STUDIES OP CEREBKALi FUNCTION IN LEARNING, VII 39 



of lowered tonus, the habit mechanism is incapable of pro- 

 viding a sufficient mass of excitation to activate the motor 

 centers except under special conditions of reinforcement. 



Forgetting might then be stated as a general weakening of 

 the habit mechanism in the sense either that its power to 

 dominate all other cerebral processes in the problem situation 

 is lowered or that it can excite motor centers only when they 

 are in a condition of hyperexcitability or tonic activity. The 

 results of these experiments may thus be translated from 

 terms of the number of trials required for relearning to those 

 of efficiency of the engram in dominating the cerebral field or 

 in overcoming resistance of relatively inexcitable motor cen- 

 ters, and we are justified in speaking of gradations in the 

 strength of the engram. 



This leads to the conception of a general weakening of a 

 unitary habit mechanism following injury in the visual area, 

 which parallels such conditions as nominal aphasia, where 

 there is not a loss of a larger or smaller number of words, 

 but a greater or lesser difficulty in recalling all words of a 

 given type. The correlations between extent of lesion and 

 loss of the habit show that the degree of weakening of the 

 engram is proportional to the extent of the lesion, irrespective 

 of its position. As a corollary of this, the larger the mass 

 of intact tissue (or the greater the number of neurons) within 

 the functional area, the greater the efficiency of the habit 

 mechanism, no matter what particular neurons within the 

 system are preserved or destroyed. This can only mean that 

 the equipotential parts of a habit system exert some sort of 

 mutual reinforcement under normal conditions; that there is 

 a summational effect in the production of the habitual acts. 

 The conditioned reflex arcs or postulated elements of the habit 

 system are therefore not independent, but closely organized 

 in this dynamic relationship. 



The lesions described in this study almost without excep- 

 tion penetrated the cortex and destroyed the underlying sub- 

 stantia alba throughout their greater extent. They thus cut 

 any association fibers which underlay the injured cortex and 



