NATURE AND BASIS OF FUSION 337 



of vision persist even after the loss of the whole of both occipital lobes. 

 Rats, and even monkeys, can make discriminations of differences in 

 intensity after bilateral occipital lobectomy. Removal of one lobe should 

 theoretically produce no change except to create hemianopia — vision in 

 only half of the field — but it has been claimed that in the chimpanzee 

 there is a slight but permanent impairment of visual acuity following 

 unilateral occipital lobectomy. In man, mild lesions of one or both areae 

 striatae alter or destroy color vision in half or all of the visual field, but 

 achromatic sensations remain intact unless the lesion is more serious — 

 the sense of brightness being particularly durable. Here again, we have 

 evidence that hue is recorded centrally by a mechanism distinct from 

 that mediating the remainder of vision. 



As regards intra-hemispheric binocular fusion, it might seem that both 

 color-fusion and pattern-fusion would have to occur in the mesogennari 

 lamina. Consciousness of the products of the fusion-processes assuredly 

 occurs only there, in man. But it is quite possible that some visual 

 information from the two eyes is mixed together below the brain-level 

 at which it gets into consciousness. In the highest vertebrates, the lateral 

 geniculate nucleus affords a one and only opportunity for such pre- 

 conscious mixture: 



The lateral geniculate (Fig. 123) is the only way-station on the path- 

 way of visual sensory impulses from retina to cortex. In the optic nerves, 

 the fibers are in fascicles, each representing a spot of retina; but in 

 going through the chiasma these bundles fray out. By the time the 

 crossed and uncrossed fibers enter the lateral geniculate body, they are 

 so intermingled that just about every crossed fiber has an uncrossed one 

 running alongside it. Within the lateral geniculate, the synapses with 

 geniculocortical fibers are intimately intermingled in an elaborate lamin- 

 ation which gives the geniculate a rather more complex structure than 

 even the cortex itself. This multiple lamination of crossed and uncrossed 

 synapses implies that some aspect or aspects of binocularity are handled 

 in the geniculate, as otherwise the interweaving, there, of the right- and 

 left-eyed optic pathways seems meaningless in view of the fact that they 

 must later be untangled again in order to enter the laminae of layer IV 

 independently. 



It appears, then, that the synthesis of monocularly or binocularly 

 mixed colors may very well be accomplished in the geniculate, so that 

 although there is no awareness of the color until the cortex is attained, 

 the information carried through the optic radiations already has the 



