1932] Poliak: Afferent Fiber Systems, Primate Cerebral Cortex 191 



visual system, a ver^' sharp projection must be accepted, seg'ment for 

 segment, point for point. 



The visual projection cortex, according to my views, represents a 

 fine mosaic of elementary units. These units are probably smaller in 

 the portion representing the macula, than in those representing extra- 

 macular portions of the retinae. The individual elementary units of 

 the visual projection cortex are each related to a definite "brush" of 

 fibers since it must be accepted that the smallest conducting unit of 

 the central link of the afferent visual path does not consist of one 

 single neuron, but of a fascicle of several isodynamic neurons. We 

 may look upon each such fiber bunch together with its correlated 

 cortical cell group as representing in some sense an independent struc- 

 tural and functional unit or "element," a kind of a primitive visual 

 system. Such a unit would be comparable to an elementary unit of 

 the composite visual system of insects. A certain small peripheral 

 stimulus, for instance, a light ray from a star falling upon a single 

 retinal receptor, upon a cone of the fovea centralis, will be transmitted 

 to the visual projection cortex by means of one single unit of con- 

 ductors in the sense as defined above. In such a case only a single 

 chain of neurons — that connected with the stimulated peripheral 

 receptor — will be involved in the physiological process without sub- 

 stantial dynamic alteration of the next neighboring elementary units. 

 An isolated reception, conduction, and transmission of the smallest 

 discernible visual stimulus to the cortex by the above units appears 

 as the structural mechanism and the functional principle underlying 

 all visual "spatial" discrimination on which ultimately depends the 

 complex process of vision in animals with a highly perfected visual 

 apparatus. The "localizing signs" in the process of vision would, 

 accordingly, be primarily due to the isolated arrangement of the above 

 mentioned elementary' visual units. The totality of these elementary 

 units would give an appropriate mechanism for the perception, con- 

 duction, etc., of a multitude of light stimuli coming simultaneously to 

 the peripheral receptor surface, the retina, that is, for a spatially 

 continuous seizing and grasping of "pictures" or "figures" of larger 

 external objects, and, with the lapse of time, for the change of shape, 

 position and so forth (visual appreciation of space and time) . Accord- 

 ing to this point of view the unity of a sum total of elementary visual 

 stimuli derived from a larger object, or what is the same thing, the 

 unity of the numerous stimulations of individual elementary struc- 

 tural visual units, would be achieved onlv after stimuli have reached 



