STUDIES OF CEREBRAL FUNCTION. IX 9 



of animals with cerebral lesions to the relative difficulty of 

 the tasks for normal animals ? 



PROGRAM OF EXPERIMENTS 



Since a primary object of our work was to test the influence 

 of extent of lesion upon the rate of learning tasks of different 

 complexity we planned to train animals with equal lesions 

 in four mazes with 4, 8, 12, and 16 culs de sac. In such ex- 

 periments on learning with animals we are confronted with 

 two variables which cannot be controlled simultaneously; 

 chance individual variations in capacity and transfer of train- 

 ing. Thus, if we wish to compare learning records on two 

 different problems we may either train the same animals 

 on both problems, in which case we control individual varia- 

 tion but ignore transfer, or we may compare the records of 

 different groups of animals, each trained on a single problem, 

 in which case transfer effects are eliminated but individual 

 differences are uncontrolled except statistically. In the first 

 case transfer effects may be controlled by subdividing the 

 subjects and reversing the order of training with a part, but 

 where four problems are to be compared the arrangement of 

 tests for transfer becomes impossibly cumbersome. We de- 

 cided, therefore, to use separate groups of animals, one with 

 each of the four mazes, attempting to get comparable groups 

 with respect to extent of lesion and to control individual dif- 

 ferences to some extent by subsequent training of all groups 

 on the same task (maze V). The validity of this control is 

 somewhat lessened by the possibility of differential transfer 

 from the different mazes and the evidence for equality of 

 the groups is not as clear as we could wish, but the differences 

 are small in comparison with other differences revealed by 

 the experiments. 



For comparison with the operated cases and for standardi- 

 zation of the mazes we have trained groups of normal animals 

 under parallel conditions with the operated ones. The vari- 

 ous groups and the number of animals included in each are 

 shown in figure 1, together with the ground plans of the 



