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



Fig. 10. Oscillograms of electric potential changes recorded by a micropipette 

 electrode in an eccentric cell showing slight hyperpolarization (downward dis- 

 placement of base-line) concomitant with inhibition of spike discharge, caused by 

 illumination of nearby ommatidia during period between arrows (small switch 

 artifact at second arrow). Tops of spikes marked by white dots. See text. 



cussion with the consideration of the mutual action of the receptor units on 

 one another. 



A survey of single optic nerve fibers sampled successively from any one eye 

 has shown that any ommatidium one picks can be inhibited at least to some 

 degree by illumination of any retinal area within a few millimeters of it. 

 More specifically, it has been shown by recording from two optic nerve fibers 

 simultaneously that any two individual ommatidia, sufficiently close to one 

 another in the retinal mosaic, inhibit one another by direct mutual action 

 (Fig. 11). Often the inhibitory action is unequal in the two directions; occa- 

 sionally it is quite one-sided, but it is safe to say that as a rule any omma- 

 tidium is inhibited by its neighbors and, being a neighbor of its neighbors, 

 inhibits them in turn. This mutual interaction gives special interest to the 

 phenomenon of inhibition in the eye of Limulus. 



The first step in the quantitative analysis of this inhibitory interaction 

 reveals a crucial point. The strength of the inhibitory influences exerted by a 

 given ommatidium on other ommatidia in its neighborhood has been shown 

 to depend not just on the stimulus to it, but rather on the net level of its 

 activity. This level is the resultant of the excitation this ommatidium receives 

 from light shining on its facet, diminished by whatever inhibitory influences 

 are exerted on it by its neighbors. Now, the strength of those inhibitory in- 

 fluences from the neighbors depends in turn on the level of their activity, 

 which is partially determined by the inhibition that the ommatidium in 

 question exerts on them. Since these statements apply simultaneously to each 



