254 



H. K. HARTLINE, F. RATLIFF AND W. H. MILLER 



u 



c ^ 

 (1) o 



10 20 30 40 



Fiber 2 

 Tpequency (impulses per sec.) 



Fig. 12. Mutual inhibition of two receptor units. In each graph the magnitude of 

 the inhibitory action (decrease in frequency of impulse discharge) exerted on one 

 of the ommatidia is plotted (ordinate) as a function of the degree of concurrent 

 activity (frequency) of the other (abscissa). See legend, Fig. 11. The different 

 points were obtained by using various intensities of illumination on ommatidia 

 1 and 2, in various combinations. The data for points designated by the same 

 symbol were obtained simultaneously. (From Hartline and Ratliff, 1957.) 



frequency of each was an unambiguous function only of the response of the 

 other (Fig. 12). 



A more direct experimental proof that the inhibition exerted by receptors 

 depends quantitatively on the level of their activity comes from experiments 

 demonstrating "disinhibition". A region of the eye too far distant from a 

 test receptor to affect it by direct action can nevertheless influence its response 

 indirectly. If a nearer region of the eye, properly chosen in a position between 

 the test receptor and the distant region, is illuminated at a fixed intensity, it 

 will inhibit the discharge of the test receptor. If now the more distant region 

 is illuminated, it will by its direct action on the receptors in the intermediate 

 region inhibit their activity and as a consequence release the test receptor from 

 the inhibition they exert on it (Fig. 13). Quantitatively, the degree of release 

 is just that which could be produced by lowering the intensity on the inter- 

 mediate receptors (in the absence of illumination on the distant region) by 

 the amount necessary to reduce the frequency of their discharge to the level 

 they had when illuminated at full intensity but inhibited by the distant region. 



