HOW DO RATS LEARN? 175 



essary link in a chain of neurons which are functional 

 in the habit without being actually essential in the 

 formation of the habit. 



The fact that a brightness-discrimination habit ac- 

 quired by a normal animal is totally lost after destruc- 

 tion of the visual cortex and the further fact that this 

 habit may be readily acquired by a rat whose visual 

 cortex has previously been completely removed sug- 

 gest that the mechanism for acquisition of this habit, 

 that is, for making the conditioned reflex associations 

 involved, lies in the thalamus, but the participation of 

 the cortical visual area is a necessary factor in the 

 successful retention and performance of the habit pro- 

 vided this cortex was present when the habit was 

 originally acquired (cf. p. 199). And yet this habit 

 can be acquired de novo in the complete absence of 

 visual cortex. 



This statement raises more questions than it 

 answers, and the part actually played by the cortex 

 in habit formation is not demonstrated. Evidently, 

 from both the anatomical and the physiological evi- 

 dence, there are in the cerebral cortex of the rat more 

 or less well-defined specific projection areas possessing 

 direct fibrous connections with their respective sub- 

 cortical sensory and motor centers. But the physi- 

 ological relations of these projection centers to one 

 another and to supposed associational tissue of the 

 cortex in habit formation are still obscure. 



8. The frontal region of the cortex in particular is 



