Photoreception 



415 



FLICKER FUSION FREQUENCY. The flicker fusion frequency is the frequency 

 at which a flickering Hght appears to be continuous. This frequency varies 

 with the intensity of the hght, as shown in the curves of Figure 122. In ex- 

 periments on man, the subject is usually confronted with a uniformly and 

 continuously illuminated field, and in the center of this field is an area which 

 flickers (as shown in the upper left insert of Fig. 122) with the same aver- 

 age intensity as the surrounding field. The lower right insert shows the ar- 

 rangement used for animals. The animal is placed at E, C is a rotating glass 

 cylinder with longitudinal opaque black stripes, and S is a circular source of 

 light. The animal is free to look along any radius, and as the cvlinder ro- 

 tates the black stripes give the appearance of flicker. Many animals (turtles, 

 frogs, fish, isopods, insects) react to the light as long as it appears to flicker 

 and cease to react as soon as it appears to be continuous. If the light to dark 

 ratio of the flicker cycle is altered the curve is shifted as shown (Fig. 122). 

 If the temperature of a cold-blooded animal is raised (e.g., from T] to T2), 

 the curve is shifted to the left (Fig. 122). 



BRIGHTNESS DISCRIMINATION. By brightness discrimination we mean the 

 ability to detect a change in brightness. The observer is confronted with a 



_1_ 



LOG INTENSITY CI) 



Fig. 123. Diagram showing the rod and cone curves for brightness discrimination 

 plotted against log intensity. The insert (upper left) indicates the type of visual field 

 used in these experiments. From Jahn.^'" 



uniformlv illuminated field of intensity Z. Then an additional intensity 

 (Al) is briefly superimposed on part of this field, and the observer deter- 

 mines the value of Al which can just barely be detected. A plot of 7/AZ 

 against log I is shown in the curve of Figure 123, and one form of the ex- 

 perimental field of view is shown in the insert. 



THE KINETICS OF DARK ADAPTATION. Since Hccht measured the time course 

 of the dark reaction in the clam, he and his students and numerous others 

 have determined the time course of the dark reaction of photoreceptors in 

 great numbers of organisms (see page 406, also Jahn"*). These studies have 

 been confirmed by determination of the time course of the resynthesis of 

 bleached rhodopsin in vitror^ 



