THE FRONTAL LOBES 221 



support Lashley's conclusion that in the rat the 

 dominant functions of both the cerebral cortex and 

 the underlying corpus striatum are dynamogenic. 

 But, as already pointed out, this cerebral reinforce- 

 ment, unhke the cerebellar, has a differential quality. 

 The various parts of the cerebral cortex are struc- 

 turally and physiologically unlike, and this dissimi- 

 larity is of two sorts whose significance in behavior is 

 very different. 



First, the primitive cortical areas, that is, the 

 projection centers, differ fundamentally from one 

 another in their subcortical connections. Each is knit 

 in with a different complex of more ancient archi- 

 tectural patterns of the brain stem. Whatever dyna- 

 mogenic or other influence these projection fields may 

 exert upon subcortical behavior patterns, it differs 

 physiologically, depending upon which cortical field 

 (optic, olfactory, or whatever) is prepotent in the 

 process; and this, in turn, seems to depend primarily 

 upon which sensory system is at the moment physio- 

 logically dominant in the lower centers. Thus, in 

 learning a brightness discrimination, at the beginning 

 the difference in illumination has no special signifi- 

 cance in the behavior, but as the learned reaction 

 becomes automatized the brightness difference be- 

 comes the factor which alone can release the ap- 

 propriate behavior and so in cortical participation 

 in the act the visual cortex acts as the trigger upon 

 which the visual stimulus must play. 



