CENTRAL MECHANISMS OF VISION 



717 



projections. In addition to these, there are e) the 

 association areas. Just where to delimit the visual sys- 

 tem is problematic, depending upon how one views 

 neural functions. 



Omitting retinal structures, the first way station is 

 the geniculate body. Saggitally, the dorsal nucleus of 

 the lateral geniculate body of the cat possesses three 

 layers : A, Aj and B. The middle layer has been further 

 diflferentiated by Rioch (62). Layers A and B receive 

 terminals of tract fibers of the contralateral retina. 

 The middle layer, Aj, is the terminus of fibers from 

 the homolateral retina. It would seem that the de- 

 velopment of binocular vision has involved increased 

 stratification of the dorsal nucleus of the geniculate. 

 The rabbit nucleus possesses scarcely any, if any, and 

 those of monkey and man present six layers. Part of 

 the process of development seems to have involved an 

 increase in homolateral representation of the retina. 



There are four groups of fibers in the optic tract 

 (24). These groups distribute to four different regions 

 and are unlike in range of cross-section size and in 

 conduction rate. The fastest conducting groups inner- 

 vate layers A and A] of the lateral geniculate. These 

 fibers relay to the projection area of the striate cortex. 



CORTEX] 



PRETECTAL. 



THALAMUS 



GENICULATE 



CULUS 



FIG. I. Diagram to indicate distribution of optic activity to 

 structures beyond the optic tract. The first relay neurons in the 

 geniculate, pretectal area and colliculus are indicated for four 

 tract components. Neurons for projection are represented in 

 the striate cortex, lateral nucleus of thalamus and tectum. 

 From the striate cortex paths are indicated to elaborative 

 structures of opposite cortex, association cortex, pulvinar, etc. 

 In cortex a short-axon cell is inferred on the basis of other 

 work to relay the impulse from afferent fiber to pyramid cell. 

 [From Bishop & Clare (24).] 



The next slower group synapses in layer B of the 

 geniculate, and relays to the lateral nucleus of the 

 thalamus. The third group goes to the pretectal area 

 and the fourth group terminates in the superior col- 

 liculus. Figure i is Bishop & Clare's schematization 

 of these connections. (See also figs. 2 and 3.) 



The conduction rates step down by ratios of one 

 half from group to group. Neither in the frog nor the 

 cat, for example, can four distinct fiber-size maxima 

 be demonstrated. No qualitative sensory differences, 

 such as are found to be correlated with fiber size in 

 the somesthetic system, have yet been found in the 

 visual system. Sensation is likely mediated by the 

 direct path to the cortex via the large fiber group. 



Strong stimulation of the contralateral optic nerve 

 elicits two definite spikes, sometimes followed by a 

 prolonged diminishing potential recordable just prior 

 to the dorsal nucleus of the lateral geniculate. The 

 second spike is propagated at about one half the rate 

 of the first, and the threshold of its elicitation is about 

 •2}/^ times as high as for the first. Corresponding to 

 these two spikes, there are two postsynaptic spikes 

 manifested by the cells and axons of the dorsal nucleus 

 (27, 28). 



Single shocks to the optic nerve induce complete 

 cortical responses even when such stimuli are weak 

 enough to activate onK the large-fiber group, in- 

 ducing the first of the two tract spikes. The time of 

 arrival at the cortex of the response to the second 

 group of radiation fibers does not tally with any of 

 the prominent spike components of the cortical record. 

 Instead, the activity induced in layer B of the genicu- 

 late is propagated to the lateral nucleus of the thala- 

 mus. It first emerges when stimuli just strong enough 

 to elicit the second spike are used, and thus the 

 activity is not a response induced there by activity 

 coming back from the cortex. 



When records are obtained from the postsynaptic 

 elements in the geniculate, the response to the first 

 tract spike from the contralateral eye arises mainly 

 from layer A and the response to the second spike 

 from layer B. Thus the ensuing responses from the 

 activation by the first tract spike reach the cortex, 

 and those from the second spike reach the thalamus. 

 This represents a functional differentiation of the two 

 geniculate layers. These two lasers are also different 

 histologically (59), but we do not yet know the sig- 

 nificance of the difference. 



The response of the middle layer Ai to the stimu- 

 lation of the homolateral optic nerve is mainly to the 

 first tract spike. When occasionally a second post- 

 synaptic spike is elicited, its threshold is the same as 



