HOW DO RATS LEARN? 173 



That the cortex, even in the rat, is not physio- 

 logically equipotential in learning is suggested by the 

 local participation of the frontal pole in the inclined 

 plane latch-box experiment to be described later, and 

 this is clearly proved by the brightness-discrimina- 

 tion experiments of Lashley to be taken up shortly. 



It should, however, be borne in mind that there 

 is much evidence, of which the experiments of Lash- 

 ley are the most significant, that in addition to such 

 localizable functions as may be demonstrable, the 

 cortex as a whole does exert a profound influence upon 

 many (perhaps most) subcortical activities. These 

 general functions may well be expressed in terms of 

 the mass of the cortex involved without specific local- 

 ization. It is suggested by the experiments that in the 

 rat they are but little affected by the removal of less 

 than 50 per cent of the cortex, but that they may be 

 profoundly disturbed by more extensive injury. 



The relation between cerebral mass, learning, and 

 retention of the visual discrimination habit has been 

 investigated by Lashley (1926) with especial care. He 

 concludes that after acquisition of the habit subse- 

 quent injury to the visual areas of the cortex results 

 in impairment of the habit which is closely propor- 

 tional to the extent of the injury and independent of 

 its locus within the occipital third of the cortex. 



There are important efferent systems of fibers 

 from the cortex to the corpus striatum, thalamus, 

 cerebral peduncle, and cerebellum, in addition to 



