THE PHOTORECEPTOR PROCESS IN VISION 



68 n 



}.0 - 



^ 



t oQ- porphyropsin^ 



C 



c 

 o 



■ ^04 



Q 



02- 







1 \ \ 1 \ \ 1 \ \ \ \ \ \ \ I \ 



T'orphyropsin, cyanopsin, and spectral sensitivity of the tench 



scotopic 

 -sensitivity 



cyanopsin 



photopic 

 sensitivity 



A L 



_L 



_L 



A L 



SOO 600 



Wavelengrt/i - m/j 



700 



FIG. 1 6. Absorption spectra of porphyropsin and cyanopsin Clines, small circles^ compared with 

 the spectral sensitivities of rod and cone vision in a fresh-water fish, the tench (broken line, large 

 circles'). The spectral sensitivities were measured electrophysiologically by Granit (i6) in opened 

 eyes from which cornea and lens had been removed. The photopic sensitivity agrees well with the 

 absorption spectrum of cyanopsin, but the scotopic sensitivity is displaced about lo mM toward 

 the red from porphyropsin, perhaps because of some yellow pigmentation in the retina or oculai" 

 fluids. 



better its reciprocal, the visual sensitivity, with the 

 concentration of visual pigment. It has been as- 

 sumed that in a steady illumination the visual pig- 

 ments bleach to steady levels, maintained thereafter 

 by regenerative processes. Simultaneously the visual 

 sensitivity falls to a steady state value; this is light 

 adaptation. Conversely, in the dark the vi-sual pig- 

 ments are synthesized to their maximal concentra- 

 tions. Simultaneously the sensitivity rises to a maxi- 

 mum; this is dark adaptation. 



Lately it has become apparent that whatever 

 relation obtains between visual sensitivity and con- 

 centration of visual pigment is not as direct as 

 simple proportionality. On the contrary, the bleach- 

 ing of a very small fraction of rhodopsin in dark- 

 adapted rods results in an extraordinarily large fall 

 of sensitivity (51). Parallel 'light adaptations' con- 

 ducted on a human subject and on a solution of 

 cattle rhodopsin in a water model of the human eye 

 show that, to a first approximation, the bleaching 



of 0.006 per cent of the rhodopsin lowers the visual 

 sensitivity 8.5 times; and the bleaching of 0.6 per 

 cent of rhodopsin lowers the sensitivity 3300 times 

 (65). Conversely the resynthesis of the last small 

 fraction of rhodopsin must raise the sensitivity 

 greatly. Indeed much of light and dark adaptation 

 in the rods seems to involve the first small fraction of 

 rhodopsin to be bleached, and the last small fraction 

 to be resynthesized (cf. 23, 24). 



Recently Rushton and his co-workers have suc- 

 ceeded by a most ingenious procedure in measuring 

 directly the rise and fall of visual pigment in the 

 living human eye (8, 49, 50). This permits a direct 

 comparison between the rates of bleaching and syn- 

 thesis of photosensitive pigments and the course ol 

 light and dark adaptation. For measuring rhodopsin, 

 the method depends on comparing the reflection 

 from the retina of a blue-green light strongly ab- 

 sorbed by rhodopsin with an orange light scarcely 

 absorbed by rhodopsin. No change of retinal re- 



