796 LIGHT AND LIFE 



workers have developed an ingenious method for studying those pig- 

 ments in the intact eye by means of retinal spectrophotometry. By 

 this means they have identified several cone pigments in subjects hav- 

 ing both normal and attenuated color vision, have measured the rate 

 of bleaching and regeneration of these pigments, and have related 

 the sensitivity of the eye to the amount of pigment present. 



We still do not know how the light-induced chemical reactions of 

 the pigment cause the excitation of the neuronal elements of the 

 retina. This is one of the most challenging problems in visual re- 

 search. Another problem concerns the mechanism of amplification 

 of the light-induced signal and its conduction along the receptor cell. 

 We only know that the absorption of a very few light quanta causes 

 the release of much larger amounts of energy in the retina (31) and 

 that the signal can be transmitted a distance of 100 microns or more, 

 from the outer segments of the rods, in as little as a few milliseconds 

 (14). 



In order to intercept the visual message at various points along its 

 path, the methods of electrophysiology have been extensively used 

 and progressively refined. Large diffuse electrodes placed on opposite 

 sides of the retina will record a series of complex potential changes 

 in response to light. These electrical recordings are called electro- 

 retinograms (ERG) and have been studied extensively under a 

 variety of conditions. In the beginning, when the methods were crude, 

 it was thought that practically all of the electrical activity of the retina 

 originated within the receptors (15). As electrical localization 

 methods were inijiroved, mainly through the introduction of micro- 

 electrodes which record activity from highly localized regions, a large 

 part of the electrical activity was found to originate in the inter- 

 mediate neuronal layers of the retina. At the present time none of 

 the recorded activity can be said with certainty to arise within the 

 receptors themselves, although the possibility has not been definitely 

 ruled out (2, 3, 4, 21, 29) . It may be that the conductive pathway 

 for electrochemical changes lies within the receptors instead of on 

 their surface membranes, as is the case with ordinary neurons. Such 

 a possibility is suggested by the fact that the receptors are ontogeneti- 

 cally derived from ciliated cells and that electron microscopy has 

 tlemonstrated typical ciliary fibrils embedded in the cytoplasm of the 

 rod and cone cells. Conduction might take j^lace along these fibrils 



(23). 



The most perijiheral localized sign of electrical activity discovered 

 in the retina to date was found some years ago by Svaetichin (24) , 



