CHAPTER X 

 MECHANISMS OF LEARNING IN THE RAT 



The physiological gradient is a case of protoplasmic 

 memory since it represents the persistence of the ejffects of en- 

 vironmental action. The establishment of a gradient in a pro- 

 toplasm may be regarded as a process of learning. In some 

 protoplasms the memory persists only for a short time and 

 gradients are only temporary. In other cases ^ once the gradi- 

 ent is established^ the protoplasm never forgets^ unless sub- 

 jected to some environmental action powerful enough to 



obliterate the former memory The establishment of a 



persistent gradient is, in fact, the first step in the education 

 of a protoplasm and the foundation of later behavior to the 

 end of individual life. 



— C. M. Child 



THE significance in normal behavior of the 

 spasmodic movements resulting from elec- 

 tric stimulation of the "motor cortex" is still 

 obscure. To this we shall return. The clearest evi- 

 dence of definite localization within the rat's cortex of 

 a particular physiological function is presented by 

 Lashley's brightness-discrimination tests (1920, pp. 

 94, 103; 1921, 1921^, 1922, 1926). The habit was the 

 simplest possible. The rats were given a choice of two 

 alleys through which to reach food. One was illumi- 

 nated, one was dark, and food was always given by 

 way of the light alley, never through the dark one. 

 Normal rats learn perfectly to take the lighted way in 

 about one hundred trials. 



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