8 K. S. LASHLEY 



A more certain test of the reliability of the discrimination 

 method is offered in the present study. The animals of group 

 B show a high correlation between retention tests and the 

 extent of brain injury. The latter variable is in no way 

 modified by the training method so that we must conclude 

 that the method really does measure with some accuracy a 

 function of the brain injury. 



The animals were trained with punishment in the non-illu- 

 minated compartment and food in the illuminated one. Ten 

 trials per day were given, the animals being allowed to reach 

 the food in every trial. Training was continued until ten 

 successive errorless trials were obtained on each of three 

 successive days — thirty consecutive errorless trials in all. 

 These thirty consecutive errorless trials constitute practically 

 perfect learning, since errors are rarely made during continu- 

 ous training after such a record has been established. Ani- 

 mals given 1200 trials overtraining after this standard was 

 reached averaged only seven errors in the entire period of 

 overtraining (Lashley, '21). 



The same method was used in the retention tests, which 

 were thus essentially retraining tests and resembled the ' sav- 

 ings ' method rather than the 'Treffer' method in human 

 studies. Any trial in which the animal entered the darkened 

 alley of the apparatus one or more times was counted as 

 one error. Of the two criteria, total trials required for learn- 

 ing and total errors made during practice, the latter is prob- 

 ably the more reliable, since one error due to chance distrac- 

 tion made after twenty-nine errorless trials may increase the 

 total number of trials out of all proportion to the seriousness 

 of the mistake or the increase in total errors. The two cri- 

 teria correlate highly (p = 0.820 for group A), so that it 

 probably makes little difference which is employed. 



Where lesions of different sizes were compared in the same 

 group, the experiment was arranged so that both animals with 

 slight and extensive lesions were trained on the same days 

 and with identical methods. Similarly, the training of ani- 

 mals from both groups A and B (v.i.) was carried on at the 



