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LIGHT AND LIFE 



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GOLD FISH RETINA 



DARK ADAPTED 6-R UNIT 



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500 

 WAVELENGTH 



600 700 



MILLIMICRONS 



Fig. 8. Intensity necessary to elicit "on" and "off" thresliold responses from a 

 single ganglion cell at different wavelengths before, during, and after exposure 

 to a red-adapting light. Heavy solid line indicates thresholds before adaptation, 

 dotted lines thresholds during red-adapting light, thin solid line thresholds ap- 

 proximately 10 minutes after extinction of the adapting light. Test stimuli 1.0 

 sec in duration were repeated at 2-sec intervals. For intensity of test stimulus 

 log units = 2.3 X 1^"" Atw/cm= for all wavelengths. Energy of red-adapting light, 

 5.2 fivf/cm-. Duration of adaptation = 27 min. Adapting light was obtained by 

 filtering incandescent light 28.')0K color temperature through a Wratten 898 B filler. 



live but was again inhibited completely in the red portion of the 

 spectrum. Thus, inhibition appears to be an active process which 

 always appears when there is an "off" discharge. More evidence for 

 this a.ssertion is shown in Fig. 9. Here the wavelength was kept con- 

 stant in a region giving a low "on" threshold. As the intensity was in- 

 creased, an "off" discharge was produced. When the "off" discharge 

 became very vigorous, the "on" response was completely inhibited. 

 Fig. 10 illustrates an experiment in which both the "off" response and 

 the inhibition of activity during illumination were employed as 

 threshold criteria for the long-wave process. Beyond 625 m^j, the 

 thresholds obtained using these two criteria were essentially indis- 

 tinguishable. Between 575 and 600 ni/x the inhibition during illumina- 

 tion had a higher threshold than the "off" response, presumably be- 

 cause of the very vigorous "on" excitation present in this region of the 

 spectrum. The "off" response is intimately associated with inhibition 

 during illumination not only of the "on" response but of the "off" re- 

 sponse to a preceding stimulus and of spontaneous activity as well. 

 Therefore, the "off" discharge itself is believed to be the excitatory 



