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After reading this article you will learn about Localisation of Mental Functions in the Cerebrum:- 1. Subject-Matter of Cerebrum 2. Integrative Function of the Cerebral Cortex.
Subject-Matter of Cerebrum:
Each cerebral hemisphere is divided into four lobes. The frontal lobe extends from the front of the brain in the forehead to the central fissure. The parietal lobe extends behind the central fissure and passes over into the occipital lobe which lies at the back of the head. The temporal lobe lies below the fissure of Sylvius in the region of the temples and rear-ward.
The different parts of the cerebrum are connected with different mental functions. The visual area lies in the occipital lobe at the rear of the brain. It is connected with the visual organs or eyes through the optic nerves. It is the seat of visual sensations.
The auditory area lies in the temporal lobe where it dips into the fissure of Sylvius. It is connected with the auditory organs or ears through the auditory nerves. It is the seat of auditory sensations. The olfactory area and the gustatory area are not visible from the surface. The olfactory area lies in a secluded position covered by the temporal lobe.
The somesthetic area lies just behind the central fissure. It is connected with the tactual organ of the skin through the tactual nerves. It is also connected with the muscle senses through sensory nerves.
It is the seat of tactual or cutaneous sensations. It is also the seat of muscular or kinesthetic sensations.
The motor area lies just in front of the centre fissure. It is connected with the muscles through the motor nerves. It is the seat of muscular movements or voluntary actions. In the motor area the highest region is connected with muscles of the legs. The next lower region is connected with the muscles of the trunk of the body.
The next lower region is connected with the muscles of the arms. The lowest region is connected with the muscles of the face. Thus the highest part of the motor area is connected with the lowest part of the body and the lowest part of the motor area is connected with the highest part of the body. The motor centres of speech lie in the lowest part of the motor area.
There is one large area in the parietal, temporal, and occipital lobes, lying between the several sensory areas. There is another large area in the frontal lobe lying forward of the motor area. These are called association areas. They have a synthetic function, and combine the activities of the sensory areas and the motor areas.
These association areas are supposed to be the seats of higher mental processes, e.g., reasoning, volition, learning by experience, etc. So the different parts of the brain are connected with different kinds of consciousness. Different mental functions are localised in different parts of the brain.
The localisation of different mental functions in different parts of the cerebrum is proved by the following evidence. First, weak electric currents are applied to a particular region of the
exposed cerebral cortex, and a movement of the legs is produced. This shows that the stimulated area is the motor area controlling the movements of the legs. This is the method of stimulation. Secondly, a particular region of the occipital lobe is extirpated and vision is impaired or destroyed. This shows that this portion of the cerebrum is the visual area. This is the method of extirpation.
Thirdly, a patient suffering from impaired vision dies. Postmortem dissection of a portion of the occipital lobe of the cerebrum reveals an injury in it. This proves that it is the visual area. This is called the pathological method.
Fourthly, the optic-nerve is observed by dissection to be connected with the visual area, the auditory nerve, with the auditory area, and so on. It is called the method of nerve-fibre tracing.
The theory of localisation of mental functions has been weakened by K. S. Lashley’s experiments on rats and other animals. He made the following observations. Ordinary mazes could be learned with one part of the cortex equally well as with another which was removed. Even if half the cortex was removed, simple movements were learned with equal speed, but complicated movements could not be learned.
From these observations he enunciated the two principles about the functioning of the brain:
(i) The Principle of Equipotentiality:
One part of the cerebral cortex is potentially the same as another part in the capacity of learning.
(ii) The Principle of Mass Action:
The cerebral cortex functions as a whole, so that the more cortex is available, the greater is the capacity of learning. If a large part of the cortex is intact, a rat learns complex trick more quickly.
The human brain is more evolved and differentiated than the animal brain. So Lashley’s findings on animal learning may not apply to the human brain. So the theory of localisation of mental functions has not been disproved by Lashley’s experiments.
Integrative Function of the Cerebral Cortex:
The cerebral cortex has primary areas which control the incoming sensory stimuli and the outgoing motor responses. An individual is able to adjust himself effectively to the environment because the various nerve impulses are systematically integrated by the brain.
There are millions of nerve-fibres which connect the various neurons of the brain. The connecting nerve-fibres are known as ‘associative fibres’. The associative fibres are the foundations of memory, language, reasoning and other higher mental processes. There is great coordination between the various parts of the brain.