34 JOSEPH U. YARB ROUGH 



backward direction when the two members to be associated are 

 presented continuously and when they are presented with an 

 interval of one second between them. The data for this group 

 are given in Table XVI. The last column of this table is mis- 



TABLE XVI 



Rat Training Trials Test Trials % on Last 100 



57 320 70 73 



58 340 80 77 



59 380 90 68 



60 320 70 76 



Average 340 77-5 73-5 



leading unless it is understood that this last 100 trials include 

 four series (80) of the regular training trials and only two series 

 (20) of the test trials. This is accounted for by the fact that 

 the test trials were interpolated after the first 60 regular trials 

 and after each 20 subsequent trials. This was done because of 

 the apparent slow rate of making the association under these con- 

 ditions. The rate of increase in the strength of the association 

 can be observed more accurately by reference to Graph 14 in 

 Fig. 8. This curve is constructed on the basis of percentages of 

 correct response on the test trials only. Although all the animals 

 made occasional correct responses to the sound stimulus during 

 the test trials no real learning is observable at the end of 380 

 training trials. It is to be regretted that the work of this group 

 was not continued, but sickness caused the experiments to close 

 at this point. Yet, several features of the results deserve com- 

 ment: i) Measured in terms of the number of trials necessary 

 to make the association function in the backward direction, it is 

 much more difficult to form the association when the terms pre- 

 sented are separated by one second than when they are presented 

 continuously. The animals in the group to which the stimuli 

 were presented continuously learned the association in an average 

 of 130 trials, while those in the former group showed no im- 

 provement at the end of 340 trials. With more training the 

 animals of the group might have learned the association, but we 

 have no evidence that they would have done so. 2) Judged upon 



