STUDIES OF CEREBRAL FUNCTION. IX 7 



Since there is never an exact duplication of the fields in- 

 volved in the two hemispheres, it is desirable to determine 

 whether this lack of symmetry affects the correlations between 

 extent of lesion and learning records. 



Organic dementia in relation to the complexity of the task to 



be performed 



Clinical studies of organic dementia frequently suggest that 

 a nearly normal ability in the execution of simple acts may 

 accompany a marked deterioration in the performance of 

 more difficult tasks. Comparing the rate of learning for three 

 mazes of 1, 3, and 8 culs de sac by partially decerebrate rats, 

 Lashley ('29) found that whereas the ratios of difficulty of 

 the mazes for normal animals were as 1 to 2.2 to 6.5, for 

 animals with cerebral lesions the ratios were as 1 to 3.5 to 

 20.6. This seemed to conform to the clinical evidence in show- 

 ing that a given amount of cerebral destruction resulted in 

 a much greater retardation in the performance of complex 

 than of simple tasks. There was some indication also that 

 large lesions produced a disproportionately greater retarda- 

 tion in the more difficult mazes than in the simpler ones, al- 

 though the evidence for this was not statistically valid 

 (Lashley, '29, table 11). 



The meaning of these data is by no means clear. If we 

 attempt to define difficulty merely in terms of the number 

 of trials required for learning, then certain tasks, the learn- 

 ing of which is unaffected by cerebral lesions, appear more 

 difficult than maze learning. The difficulty of a task may de- 

 pend upon the number of similar elements which must be 

 integrated in order to give an efficient performance. In logi- 

 cal processes the ability to keep in mind a number of separate 

 elements and at the same time manipulate them in thought 

 is essential, and, according to Boumann and Grimbaum ('25), 

 this capacity primarily suffers in organic dementia. In 

 learning relatively meaningless material the difficulty, as 

 measured by practice necessary to secure perfect reproduc- 

 tion, increases as the 3/2 power of the length of the series 



