REFLEX AND CONDITIONED ACTION 6i 



peripheral stimulation or internal state of the central 

 neurons may divert the flux of nervous transmission 

 now in one direction, now in another. The prevailing 

 view is that the mechanism of this central readjust- 

 ment inheres chiefly in variable permeabilities at the 

 synaptic junctions, though this is doubtless only one 

 of many factors in the process. Johnson^s hypotheti- 

 cal neural scheme, as he points out, "would be pre- 

 posterous if taken too literally," yet to the present 

 writer some such analysis of the mechanisms of 

 correlation seems to be quite in harmony with our 

 best available evidence. 



That there is a diflfuse irradiation of any nervous 

 excitation throughout the correlation center is sug- 

 gested by the fact that in the early stages of training 

 the acquired or conditioned reaction is generalized, 

 that is, it can be eHcited not only by the specific 

 stimulus by which it was established, but also by 

 other stimuli of a similar kind. If the association is 

 made for only a tone of particular pitch, it may at 

 first appear in response to tones of other pitch, 

 though later these "generalized" responses disappear. 

 At any time, moreover, an increase in the excitability 

 of the brain, no matter by what means brought about, 

 will favor the appearance of generalized responses. 



The map of these pathways presented in Figure 

 5 is schematic, and some of the anatomical details 

 have not been fully established. But enough experi- 

 mental evidence has been accumulated regarding this 



