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Fig. 20. Solutions generated by an analog computer (constructed by C. C. 

 Yang) imitating the responses of three interacting receptor groups. The traces 

 are analogous to the experimental plots of Fig. 19. The decrease in response of a 

 test element (X) inhibited by two interacting elements, A and B, in combination 

 minus the decrease produced by B alone is traced (ordinate) as a function of the 

 decrease in response of X when inhibited by A alone (abscissa). In the upper figure, 

 A and B were caused to inhibit one another to varying degrees (increasing from 

 top to bottom). In the lower figure, various degrees of interaction between A and 

 B are portrayed. The lowest trace (negative slope) illustrates disinhibition. The 

 topmost trace is the only one for which X was caused to inhibit A and B. In this 

 latter case A and B did not interact : this illustrates how their combined effect can 

 sometimes exceed the sum of their separate effects, as in the points above the line 

 in Fig. 14. In both figures the dotted line represents the case for equality of the 

 combined and separate effects of A and B (solid line of Fig. 14). 



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