Photoreception 



423 



wave length yields a curve which describes the spectral sensitivity of the 

 electrical response (Fig. 128). Numerous such measurements have been 



j^ade 50.51.95,1.34 



The spectral sensitivity curve of the electric response of the frog eye, using 

 intensities below the cone threshold, yields a curve which is identical with 

 the spectral absorption curve of rhodopsin, the rod pigment in the frog eye 

 (Fig. 128). Despite the relative abundance of electrical spectral sensitivitv 

 measurements on diverse animals, no other direct comparisons with spectral 

 absorption of photolabile pigments can be made because the latter have not 

 been measured in vitro. The action spectrum of the squid eye''^^ resembles 

 the absorption spectrum of rhodopsin, which agrees with the presence of a 

 rhodopsin-like substance in these eyes.^'"*' '■* 



0-400 



500 



0-600 <i 



Fig. 128. Spectral absorption curve of visual purple (open circles), and the action 

 spectrum of the dark-adapted frog eye as measured by the size of the b-wave of the 

 retinal action potential (closed circles). From Granit." 



Spectral sensitivity curves of photoreceptors obtained by the measurement 

 of different responses (e.g., electrical and behavioral) are in general agree- 

 ment. Thus the spectral sensitivity of insects determined by behavior agrees 

 with the electrical measurements. In the human eye the scotopic and 

 photopic (rod and cone) luminosity curves, obtained by verbal response, 

 agree well with similar electrical determinations.'*^ 



TEMPERATURE EFFECT ON THE RETINAL ACTION POTENTIAL. A SCCOnd 



point of comparison between the photochemical process and the electrical 

 event in the retina concerns the effect of temperature on these reactions. It 

 is well known that photochemical bleaching reactions are affected not at all or 

 very little by temperature. It will be remembered that Hecht determined 

 the effect of temperature on the photochemical process in the clam, obtaining 



