6o BRAINS OF RATS AND MEN 



There are many experimentally established facts 

 about conditioned reflexes which are not explained 

 by the much-simplified statement here presented, but 

 into these details we cannot enter. Beritoff (1924) 

 gives a good account of some typical experiments and 

 discusses in detail the laws of the spread or irradiation 

 of natural and conditioned excitations within the 

 cerebral cortex and the activating and facilitating 

 effects of repeated excitations, both simultaneous and 

 successive. 



In the course of the discussion of the neural 

 mechanisms of attention, Johnson (1925) presents 

 an elaborate diagram of the nervous circuits that may 

 be involved in a simple response to sensory stimula- 

 tion, and of the related circuits which may modify 

 this response through simple reinforcement by addi- 

 tional stimulation, through conditioning of the re- 

 sponse by simultaneous or preceding responses, 

 through preservation (with the aid of reflex arcs 

 related with muscle spindles) of enduring motor 

 "sets'* or "cues" in delayed reactions, through dis- 

 traction (inhibition) or reinforcement (facilitation) 

 by intercurrent stimuli, diffusion, and "drainage." 

 Throughout this entire sequence of events, particular 

 neurons related in specific patterns are successively 

 activated. The facile modifiability or plasticity of the 

 behavior pattern is correlated with the complexity 

 of the interneuronic connections available in the 

 correlation center and the ease with which changes in 



