STUDIES OP CEREBRAL FUNCTION IN LEARNING '261 



Retention tests. — Animals trained before operation were tested in 

 the problem box as soon as their general condition permitted ; usually 

 twenty-four hours after operation. In these tests for retention of the 

 habit the same method was employed as in training, except that no 

 punishment was given for errors during the first fifty trials ; that is, 

 the retention test is essentially a retraining in the habit and evidence 

 for retention consists in the reappearance of accurate discrimination 

 in significantly less time than was required for the original learning. 



When a habit is lost after operation the question of post-operative 

 shock must be considered. In earlier papers I have discussed methods 

 of distinguishing between loss due to shock and that due to destruction 

 of a functional area. In the present experiments positive evidence for 

 retention following operation was obtained in every case, so that the 

 question of shock does not enter, save as an explanation of the few 

 errors made in the trials immediately following operation. 



Reconstruction of lesions. — After completion of the retention tests 

 the brains were removed and serial frontal sections stained with iron 

 hematoxylin were prepared. Camera outlines of sections at intervals 

 of one half millimetre were made and the extent of the lesions indicated 

 on them. The positions of the sections were determined from internal 

 structures and indicated on diagrams of these structures projected to 

 the surface of the brain. The dimensions of the lesions were then 

 transferred with proportional dividers to the diagrams, the points so 

 determined connected by lines and the areas inked in. The reconstruc- 

 tions were finally verified by reference to the sections. 



Only easily recognizable lesions, absorption or complete degenera- 

 tion of the cortex and subcortical nuclei were recorded. The diagrams, 

 therefore, represent the minimal destruction produced by the operations. 

 In all cases, probably, larger areas than those shown were destroyed, 

 but, since the experiments deal with the non-function of given areas 

 and do not seek to localize functions accurately, this fact does not in- 

 validate the results. 



Graphic and tabular presentation of results. — In the preparation of 

 the figures a uniform system has been followed. Diagrams of the 

 superficial lesions made from camera outlines of an average brain are 

 given at the top of the figure. Below at the left is a diagram of a 

 horizontal section through the cerebrum at the level of the maximum 

 area of the caudate nucleus. Practically all of the ganglion cells of 

 that nucleus are included within the dotted line, the posterior horn 

 being made up almost altogether of the fibres of the internal capsule. 



