The cerebrum

The cerebrum has a left and right hemisphere that are united by a bridge-like structure called the corpus callosum, through which nerve cells in one hemisphere communicate with their counterparts in the other. In general the left hemisphere is concerned with the right part of the body and vice versa. The surfaces of the hemispheres are intricately folded to form a pattern of ridges called gyri and furrows called sulci. This folding pattern provides a much greater area of cortical grey matter than would be expected. Each hemisphere is subdivided into four lobes by prominent furrows in the cortex. The lobes are further divided into relatively constant gyri. These all have specific functions. Although anatomically symmetrical, the hemispheres do not function in the same way. One of them – usually the left – is the dominant hemisphere. Because each hemisphere is responsible for the movement of the limbs of the opposite side of the body, this explains why most of us are right-handed. The ability to speak also resides in the left hemisphere.

As a consequence of this right-left difference the effect of brain damage depends on the side that is involved. A stroke in the left hemisphere can disturb speech and movements of the right arm and leg. A stroke in the right hemisphere in general has no effect on speech; it will however effect the left limbs. The dominant left hemisphere is also regarded as the seat of logical thought. It is skilled at processing information sequentially, which is essential for the way we read, write and speak. It is also necessary for deductive reasoning. The non-dominant hemisphere, on the other hand, specializes in dealing with the orientation of the individual in space. It is concerned with the appreciation of visual art and music. The dominant hemisphere deals with algebra and the non-dominant with geometry. The individual lobes have specialized functions too.

The frontal lobes

The foremost lobes are the frontal lobes. They are the site of constructive thinking, planning ahead and the weighing of the consequences of ones deeds. Therefore the frontal lobes are concerned with the regulation of social behaviour. Damage to this area results in uninhibited, inappropriate behaviour and lack of initiative. (In former times in psychiatry an operation to cut fibres in the lobes, called lobotomy, was performed in very aggressive chronic psychotics. The results were less aggression, lack of initiative and less personality.) The motor cortex is a strip of the frontal lobe bordering the parietal lobe. This area is responsible for voluntary movements of muscles on the opposite side of the body. Because the hands and the mouth and tongue make very many complex movements, their control occupies a very large part of the motor cortex. Impulses for movement are generated here and nerve fibres from this area converge on the brain stem where they form a compact bundle called the pyramidal tract. The fibres of the pyramidal tract terminate on the nerve cells of the spinal cord that bring about muscle contraction. Part of the speech area in the dominant hemisphere is located on the lower border of the frontal lobe. This area is con- cerned with the expressive component of speech. Damage to it results in speaking difficulties even though the tongue muscles are intact. The ability to understand speech is not damaged.

The parietal lobes

Lying behind the central sulcus, the parietal lobes are responsible for the conscious appreciation of sensory information. This includes the location of touch, the awareness of the position of the joints and the ability to recognize objects by touch. Sensory information of the whole body is mapped out here. Because the fingers and the lips have many touch corpuscles, they are allocated a large part of this area. Damage to it can result in a lack of sensation in the opposite side of the body. Such a patient can strike a match, but does not know without looking if he is burning his fingers.

The occipital lobes

The hindmost part of the cortex is responsible for vision. The right occipital lobe sees the left half of the field of vision and the left lobe the right half. Integration of these halves gives us the ability to see stereo-scopically, in depth.

Nterpretation of visual impulses takes place there, as does co-ordination of ocular reflexes. That is why a blow on the back of the head causes us to see stars. Permanent damage to this area leaves the patient able to distinguish only between dark and light.

The temporal lobes

The hearing cortex is in the temporal lobe. Like the visual fields, the cochlear impulses from the inner ear are mapped out in the temporal lobe. The understanding of speech is a property of the dominant temporal lobe (and a part of the parietal lobe). The temporal lobes are also concerned with memory, but the dominant lobe specializes in verbal memory. The non-dominant lobe deals with visual memory. This area is connected with an underlying primitive part of the brain, the limbic system, which regulates aggressive and sexual behaviour, and is responsible for the conscious appreciation of sound and smell.