334 ADAPTATIONS TO SPACE AND MOTION 



The fusion of pattern and the fusion of color thus seem to be two 

 very different kinds of fusion. But though both pattern vision and color 

 vision are equally attributes of the retinal cones, there is room for be- 

 lieving that they reside in different parts of the central nervous system 

 (see also pp. 521-3). On each side of the brain there may be two distinct 

 fusion centers, one being for pattern and the other for color. In such 

 centers, should we find them anatomically, we should have a basis for a 

 perhaps entirely physiological fusion of impulses stemming from both 

 retinae. The basis of the psychic act of fusion (see p. 322) of the two 

 fusion-images (one in each side of the brain) into one cyclopean image — 

 and the basis of this, the only kind of fusion present in species with 

 binocular fields but with totally-decussated optic nerves — would be still 

 to be sought, presumably in an inter-hemispheric interchange of infor- 

 mation through commissures. 



There is, indeed, good neurological evidence for the existence of two 

 binocular fusion-loci in each half of the mammalian brain. One of these 

 may be the residence of pattern fusion, the other of color fusion, and we 

 can even hazard a shrewd guess as to which is which. In the past two 

 or three decades many neuro-anatomists and neuro-physiologists have 

 come to agree that the right- and left-eye pathways, which are separate 

 in each optic tract, maintain their separateness past the synaptic center 

 in the lateral geniculate nucleus, all the way to the visual cortex in the 

 area striata of the occipital lobe. Here, 'layer IV' of the general sensory 

 cortex — the layer in which awareness in general resides — is triply lam- 

 inated in primates, and locally presents so-called supragennari (IVa), 

 mesogennari (IVb), and infragennari (IVc) sub-layers (Fig, 123). 



Studies of the brains of traumatic and experimental one-eyed indi- 

 viduals, in man and other species, have shown that the infragennari 

 lamina receives only fibers which, coming from the lateral geniculate 

 body of the same side of the brain, are there connected with optic-tract 

 fibers hailing from the retina on the other side of the head. Total decus- 

 sation being the primitive situation, the infragennari layer is likewise 

 primitive, and its physiological counterpart could presumably be iden- 

 tified in any vertebrate which has a visual cortex at all. But the supra- 

 gennari layer, or its equivalent in non-primates, receives only fibers con- 

 nected with uncrossed optic-tract fibers. This layer is lacking in the 

 cortical areas upon which the uniocular fields are projected, and is of 

 course greatly reduced in species whose binocular visual fields are narrow. 



