MECHANISMS OF LEARNING i8i 



result of operative shock, or of the production of scotoma. The 

 degree of retention is a direct function of the amount of nervous 

 tissue intact. 



Lesions in the optic radiations and optic nuclei of the thala- 

 mus seem to retard learning of visual habits. 



The relation between cerebral mass and the efficiency of 

 retention is interpreted as indicating a summation of the activi- 

 ties of different parts of the visual area. This summation takes 

 place in spite of the cutting of any particular group of associa- 

 tional fibers. 



Fig. 42. — The total extent of the lesions in animals which formed 

 the habit of visual discrimination after operation. The stippled area 

 represents an animal in which training was not completed. After Lashley. 



It is clear that visual habits are related with the 

 occipital cortex in a much more specific way than are 

 other habits. 



The destruction of the occipital pole did not result in a loss 



of all habits, but only the loss of the visual one The 



tactile and kinesthetic motor habits were retained, only the 

 visual habit was lost [1920, p. 113]. 



In another series of experiments, portions of the 

 cortex other than the posterior third (and the olfacto- 

 ry cortex which was not significantly involved in any 

 of the cases) were removed and the effects upon the 

 brightness-discrimination habit noted. Removal of 



