RATE OF LEARNING IN THE WHITE RAT 45 



a temporal sequence. The continued running activity of the 

 animal and the external environment were possible third factors 

 in our experiments. If the running activity, which was almost 

 always present, did serve such a mediating function no increase 

 in difficulty in forming the association over different time inter- 

 vals should have been evidenced. Nor can this variation among 

 the different groups be accounted for on the basis of the external 

 environment, for it remained constant. 3) The third theory 

 assumes that the two terms may be associated directly, i.e., when 

 the first member of a pair of stimuli ceases the actual nervous 

 excitation is not finished but continues for a time, gradually 

 diminishing in strength. The second member coincides with the 

 diminishing phase of the preceding one, and the association is 

 due to this simultaneity of the two experiences. Although the 

 external stimuli are presented in succession the internal ex- 

 periences are in a large measure coexistent. 



In the light of our data, this "gradually diminishing nervous 

 excitation" continues for at least six seconds. This after-phase 

 may account for the distinct, though disproportionate, increase 

 in difficulty in making the association over larger and larger 

 intervals. If this difficulty in making the association is a measure 

 of the waning strength of the after-phase of the first term, our 

 data warrant the statement that little or no loss in strength is 

 experienced during the first second, but by the end of two seconds 

 a marked weakening is observed. For longer intervals the diminu- 

 tion is gradual with no marked breaks observable. This is in 

 agreement with Froeberg's finding that association can be formed 

 over an interval of at least five seconds. 



Although the hypothesis of direct association seems to be the 

 best explanation of successive association, the following pos- 

 sible objections to its acceptance should be pointed out: a) 

 When human subjects are used it may be assumed that all 

 memory and thought connection are not wholly excluded. These 

 connections are many, and some are very difficult to overcome, 

 b) Where activities are interpolated to destroy the memory image 

 of the preceding term it may be assumed that these activities 



