BENTLEY GLASS 901 



very much sinallci pulses. Rusluoii therefore suggests that the mechan- 

 ism of adaptation is such that every quantum produces an effect, 

 but that the effect diminishes rapidly and exponentially with the 

 proj:)ortion of pigment blcadied. The theory of vision nuist in the 

 future therefore reckon with the fact that a quantum of light ab- 

 sorbed anywhere in a rod or cone can produce an effect a tenth of a 

 millimeter away which adds to similar effects from other receptors 

 in stimidating a nerve, and all within a second. It must also accord 

 with the equation 



V - Vn = 



log(f^e-«^+l) 



where / is the light intensity, V the nerve signal, V ,^ the value of V 

 in the dark, !„ the absolute dark-adapted threshold, a a constant that 

 is about 4 for cones and 40 for rods, and x the fraction of visual 

 pigment bleached. How to relate the information content of the 

 nerve signal as well as its magnitude to the quantum content of the 

 light remains for the present a puzzle. 



The Molecular Orgojiization 



In vertebrate eyes, vitamin A^ or A2, oxidized to the aldehyde forms 

 known as retinenes, and combined with specific proteins called opsins, 

 generate the four visual pigments: rhodopsin and porphyropsin, found 

 in rods; iodopsin and cyanopsin, found in cones. Invertebrate animals 

 possess visual pigments of a similar type, made of vitamin A^ and 

 retinenci combined with a variety of opsins; but they lack the DPN 

 — alcohol-dehydrogenase system that functions in vertebrates in the 

 formation of retinenes. According to George Wald, whose compre- 

 hensive review covers the chemistry and behavior of these visual pig- 

 ments, rhodopsin, the most fully studied of the four named above, 

 changes upon exposure to light to a transient orange-red form, lumi- 

 rhodopsin. Lumirhodopsin transforms into another intermediate, also 

 orange-red, and called metarhodopsin; and metarhodopsin is hy- 

 drolyzed into retinene and opsin (Fig. 13) . The regeneration of 

 rhodopsin requires two strongly coupled reactions, the oxidation of 

 vitamin to retinene and the union of the retinene with opsin. The 

 second reaction is sufficiently exergonic to drive the first, and the 

 continuous removal of the aldehyde by opsin displaces the equilibrium 

 of the first reaction in the oxidative direction. Retinene arising from 

 the decomposition of rhodopsin is not immediately reusable in forming 

 rhodopsin. 



