INHIBITORY INTERACTION IN THE RETINA 



265 



in the experiment reported in Fig. 6 of the paper by Harthne et al. (1956). 

 The curve drawn in that figure is theoretical, based on equation (5a), but 

 neglecting r^. Too much weight must not be given to the exact form of the 

 fitted curve, for the larger areas were great enough in linear extent that the 

 falling off in inhibitory action between widely separated receptors undoubtedly 

 contributed to the flattening of the upper part of the curve. 



Experiments on the effect of area necessarily depart from the ideaUzed 

 situation, but the principle involved can usually be illustrated to a good 

 approximation: as area is increased, receptors recruited in each new incre- 

 ment are subjected to increased inhibition from receptors in the rest of the 

 area and so themselves add less and less to the total inhibition. We have 

 shown this effect directly in the following experiment. A small fixed area 

 was used as an "increment" to a contiguous area, the size of which was varied. 

 The contribution of tliis fixed increment of area to the total inhibition exerted 

 on a test receptor became less and less as the area of the contiguous region 

 was increased (Fig. 17). 



We now turn to the consideration of two groups of receptors, ideaUzed by 

 the assumption that all the receptors in any group have uniform properties, 

 interact equally with one another in that group and also interact equally with 



50 



4.0 - 



3.0- 



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Units of inhibiting area illuminated 

 tincluding B) 



Fig. 17. Increment in the inhibitory effect on a test receptor (A) produced by 

 adding a small retinal region (B, stippled) to various numbers of other small 

 regions (arranged around B, as diagrammed). The decrease in A's frequency pro- 

 duced by the region B in combination with various other regions minus the 

 decrease produced by those other regions alone is plotted (ordinate) as a function 

 of the total number of regions illuminated (abscissa). 



