MacNICHOL, WOLBARSHT, AND WAGNER 



809 



niaxinumi sensitivity in the long-wave region. Many units have been 

 found in \\hi(h the converse is true. A typical response of this type 

 is shown in Fig. II. Still other units have been found in which the 

 "on" and "off" responses had nearly identical spectral sensitivities. 

 Such a imit is shown in Fig. 12. 



Thus, it appears that the "on-off"-type ganglion cells giving rise 

 to the messages in the individual optic nerve fibers can be classified 

 into at least three subtypes: (1) a short-wave "on," long-wave "off" 

 unit; (2) a short-wave "off," long-wave "on" unit; and (3) a unit 

 which gives both "on" and "off" responses more or less equally 

 throughout the spectriuii. 



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 WAVELENGTH 



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Fig 12. Intensity necessary to elicit various types of threshold responses from a 

 single ganglion cell at different wavelengths. Experimental conditions same as 

 those for Fig 11. Intensity plotted on a log quantum scale. log units = 2.6 

 X 10" quanta/cm-/sec. The thin line is a curve calculated from Dartnall's nomo- 

 gram (5) for a photopigment having a wavelength maximum of 600 m^^. 



Discussion 

 The ganglion cell spectral sensitivity curves determined by the 

 constant response criterion discussed previously can be taken as a 

 working indication of the action spectra of the photopigments in the 

 primary receptors (36, 37) . Although the action spectrum of a photo- 

 pigment in a receptor may differ from the absorption spectrum of the 

 same photopigment after it is extracted and purified, the differences 

 tend not to be large (1, 2, 6, 35). Thus, some correlation between 



