568 



IX. CAROTENOIDS AND VITAMINS A 



H3C CH; 



CH3 



HsC CH 



CHs 



Suggested Formulae for Antivitamin A^*^* 



8. The Importance of Carotenoids and of Vitamins A in Vision 



It has been recognized for many 3'-ears that acuity of vision is related to a 

 dietary component. Thus, it has been known since early Eg^^ptian times 

 that night blindness could be alleviated by the ingestion of animal livers; 

 this was one of the first nutritional disorders for which a specific therapy 

 was indicated. Night blindness has made its appearance sporadically in 

 populations subjected to restricted diets, especially as a result of wars or 

 famine. Nyctalopia denotes impaired vision in dim light. This "scoto- 

 pic" vision is related to the activity of the rods in the retina. In contrast, 

 hemeralopia is impaired vision in bright light; this is called "photopic" or 

 daylight vision, and it is dependent upon cone receptors. However, the 

 two terms have ordinarily been used interchangeably; hemeralopia has 

 been employed as synonymous with night-blindness although, strictly 

 speaking, it is an incorrect term. However, there is increasing evidence 

 that both rod vision and cone vision are dependent upon the presence of 

 vitamin A. Granit^-^ proposed a theory to explain scotopic and photopic 

 vision which postulates the presence of dominators (a sensory mechanism 

 characterized by a broad spectral sensitivity curve which makes a wide 

 range of wave lengths available for vision), and modulators, which are charac- 

 terized by a narrow^er response curve than that of the dominators; they 

 are relatively light-resistant, and are suited to mediate wave length dis- 



'*' R. Granit, Sensory Mechanism of the Retina, Oxford Univ. Press, London, 1947, 

 pp. 298 ff. 



