46 K. S. LASHLEY 



fitting theory of the mechanism of development is that which 

 appeals to physiological stresses, as elaborated by Child ( '23) 

 in his discussions of physiological gradients, and it seems 

 probable that further development of theories of neural func- 

 tion must proceed along somewhat similar lines. 



SUMMAEY 



To test the influence of the extent of brain injury upon the 

 learning and retention of reaction to brightness, rats were 

 trained before and after cerebral lesions of various extents 

 and loci. Enough cases were obtained to permit of statistical 

 treatment of the results. The results indicate that : 



1. Injuries to the occipital region inflicted before training 

 and including every possible part of the occipital third of the 

 cerebrum have no effect upon the ability of the animals to 

 form the habit of brightness discrimination. Total destruc- 

 tion of the 'visual' area does not reduce the speed with which 

 a simple visual habit may be formed. 



2. Injuries in the same region produced after the habit was 

 established resulted in a weakening or total loss of the habit. 



3. The loss, as measured by the amount of practice neces- 

 sary to reestablish the habit, was closely proportional to the 

 extent of the injury and independent of its locus within the 

 occipital third of the cortex. 



4. Evidence is given to show that the loss of the habit is 

 not the result of operative shock or of the production of 

 scotoma. The degree of retention is a direct function of the 

 amount of nervous tissue intact. 



5. Lesions in the optic radiations and optic nuclei of the 

 thalamus seem to retard learning of visual habits. 



6. The relation between cerebral mass and the efficiency of 

 retention is interpreted as indicating a summation of the 

 activities of different parts of the visual area. This summa- 

 tion takes place in spite of the cutting of any particular group 

 of association fibers. 



7. From this it is argued that the theory which makes the 

 conditioned-reflex arc the unit of cerebral organization is 



