STUDIES or CEREBRAL FUNCTION IN LEARNING. VII 41 



Brightness discrimination is not a simple reaction which 

 can be stated readily in terms comparable to the descriptions 

 of spinal or of conditioned reflexes. The stimulus is complex. 

 The simultaneous or successive applications of at least two 

 optical stimuli is necessary to activate the habit. The reac- 

 tion is not merely an advance to light or retreat from dark- 

 ness. If both alleys of the discrimination box are darkened 

 the trained animal will not enter either, as is to be expected, 

 since he is trained to avoid the darkened alley. But if both 

 alleys are illuminated the animal also frequently refuses to 

 advance. His response is thus not merely tropistic to a lighted 



Text fig. 7 Composite diagram, showing by lines the long axes of lesions which 

 separated parts of the occipital area in animals which made not more than five 

 errors in retention tests. 



alley, but is conditioned by the presence of both stimuli. In 

 addition to this, the habit is conditioned by the tactile and 

 olfactory stimuli of the problem box and by hunger. It thus 

 involves a complexity of integration which seems unlikely 

 to be carried out without the activity of transcortical asso- 

 ciation paths. 



The relative importance of the cerebral cortex and thala- 

 mus in the performance of such acts of discrimination is as 

 yet an open question. Discrimination habits have been estab- 

 lished in animals with a very primitive cerebrum (e.g., White, 

 '19) and even in invertebrates (e.g., von Frisch, '14) so that 



