and parahippocampal gyrus, forming part of the limbic system. The four lobes have a dedicated page.

Grey matter, white matter, and the tracts

Cut a fresh cerebrum in half and the two tissues are immediately visible. A darker rim of grey matter follows the folded surface; a pale mass of white matter fills the interior; and within that pale mass are further islands of grey. The colours are not arbitrary. Grey matter is grey because it is crowded with neuron cell bodies, dendrites, and synapses. White matter is white because it is packed with axons wrapped in myelin, a fatty insulating sheath.

The white matter of the cerebrum is not an undifferentiated mass. It is organised into three classes of fibre bundle, and once you know the three, the wiring diagram of the cerebrum falls into place.

The islands of grey matter inside the white are the deep nuclei: the caudate nucleus and putamen and globus pallidus, which together form the basal ganglia; the amygdala in the medial temporal lobe; and the hippocampus curling along the floor of the lateral ventricle. These are not part of the cortex, but they are unmistakably part of the cerebrum, and any account of the cerebrum that stops at the cortex has left out the machinery of movement selection, habit, and emotion.

The corpus callosum and other commissures

The two hemispheres would be of limited use as separate machines. They are joined by a set of commissures, and the dominant one is the corpus callosum, a broad, thick, C-shaped sheet of white matter arching over the ventricles. It carries on the order of 200 million myelinated axons, making it the largest fibre tract in the human brain. Anatomists divide it into the rostrum and genu at the front, the long body, and the splenium at the back, and the divisions matter because different regions of cortex send their fibres through different parts: frontal traffic through the genu, occipital traffic through the splenium.

Smaller commissures supplement it. The anterior commissure links parts of the temporal lobes and the olfactory regions, and it is thought to be evolutionarily older than the corpus callosum. The posterior commissure and the hippocampal commissure connect further specific territories.

What happens if the corpus callosum is cut? This is not hypothetical. In the twentieth century, surgeons severed it in a small number of patients with otherwise untreatable epilepsy, to stop seizures spreading from one hemisphere to the other. The operation worked, and the patients were, to casual observation, unchanged. But careful testing revealed something remarkable: when information was presented to one hemisphere alone, the other genuinely did not have access to it. A patient might be able to select an object with the left hand that the right hemisphere had seen, yet be unable to name it, because the language areas in the left hemisphere had not received the information.

Lateralisation, and the myth it spawned

Lateralisation is real. It refers to the fact that certain functions are handled more by one hemisphere than the other. The best-established example is language: in about 95 per cent of right-handed people, and in roughly 70 to 75 per cent of left-handed people, the main language areas lie in the left hemisphere. That gap is worth noticing. Left-handedness shifts the odds of right-hemisphere or bilateral language, but it does not reverse them, and the majority of left-handers are still left-lateralised. The clinical evidence is unambiguous, since damage to these left-hemisphere regions produces aphasia while equivalent damage on the right typically does not.

Other functions lean the other way. Aspects of spatial attention, the perception of faces, and the interpretation of the emotional tone of speech are more associated with the right hemisphere, which is why right parietal damage can produce hemispatial neglect, a striking condition in which a patient ignores the entire left half of space.

From these genuine findings a wholly unsupported idea took hold in popular culture: that people are either left-brained, meaning logical, analytical, and verbal, or right-brained, meaning creative, intuitive, and artistic. This is not what lateralisation means and it is not what the data show.

The accurate picture is of a cooperative system with a modest and specific division of labour. The two hemispheres are not rivals and not different kinds of mind. They are two halves of one organ, wired together by the largest fibre tract in the body precisely so that they can work as one.

What the cerebrum does

Summarising the function of the cerebrum risks sounding like a list of everything the brain does, and that is close to the truth. Still, its contributions can be grouped.

Perception. Sensory information from the eyes, ears, skin, and tongue arrives in the cortex, mostly by way of the thalamus, and is analysed there. The occipital lobe turns retinal signals into seen objects; the temporal lobe turns sound waves into recognised words; the parietal lobe turns skin pressure into felt touch.

Voluntary movement. The frontal lobe plans and issues motor commands. The primary motor cortex sends the final signal, but a wide network of premotor, supplementary motor, and parietal areas prepares and sequences the action first, and the basal ganglia and cerebellum tune it.

Language. Producing and understanding speech and writing depends on cortical regions, mostly on the left, and on the white-matter tracts connecting them. Cutting the connection alone, without damaging either region, is enough to disrupt language.

Reasoning, planning, and control. The prefrontal cortex supports executive function: holding a goal in mind, resisting a tempting but wrong response, switching between tasks, sequencing steps toward an aim.

Memory and emotion. The hippocampus, deep in the medial temporal lobe, is essential for laying down new conscious memories; the amygdala attaches emotional significance to events; and the cortex is where memories are ultimately stored and re-activated.

The picture that emerges is not one of separate centres each doing a job. It is one of distributed networks, made of cortical patches linked by white matter, with the deep nuclei modulating them. The cerebrum is best understood not as a collection of organs but as a very large, very well-connected network, and that is why disconnection can be as damaging as destruction.

Related topics