MacNlCHOL, WOLBAliSHT, AND WAGNER 



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WAVELENGTH IN MILLIMICRONS 



Fig. 7. Intensity necessary to elicit various types of threshold responses from 

 a single ganglion cell at different wavelengths. The duration of the stimulus was 

 1.0 sec. log units — 5.5 X ^O"" MW/cm= for all wavelengths. In this and in 

 subsequent illustrations, the thresholds of "on" responses are indicated by circles 

 and "off" responses by crosses. 



the long wavelength limit of our monochromator. Beyond 610 m/x 

 no "on" response could be obtained even at the maximum output of 

 our stimulator, although a vigorous "off" discharge was present. An 

 "off" response could be obtained at wavelengths shorter than 600 xa^ 

 even in the presence of a vigorous "on" discharge. In this unit the 

 "off" discharge appeared to be combined with a strong inhibitory 

 effect on the "on" discharge. However, the "on" discharge was not 

 associated with inhibition of the "off" discharge, which could be ob- 

 tained over nearly the entire spectrum. 



Further evidence that it is in fact the "off" process that inhibits 

 the "on" response was obtained from the experiment represented in 

 Fig. 8. Here typical blue sensitive "on" and red sensitive "off" re- 

 sponses were found (solid black lines). After adaptation to deep red 

 light, the sensitivity of the "off" response was greatly decreased while 

 the threshold of the "on" response was slightly increased (dotted 

 lines) . However, an "on" threshold could now be obtained through- 

 out the entire visible spectrum. Thus, the effect of the adaptation 

 to red light was both to depress the .sensitivity of the "off" process 

 and to release the "on" process from inhibition in the red end of the 

 spectrum. After dark adaptation, the "off" response returned to its 

 original threshold and the "on" process became slightly more sensi- 



