STUDIES OF CEREBRAL FUNCTION IN LEARNING. VII O 



study (Lashley, '20) I tested the effects of lesions ranging 

 from 15 to 50 per cent of the total surface of the rat 's cortex 

 upon the rate of formation of a motor habit. Within these 

 limits it was found that no lesion, of whatever location or 

 extent, had any effect upon the rate of learning, although 

 there was indication that lesions more extensive than those 

 studied systematically (more than 50 per cent) did produce 

 a marked retardation. The result was based upon a limited 

 number of cases and required verification and extension to 

 other types of learning. I have therefore undertaken a pro- 

 gram which includes : 1) a study of the effects of very exten- i^ 

 sive lesions (more than 50 per cent) of the cerebrum upon 

 the learning and retention of various types of habits in the 

 rat; 2) measurement of the rate of formation and the reten- 

 tion of localized and non-localized habits after lesions of 

 lesser extent and of various loci; 3) determination of the 

 effects of lesions of various extents upon the function of a 

 sensory projection area; 4) a test of the validity of the find- 

 ings in the rat for the monkey and other higher forms; 5) a 

 review of the clinical literature to determine the relative im- 

 portance of locus and extent of lesion for the severity and 

 duration of symptoms in man. 



The present paper is a report of experiments upon the 

 third of these problems : the influence of the extent of lesions 

 within the visual area of the rat upon the formation and 

 retention of habits of brightness discrimination. Some justi- 

 fication of this choice of material is perhaps necessary. Ob- 

 jections are obvious; the rat is low in the evolutionary scale 

 and conclusions based upon it are not necessarily applicable 

 to higher forms ; the visual area shows a subordinate spacial 

 arrangement within the total area and the production of 

 scotoma may lead to ambiguous results; the habit of bright- 

 ness discrimination is a simple one and perhaps does not 

 involve any mechanisms comparable to those which function 

 in the adaptive behavior of higher forms. The importance 

 of these objections is admitted, but for a preliminary experi- 

 ment the material has definite advantages. A large number 



