Physiological psychologists from the days of Johannes Muller are trying to answer the question of whether or not the brain functions as a ‘whole’. There are differences in views. A group of researchers believed that each part of the cerebrum had a definite function. Other group of researchers believed that parts of the brain are functionally interchangeable. From the modern findings, the answer is that brain functions in parts as well as in whole. Experiments on localization by Franz and Lashley were carried out and their findings are expressed in two theories, i.e., theory of equipotentiality, and theory of mass action.

Theory of Equipotentiality:

A monkey was trained to open a door by handling a switch after continuous training. After the learning was established, a portion of the frontal lobe was removed. It was found that the monkey could not perform the learnt skill of door-opening showing forgetting.

Thus, one can conclude that motor activity is dependent on a part of the motor cortex area of the frontal lobe. However, it was found that the monkey could relearn the same trick. However, when more amount of cerebral cortex was removed the animal could not learn again. Both Larshley and Franz did such studies repeatedly on several animals. Their findings were that ordinary maze learning was possible equally well in any part of the cortex. Difficulty of task needs more amount of cortex. Lashley concluded that complex learning does not depend on any definite structure of a specific area of cortex. It rather depends on the total organization of the cortex.


The theory of equipotentiality means capacity of the injured part of the brain to function as a substitute for another lesioned part in the brain. That means, according to this theory all parts of the cortex are equally potential enough for simple learning function.

The Theory of Mass Action:

This theory put forth the findings that the brain functions as a whole, and therefore more parts of the cortex available, the better would be the learning capacity. By comparing decorticated animals with normal animal, Lashley found that animals having decorticated cortex demonstrated general reduction in sensitivity, exploratory activities and aggressiveness. Thus, the removal of any part of the brain affects the learning process in a general manner.

On the basis of the experimental findings with apes and injured persons, the two theories are found to be confirmed. Thus, according to these theories the brain functions as a whole and in parts in some respect.