The cerebral cortex

This structure consists of four main segments or lobes: frontal, temporal, parietal and occipital. Each of the four has various complex functions. The occipital lobe, at the back of the brain, does most of the processing of visual information. The parietal, across the top of the brain, co-ordinates the body image (from the sense of touch and the internal receptors that detect information about balance and posture). It also oversees body movements. The tasks of the temporal lobes, one on each side of the brain, include processing auditory information. The frontal lobe is concerned with - among many other things - cognition and abstract thought. As a very rough rule of thumb, the front half of the cortex controls outputs such as muscle movements and the back half is concerned with processing sensory inputs; but this division is far from absolute.

Sensory inputs come from two main sources: the environment, and the animal's body. In mammals, the environment is detected by seeing, hearing, touching, tasting and smelling; the relative importance of these five main sensory modalities varies from species to species. In humans, some 85% of the information about the environment processed by the brain is visual; the visual areas occupy a large part of the cortex, mainly the occipital lobe. Information from the body includes sensations of position and movement in head and limbs, sensations of gravity and acceleration, and basic physiological operations such as heart function, respiration and nutritional status. The brain - and in particular the cerebral cortex - has the task of relating all the information from these two main sources moment by moment and executing the appropriate behavioural outputs.

Throughout the cortex, neurones are organised in cylindrical "modules" with their axes perpendicular to the surface of the brain. There are about 150 neurones in each module (rather more in the visual areas of the occipital lobe), arranged vertically in six layers, with layers of horizontal neurones between. Each neurone can make connections with hundreds, thousands or tens of thousands of others, both within the cortex and in the rest of the brain. Their cell bodies bristle with dendrites and their axons have massive terminal arborisations. Because the number of connections among cerebral neurones is so astronomical, it is easy to forget that the whole structure of the cortex is organised in simple repeating patterns. But much of our knowledge of cortical function has come from studying neurones in particular cylindrical modules.

The cerebral cortex is by no means independent of the rest of the brain. For example, it is continuously activated by a "motivational" system in the centre of the brain, the basal ganglia, and in particular the corpus striatum. These structures are activated by centres in the hindbrain and cerebellum that process information from the body; they receive inputs from the cortex; and they are crucially important in controlling and co-ordinating movements.

Fig. 16-4: the human cerebral cortex and cerebellum.

The hippocampus, located below the temporal lobes, processes visual information before it reaches the visual cortex and is also of primary importance in storing memory, especially spatial memory. All parts of the brain, including the cortex, are interconnected.

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