A Tour around the Cerebral Cortex

(Date of publication 09 May 2005)

Question: why is the cerebral cortex, which comprises the external surface of the cerebral hemispheres, so highly convoluted and folded? Answer: because during the evolutionary process its volume increased more rapidly than the underlying tissue, producing grooves (sulci) and more elevated regions (gyri). Many areas of the cortex either process sensory information or co-ordinate motor output – these are divided into primary, secondary and tertiary sensory or motor areas, depending on their degree of involvement. For example, primary sensory areas receive information from peripheral receptors on the contralateral side of the body with only a few synapses between. There are also three large regions, called association areas, which make up the largest area of the cortex in primates. These are concerned with forming complex perceptions, planning voluntary movement, motivation, emotion and memory. If you are not sure of the precise location of the different functional areas described on this page, simply surf to this site and move your mouse over the interactive diagram.

Interestingly, the two cerebral hemispheres are not functionally equivalent. For example, left or right-handedness reflects an asymmetry for fine motor ability, and certain visuospatial skills are more highly developed in the right hemisphere. In over 95% of right-handers, the left hemisphere is dominant for language, while in left-handers either left hemisphere dominance or bilateral language capabilities are the commonest findings.

The major connection between the two cerebral hemispheres is the corpus callosum, which integrates the functions of the right and left sides. If this structure is completely divided, the results are bizarre; a right-handed person is unable to name aloud objects felt with the left hand (anomia), cannot read aloud text presented only to the left side of the visual field (hemialexia), and cannot execute with the left hand actions described by an examiner (apraxia). Apraxia usually diminishes within in a few months, whereas the hemialexia and unilateral anomia persist for years.

However, partial or complete surgical section of the corpus callosum can benefit epileptics who experience 'drop attacks', when they suddenly fall to the ground with a sudden jerk (myoclonic seizure), by becoming stiff (tonic seizure) or by becoming floppy (atonic seizure). About 50% of children who undergo this procedure have a reduction in the frequency and severity of their seizures. One study of patients who had been operated on at least two years previously indicated that complete division of the corpus callosum is more effective than partial division, and that children derive greater benefit from the procedure than adults.

On a microscopic level, most of the cerebral cortex (isocortex) is characterised as having six distinct layers. However, there are no actual borders between the layers, and the axons and dendrites of neurones cross the boundaries. The layers have no obvious functional significance, but it has been postulated that they form part of a laminar memory system. Pyramidal cells, which comprise the majority of neurones here and can be clearly seen in the second of these photomicrographs, span at least three layers, and sometimes all six. More information about the individual layers and the structures they contain is provided by Southern Illinois University, while a higher magnification section (with labels!) can be found here. Astrocytes have numerous sheet-like processes extending from their cell bodies and are one component of the glia, the special connective tissue of the CNS which has various supportive and nutritive functions.

In two regions the cortex has a variable number of layers and is termed allocortex. At the base of the brain it is not layered, has no pyramidal cells and is primarily involved with emotions, while in the hippocampal formation it has one layer of cells, appears to perform more sophisticated computations than the basal allocortex, and is predominantly concerned with registering new long term memories.

More detailed data about some functions of the various lobes can be extrapolated from the results of injury. For example, damage to the frontal lobes causes loss of the ability to solve problems and to plan and initiate actions. If the most anterior part of the frontal lobe is affected, there may be impaired concentration, apathy, inattentiveness and a delayed response to questions, but the impact on IQ is insignificant. Trauma to the anterior part of the parietal lobe produces numbness and impaired sensation on the opposite side of the body, but more posteriorly causes right/left disorientation and problems with calculations and drawing. Injury to the right temporal lobe tends to impair memory for sounds and shapes, whereas left temporal lobe damage can drastically affect the memory for words and the ability to understand language. If the occipital lobe is damaged on both sides of the brain, cortical blindness can result. For more information click here.

No tour of the cerebral cortex could possibly be complete without a mention of frontal lobotomy, the neurosurgical destruction of the frontal lobe which was used to treat thousands of mentally disturbed patients in the middle of the last century. With the advent of effective psychotropic drugs the use of the procedure rapidly declined. In the USA, neurologist Walter Freeman developed a lobotomy technique which involved hammering a modified ice-pick into a patient's frontal lobe through the orbit under local anaesthesia. I challenge anyone to look at this photograph without wincing!

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