INHIBITORY INTERACTION IN THE RETINA 



259 



enough to it to inhibit it but far enough from each other that they did not 

 interact. The decrement in frequency of the test receptor's discharge when 

 both regions were illuminated together was found to be equal to the sum of 

 the frequency decrements produced by illumination of each separately (Fig. 

 14). This was true with considerable accuracy for a wide range of intensities 

 on the two patches in various combinations. This is the basic fact on which 

 the law of summation is based. 



When, in an experiment similar to that just described, two patches of light 



0) r 



O +3 



z) (a 



-a CD 



o in -^ 



10.0 



15.0 



5.0 



5um of inhibitopy effects produced 

 by two spots acting separately 



(Decrease in freouency - impulses pep sec ) 



Fig. 14. The summation of inhibitory effects produced by steady illumination, 

 at various intensities, of two widely separated groups of receptors. The sum of the 

 inhibitory effects on a test receptor (steadily illuminated at a fixed intensity) 

 produced by each group acting separately is plotted as abscissa ; the effect produced 

 by the two groups of receptors acting simultaneously is plotted as ordinate. (From 

 Hartline and Ratliff, 1958.) 



were put close together, their combined effect in inhibiting the test receptor 

 became considerably less than the sum of their separate effects (Fig. 15). We 

 attribute this to the interaction of the two groups of receptors illuminated, for 

 we know that each group must have inhibited the activity of the other and 

 that this must have reduced the net inhibition exerted on the test receptor. 

 We now make the assumption that the law of spatial summation of inhibitory 

 effects stated above can be extended to this case of interacting elements. 

 According to this assumption, the reduction in intensity of inhibitory action 

 is quantitatively accounted for solely by the reduction in activity of the two 

 receptor groups (due to their mutual inhibition). In the formal description the 

 partial inhibitory terms representing the influences exerted on the test re- 

 ceptor by the two regions will each be reduced by mutual action, but are 



