684 



HANDBOOK OF PinSlOI.OCV -^ NEUROPHYSIOLOCJY I 



\ \ 1 \ \ 



ChicHen : o iodopsin 



• rhodopsin 



Human lensless peripheral 

 vision: O cones 



• nods 



rhodopsin 

 rod vision 



1 — I — I — I — r 



1.2 - 



1.0 



p 0.8 

 5 



§ 0.6 

 o 



u 



l5 



0.2- 



0- 



I r 



iodopsin 

 cone vision 



400 



500 



600 



700 



FIG. 15. Absorption spectra of chicken rliodopsin and iodopsin compared with the spectral 

 sensitivity of human rod and cone vision. The spectral sensitivity measurements were made in a 

 peripheral field in the aphakic (lensless) eye to avoid distortions caused by the yellow pigmenta- 

 tions of the lens and macula lutea. They represent as close an approximation to the sensitivities 

 of the naked rods and cones as can be achieved in the living eye (cf. 61, 63). The scotopic (rod) 

 sensitivity agrees with the absorption spectrum of rhodopsin over most of its course. The photopic 

 (cone) sensitivity is displaced some 20 m^i toward the blue from the absorption spectrum of iodopsin; 

 it represents the resultant of the spectral sensitivities of at least three groups of cones concerned 

 with color vision. [From VVald li at. (72).] 



measured electrophysiologically, are shown as large 

 circles. The lines and small circles show the ab- 

 sorption spectra of porphyropsin and cyanopsin. 

 The photopic sensitivity agrees very well with the 

 absorption spectrum of cyanopsin; but for reasons 

 which are still obscure, the scotopic sensitivity is 

 displaced about 10 m/i toward the red from por- 

 phvropsin. The corneas and lenses had been remo\ed 

 from these preparations; possibly some yellow pig- 

 mentation in the retina or a trace of blood in the 

 ocular fluids may account for this discrepancy,^ In 

 animals having vision based upon \itamin A.., the 



' Recently it has been shown that the absorption spectra of 

 visual pigments in silu lie about 7 mix toward the red from their 

 positions in solution (ga, 70a). 



Purkinje shift is unusually large: about 90 m/z, 

 from about 530 m/x in the scotopic eye to about 620 

 mix in the photopic eye. This is consistent with the 

 large displacement between the absorption spectra 

 of porphyropsin and cyanopsin. 



It can be concluded that the spectral sensitivities 

 of rod and cone vision, and hence the Purkinje 

 phenomenon, derive directly and quaiititati\ely 

 from the absorption spectra of the \isual pigments. 



Visual Adaplalion and the Bleaching and 

 Synthesis of Visual Pigments 



It has been believed for many years that soine 

 simple relation connects the visual threshold, or 



