280 



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



0.2 0.4 06 



Time - seconds 



Fig. 25. The "silent period" in the response of an optic nerve fiber. The upper 

 heavy line shows the frequency of discharge of impulses from an ommatidium 

 illuminated alone. The lower curve shows the frequency of discharge in the same 

 fiber when the area of illumination (same intensity as before) was enlarged so that 

 neighboring ommatidia were also stimulated. The time delay of their inhibitory 

 action on the test receptor was long enough that the initial peak of the discharge 

 was unaffected; it was complete before the inhibitory influences affected the test 

 receptor. Soon after, however, the inhibitory influences affected the test receptor 

 and its response dropped abruptly. Since the inhibition is mutual, similar effects 

 were produced on the neighboring receptors themselves, the inhibition they 

 exerted became smaller, and the response of the test receptor increased somewhat. 



the deep minimum observed in the response of a steadily illuminated test 

 receptor when a neighboring group of units is suddenly illuminated (Fig. 7). 



One can readily understand how several groups of receptors, under suitable 

 conditions, might respond to sudden changes of intensity with rather complex 

 transient oscillations, as the groups interact with time delays. An actual 

 experimental example is given in Fig. 26. It is easy to simulate such oscilla- 

 tions in the output of a pair of interacting amplifiers, connected through an 

 electrical delay network (Fig. 27). However, the detailed quantitative analysis 

 of these complex transient effects in the eye of Limulus must wait for a more 

 thorough experimental study of the temporal features of the inhibitory inter- 

 action. 



Vigorous and complex transient responses to sudden changes in light 

 intensity are familiar to students of visual physiology. "Charpentier's bands", 

 for example, are oscillations in brightness perceived by a human subject 

 under suitable stimulus configurations. Of much greater importance is the 

 pronounced sensitivity of animals to movements in their visual fields. A 

 physiological basis for this sensitivity is found in the "on" and "off" bursts 

 of impulses characteristic of the responses of certain ganglion cells of the 

 vertebrate retina, and in the visual pathways of many invertebrates as well. 

 Such elements are often extremely sensitive to sUght changes in intensity of 



