Ill 



PARALLEL, INTERCONNECTED NEURONS 



Color-contrast and visual illusions of shape provide well-known 

 examples of the almost universal interdependence of perceptions. Phy- 

 siologically no stimulus occurs in the absence of all others, and the 

 response to any stimulus depends in part upon the nature of the back- 

 ground against which it is presented. One may wish to say, indeed, 

 that the true stimulus to the organism is the whole situation, but 

 since we cannot discuss any whole situation, and since the whole situ- 

 ation is never duplicated, such terminology does not seem to serve 

 any useful scientific purpose. 



If two stimuli differ only in degree, it may be true that the re- 

 sponses which they evoke differ only in degree, the stronger stimulus 

 evoking the stronger response. But in many instances there is a com- 

 plete change in the form of the response, and in others it is the 

 weaker stimulus, and not the stronger, which brings forth the strong- 

 er response. 



In our schematic reacting organism, such phenomena are easily 

 understood in terms of suitable interconnections between parallel neu- 

 rons. We are reserving Part II for the precise formulations neces- 

 sary to make quantitative predictions, so that we content ourselves 

 here with a few qualitative results to indicate in general how this 

 comes about. 



In the barest terms, if two stimuli which differ only in degree 

 lead to responses which differ in form, then there are pathways — 

 neural chains from receptor to effector — which can be traversed when 

 the impulses are initiated by a stimulus within a given range of in- 

 tensities but not when these are initiated by a stimulus lying outside 

 this range on the scale of intensities. The simplest neural mechanism 

 having this property consists merely of two neurons, N e excitatory 

 and Ni inhibitory, having a common origin and a common terminus 

 (Landahl, 1939a). Let h e and hi be the thresholds of N e and Ni , re- 

 spectively, and let h be the threshold of some neuron N originating 

 at the common terminus of the two neurons. Suppose 



hi> h e ,y>(oo) > <p( oo ) f (f> (hi) >h. 



Then if S is sufficiently near to hi in value (Figure 1), asymptotically 



<r = </>(£) ~y(S) >h 



and N will become excited, whereas a somewhat larger S will result 



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