STUDIES OF CEREBRAL FUNCTION. IX 45 



The evidence from studies of the normal growth of in- 

 telligence in man and from clinical studies of dementia goes 

 far toward proving that differences in capacity are in some 

 measure qualitative. Tasks where a mere reduplication of 

 elements is involved, as in span of attention or memorizing 

 of nonsense syllables, do not reveal differences in intelligence 

 brought out by batteries of qualitatively different tasks. 

 Clinical studies, such as those of Head ('26) on semantic 

 aphasia (interpreted as dementia by Henschen, '27), indicate 

 that limitations to performance are set by qualitative char- 

 acters of the tasks. We should therefore expect to find that 

 a battery of qualitatively different tasks, graded in difficulty 

 and making up an apparently continuous series for normal 

 individuals, would show disproportionate difficulty at the 

 higher levels for demented individuals in accord with the 

 severity of the dementia. Comparison of the results with the 

 qualitatively different tasks used by Cameron, Lashiey, and 

 Maier suggests that this is actually the case for animals with 

 cerebral lesions. Extensive experiments will be necessary, 

 however, to establish the point and to reveal the nature of the 

 effective qualitative differences, if they exist. 



For tasks where difficulty is determined chiefly by the re- 

 duplication of similar elements our data seem conclusive. 

 The difficulty of such tasks for animals with cerebral lesions 

 increases at an accelerating rate with the extent of lesion, but 

 the relative difficulty of the several tasks remains the same. 



Difficulties of interpretation arising from individual 

 variations 



One of the most difficult problems in the study of cerebral 

 functions is that of accounting for the widely divergent 

 symptoms following similar lesions in different individuals. 

 In our present series, as in other similar studies, we find a 

 number of animals with extensive lesions and yet with train- 

 ing records which are far better than the averages of other 

 animals with equal amounts of destruction, and which may 

 approach the records of normal animals. Such cases are nos. 



