30 K, S. LASHLEY 



however, in group A, so that it seems certain that cortical 

 blindness could not have been responsible for the results 

 with group B, the lesions in the two groups being practically 

 identical. 3) Finally, from what we know of the conditions 

 of vision in the rat and in man, the production of scotoma in 

 the rat seems very improbable. 



Poppelreuter ( '23 ) has distinguished six levels of complex- 

 ity in the organization of visual function in man, basing his 

 conclusions chiefly on hemianopic cases. These are : Level 1, 

 amorphous quantitative sensitivity. Differences in intensity 

 of illumination are recognized, but without location or form 

 in the hemianopic field. Level 2, size perception without defi- 

 nite form or localization within the visual field. Level 3, 

 amorphous form perception. The general direction of single 

 lines crossing the visual field can be distinguished, but any 

 complication of lines or patterns appears amorphous. Level 

 4, perception of discrete objects. The number which can be 

 distinguished within the hemianopic field is very limited and 

 patterns are not identified. Level 5, mild amblyopia. True 

 pattern vision is possible. Level 6, normal vision. 



Vision in the rat is at a very primitive level. Color vision 

 is absent (Watson and Watson, '13), true pattern vision is 

 probably lacking, and the best evidence suggests that the 

 animal can distinguish differences of brightness, differences 

 of size, and gross differences in the direction of single 

 lines within the visual field (Waugh, '10; Lashley, '12). Thus 

 the limit of visual sensitivity in the rat corresponds rather 

 closely with Poppelreuter 's third level. In the present study 

 we are dealing only with the most primitive level, that of 

 reaction to great differences in brightness (level 1), and this 

 Poppelreuter finds to be retained in practically every case 

 of hemianopsia. There is, then, no reason from analogy with 

 man to believe that cerebral lesions will produce scotoma in 

 the vision of the rat for brightness, and the dissimilarity of 

 the results for groups A and B gives conclusive evidence 

 against scotoma as an explanation of the quantitative results 

 found. 



