VISUAL SYSTEM: STATE OF THE ART 



83 



Gruber (1966, 1967, 1969, 1975), Cohen et al. (1977), and Gruber and 

 Hamasaki (in preparation). The training technique associated with these in- 

 vestigations has already been described in the previous section on Eyelids. 

 These experiments have been framed in the duplexity theory and seek to 

 provide a relation between receptor type and visual capability of sharks. 

 Duplexity theory assigns separate roles to the photoreceptor systems of the 

 retina: a sensitivity mechanism resides in the rods and a chromatic, acuity 

 mechanism in the cones. Each system operates through a transitional range 

 of light intensities, the cones functioning in daylight, the rods at night. Thus, 

 depending upon which system was in operation, one might expect dif- 

 ferences between the visual functions of light- and dark-adapted animals, and 

 at the transition point a discontinuity or break in the smooth curve of a 

 particular visual parameter might signal the changeover from rod to cone 

 control of that parameter. For example, Figure 22 shows how the temporal 

 resolution of various animals increases as intensity of the flashing light 



70 r 



60 



CJ> 



20 - 



10 - 



* iguana (cone) 



man (rod- cone) 

 shark (rod -cone) 



gecko (rod* 



2 3 4 5 6 



LOG RELATIVE INTENSITY 



Figure 22 Relation between CFF and stimulus intensity in 

 four vertebrates. The nonhuman data were taken under 

 identical stimulus conditions and are thus directly com- 

 parable. The human data are shown for reference only. The 

 curves demonstrate the rod-cone break in duplex retinas 

 compared to the monotonous function of the simplex 

 retina. (From Gruber 1969. Unpublished dissertation.) 



