16 K. S. LASHLEY AND L. E. WILEY 



lesions, as determined in relation to internal landmarks. The 

 distribution of cytoarchitectural fields is variable and in a 

 large series of cases it is quite impossible to study the cyto- 

 architecture of the cortex in sufficient detail to determine the 

 limits of the remaining fields in each case. At best we can 

 only compare the diagrams of the lesions with the somewhat 

 conventionalized diagram of cytoarchitectural fields adapted 

 from Fortuyn's studies ('14). The classification of cases by 

 areas destroyed has been made by superimposing a trans- 

 parent diagram of the cytoarchitectural fields upon the dia- 

 grams of lesions and measuring the area of the lesion within 

 each field with a planimeter. These measurements were then 

 expressed as percentage of the total neocortex and used as a 

 basis for estimation of the effects of injury to different fields. 



The question of the relative effectiveness of lesions re- 

 stricted to one hemisphere and of lesions of equal magnitude 

 distributed symmetrically on both hemispheres in reducing 

 learning ability has arisen continually in experimental work 

 of this sort. It has been difficult to test the question by direct 

 experiment because of the impossibility of distinguishing 

 with certainty in maze studies between genuine reduction of 

 learning capacity and possible disturbances of orientation 

 produced by asymmetrical motor defects which frequently 

 result from unilateral lesions. The recent study by Loucks 

 ('32) of habits involving alternation of turns to right and 

 left, where the turns were recorded separately has shown 

 that even a strong motor tendency to rotation does not affect 

 the rate of formation of the alternation habit, so that with 

 a series of bilateral lesions like the present one, we may 

 proceed with assurance upon the assumption that bilateral 

 asymmetries have not influenced the training records through 

 their motor effects. 



We have therefore attempted to determine whether or not 

 the bilateral destruction of corresponding areas is more 

 effective in retarding learning than unilateral destruction. 

 To this end the lesion in one hemisphere was traced on trans- 

 parent paper which was then inverted and superimposed upon 



