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



which the restriction y i^ p must be restored in the summations. It is evident that these 

 quantities are in fact the e's and /l's (unprimed) of equation (4), according to the 

 operational definition of these observable quantities (e,, is the frequency of discharge 

 of ommatidium /?, observed when it is illuminated by itself, Kp, q is the coefficient of 

 the inhibitory action of ommatidium q on ommatidium /?, calculated from measure- 

 ments of the responses oi p and q when illuminated together at different intensities). 

 It is also clear that "self-inhibition" cannot, indeed, be detected by the kind of experi- 

 ments we have described in which optic nerve fiber activity is recorded under various 

 conditions of retinal illumination. These considerations permit us to proceed without 

 committing ourselves as to the presence or absence of "self-inhibition"; they indicate 

 how the formal theory would have to be amended in case it became desirable to 

 consider such self-action in an interacting system. 



The theory so far developed has been subjected to test in a series of experi- 

 ments in each of which three optic nerve fibers were isolated, coming from 

 ommatidia close to one another in the eye. The ommatidia were illuminated 

 independently, at various intensities in various combinations. The e's were 

 determined, for the various intensities used, by illuminating the ommatidia 

 separately and measuring the discharge frequency of each over a fixed interval 

 of time, beginning at a fixed time after turning the light on (to permit the 

 steady level of discharge to be attained). By pairwise illumination and 

 measurement, the AT's and r^"?, were determined (from plots similar to Fig. 

 12). The responses (steady frequencies) in all three optic nerve fibers were 

 then measured when all three ommatidia were illuminated together. The test 

 of the theory was the comparison of these observed responses with values 

 calculated from the solutions of a set of three simultaneous equations (equa- 

 tions (4), n = 3) using the measured values of the e-'s, A^'s and r^'s. The 

 agreement has been satisfactory in all of the experiments done so far within 

 the limits of the methods. An example is shown in Table 1. The different 



Table 1. Inhibitory effects (deficit in number of impulses over a 10 sec period) 



OBSERVED in THREE RECEPTOR UNITS ILLUMINATED SIMULTANEOUSLY, COMPARED 

 WITH effects CALCULATED BY SOLUTION OF SIMULTANEOUS EQUATIONS (SEE TEXT) 



Units illuminated pair-wise 



Three units illuminated 

 simultaneously 



60 



18-9 



20-4 220 



