The mental set for adding has previously exercised [{414}] linkages with the responses composing the addition table, while the mental set for multiplication has linkages with the responses composing the multiplication table. When the set for adding is active, a pair of numbers, seen or heard, together with this internal stimulus of the mental set, arouses the response that gives the sum; but when the multiplying set is active, the same pair of numbers gives the product as the response. All thinking towards any goal is a similar instance of the law of combination.
The Laws of Learning in Terms of the Neurone
We have good evidence that the brain is concerned in learning and retention. Loss of some of the cortex through injury often brings loss of learned reactions, and the kind of reactions lost differs with the part of the cortex affected. Injury in the occipital lobe brings loss of visual knowledge, and injury in the neighborhood of the auditory sense-center brings loss of auditory knowledge.
Injury to the retina or optic nerve, occurring early in life, results in an under-development of the cortex in the occipital lobe. The nerve cells remain small and their dendrites few and meager, because they have not received their normal amount of exercise through stimulation from the eye.
Exercise, then, has the same general effect on neurones that it has on muscles; it causes them to grow and it probably also improves their internal condition so that they act more readily and more strongly. The growth, in the cortex, of dendrites and of the end-brushes of axons that interlace with the dendrites, must improve the synapses between one neurone and another, and thus make better conduction paths between one part of the cortex and another, and also between the cortex and the lower sensory and motor centers.
The law of exercise has thus a very definite meaning when [{415}] translated into neural terms. It means that the synapses between stimulus and response are so improved, when traversed by nerve currents in the making of a reaction, that nerve currents can get across them more easily the next time.
Fig. 63.--The law of exercise in terms of synapse. A nerve current is supposed to pass along this pair of neurones in the direction of the arrow. Every time it passes, it exercises the end-brush and dendrites at the synapse (for the "passage of a nerve current" really means activity on the part of the neurones through which it passes), and the after-effect of this exercise is growth of the exercised parts, and consequent improvement of the synapse as a linkage between one neurone and the other. Repeated exercise may probably bring a synapse from a very loose condition to a state of close interweaving and excellent power of transmitting the nerve current.
The more a synapse is used, the better synapse it becomes, and the better linkage it provides between some stimulus and some response. The cortex is the place where linkages are made in the process of learning, and it is there also that forgetting, or atrophy, takes place through disuse. Exercise makes a synapse closer, disuse lets it relapse into a loose and poorly conducting state.