14 K. S. LASHLEY 



liminary retention tests). Cerebral lesions were then pro- 

 duced and, seven days after operation, retention was again 

 tested (postoperative retention tests). The first training rec- 

 ords served as a control for group A. The preliminary reten- 

 tion tests measured the amount of loss to be expected from 

 disuse in the period allowed for recovery from operation. 

 The postoperative tests provided data for determination of 

 the average effects of the lesions upon retention and for a 

 comparison of the effects of lesions of different magnitudes 

 by correlation with retraining records. 



Group C. Since there was a possibility that in severe cases 

 shock effects of operation might persist for more than seven 

 days and so give a misleading appearance of greater loss in 

 those cases, a further group was trained with longer intervals 

 between training and the retention tests. After seven days' 

 training, fourteen days after operation, all of the animals of 

 group B were giving evidence of discrimination, although 

 the retraining of some was not completed until later. This 

 fact was taken as evidence that they were no longer suffering 

 from shock or depression. The intrval of fourteen days 

 was therefore adopted for group C, as adequate to allow for 

 recovery from shock, and the preliminary and postoperative 

 rest periods were both made of this length. The procedure 

 with this group was otherwise as with group B. 



Group D. At necropsy it was found that the thalamus had 

 been injured in some cases. These were excluded from the 

 major groups, but since the results with them have some 

 bearing on the problem they are given separately. 



Summary of data 



Since the three experiments provide controls for each other, 

 the raw data will first be presented for all of the groups, then 

 summarized in relation to the various problems raised. The 

 large number of cases included in the study, 112 in all, pre- 

 cludes the publication of individual protocols, but the essen- 

 tial data are presented in the following tables and the extents 



