28 K. S. LASHLEY 



5. Since many of the cases showed lesions in subcortical 

 structures, we must further inquire into the role of these in 

 the production of the defects. Cases with thalamic lesions 

 have been excluded from the series. Lesions to the hippo- 

 campi and colliculi are indicated in table 3. Inspection shows 

 that there is no significant correspondence between the lesions 

 in the colliculi and the severity of the symptoms. The extent 

 of lesion to the hippocampi follows closely that to the cortex. 

 With a larger number of cases, the calculation of partial 

 coefficients might show the relative significance of lesions in 

 the two regions for the production of the habit disturb- 

 ances; but with so small a number of cases, these would be 

 unreliable, and we can only judge from the anatomical rela- 

 tions of the hippocampus that it is probably not concerned 

 in the production of the results. 



Scotoma versus habit interference 



These considerations seem to rule out the possibility that 

 the correlation between amnesia and the extent of injury is 

 a secondary effect of the operations, dependent upon shock 

 or temporary depression of the lower visual centers. They 

 show also that it is not due to an error in sampling a large 

 area including a smaller hypothetical visual center, and that 

 it is almost certainly not due to chance injury to lower visual 

 centers. Consequently, we must ascribe the results to the 

 actual reduction in the mass of cerebral tissue. Injury to 

 the occipital area interferes with the habit of brightness dis- 

 crimination in direct proportion to the extent of the lesion. 



The establishment of the quantitative relationship still 

 leaves the cerebral mechanism undetermined. The results 

 presented for group B are capable of two different inter- 

 pretations. The lesions may have produced a cortical blind- 

 ness or they may have interfered with the habit organization 

 without affecting the purely sensory mechanism. The former 

 is the more in accord with the traditional view of occipital 

 lobe function and will therefore be considered first. 



