336 ADAPTATIONS TO SPACE AND MOTION 



The mesogennari layer is unique in the presence there of numerous 

 'star' cells. It is not disturbed, but both the infra- and supragennari 

 layers are subject to atrophy, if the receptors connected with them are 

 removed. Thus for example, in the brain of a man who has lived for 

 some time minus his left eye, the left supragennari and the right infra- 

 gennari laminae of layer IV will be found to be atrophied. 



The implication of the sandwich-like morphology of layer IV is that 

 the mesogennari layer is the locus not only of some or all of visual 

 consciousness, but also of the fusion of ipsilateral and contralateral 

 information sent by the two retinae to the top and bottom layers of the 

 sandwich. The mesogennari layers of the two sides of the brain thus 

 constitute, taken together, the 'binocular center' which earlier (pp. 90-1) 

 we saw to be always employed even when one eye is used alone. Again, 

 both mesogennari layers are involved even in 'hemianopic' vision. Each 

 of them represents one half of the whole visual field (consult Fig. 123). 

 Now, if the right or the left optic tract is severed by injury or disease, 

 the individual is thereafter blind in respectively the left or the right half 

 of his erstwhile visual field. Even so, he can experience contrast effects 

 between his seeing and his blind fields. Thus, if he looks at a bright 

 surface long enough to develop an after-image, he will have a bright 

 after-image in the blind half. Similarly, if he looks at a colored surface 

 he 'sees' the complementary color in his blind field, and in turn the 

 complementaries of both of these colors in the two halves of a chromatic 

 after-image. Only an interaction of the two sides of the cortex could 

 account for such phenomena. 



A fusion of left- and right-eyed information thus occurs in each 

 mesogennari layer, and between the two mesogennari layers an inter- 

 hemispheric fusion takes place, creating the cyclopean image — whether 

 this be flat or stereoscopic. Presumably, if a symmetrical lesion should 

 destroy either both infragennari layers, or both supragennari laminae, the 

 individual would retain a complete visual field (so long as both eyes 

 were open), but would no longer see stereoptically. No clear-cut case 

 of this sort has yet appeared in the neuropathological literature. 



In man, layer IV appears to be the locus of the entirety of visual 

 awareness. At least, if the areas striata, or the whole occipital lobes, 

 are destroyed, the result is total blindness. Something of vision might 

 remain, of course, if only layer IV were selectively destroyed — we do 

 not know, since this never happens accidentally and would be impossible 

 to accomplish experimentally. In lower animals, certainly, some aspects 



