INHIBITORY INTERACTION IN THE RETINA 



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Time (seconds) 



Fig. 23. Graph showing simultaneous excitatory and inhibitory transients in two 

 adjacent receptor units. One ommatidium, black filled circles, was illuminated 

 steadily throughout the period shown in the graph. The other unit, open circles, 

 was illuminated steadily until time when the illumination on it was increased 

 abruptly to a new steady level at which it remained for 2 sec, then was decreased 

 abruptly to the original level. The added illumination produced a large transient 

 increase in frequency of that receptor which subsided quickly to a steady rate of 

 responding ; the subsequent decrease in illumination to the original level pro- 

 duced a large transient decrement in the frequency of response after which the 

 frequency returned to approximately the level it had prior to these changes. 

 Accompanying these marked excitatory transients are large transient inhibitory 

 effects in the adjacent, steadily illuminated receptor unit. A large decrease in 

 frequency is produced by the inhibitory effect resulting from the large excitatory 

 transient ; during the steady illumination the inhibitory effect is still present but less 

 marked; and finally, accompanying the decrement in the frequency of response of 

 the ommatidium on which the level of excitation was decreased, there is a marked 

 release from inhibition. (From Ratliff, 1961.) 



action by studying the effects of short flashes of hght appUed to one receptor 

 while a second receptor was illuminated steadily. Figure 24 shows the brief 

 burst of impulses elicited by a short flash appHed to one receptor and the 

 brief transient dip in frequency elicited by this burst of impulses in the re- 

 sponse of the second. The delay in the action is noteworthy: the dip in fre- 

 quency did not begin until about 0-13 sec after the onset of the burst of 

 impulses in the first receptor's fiber (0-20 sec after the flash of light). Indeed, 

 we chose this record to show that if the burst of impulses is short, the in- 

 hibitory effect may not begin until the burst is nearly all over. 



We can measure the total inhibitory effect produced by a burst of impulses 

 in one fiber by counting the number of impulses discharged in the second 



