Hamasaki and Bridges, 1965; and Kobayashi, 1962), and S-potentials have 

 been reported on (Dowling and Ripps, 1971; Tamura and Niwa, 1967). 

 Receptive field mapping by single unit technique (Naka and Witkovsky, 1972) 

 and tectal studies have also been accomplished. Although much data on 

 visual mechanisms has been derived from ERG studies, single unit analyses 

 are lacking, especially in the higher visual centers (see page 21). 



Little is known of the vegetative physiology of the eye. Visual 

 pigments from several elasmobranchs have been extracted and characterized 

 (Denton and Shaw; 1963; Bridges, unpublished). The more revealing 

 techniques of microspectrodensitometry have not been attempted on shark 

 retinas. 



c. Behavioral Aspects 



Few psychophysical studies on vision have been reported though train- 

 ing experiments have yielded reliable data on dark-adaptation, brightness 

 discrimination, critical frequency of flicker-fusion, visual sensitivity, 

 colior discrimination and pattern discrimination (Aronson e_t al . , 1967; 

 Cla'rk, 1959; Graeber et_ al. , 1973; Gruber, 1975). These data have tended 

 to confirm the results of physiological studies and have strengthened the 

 view that sharks have an extremely functional visual system. Any 

 discussion of visual capability must account for both visual acuity and 

 light sensitivity. Thus, when one asks if an animal has "good vision" the 

 inquiry may concern either spatial resolution (acuity) or how well an 

 animal can see in dim light (sensitivity) . We can state that sharks have 

 extremely good vision regarding sensitivity factors. Their sensitivity 

 to light spans the visible spectrum, from blue to deep red and they are 

 ten times more sensitive than man under similar test conditions. Nothing 

 at all is known of spatial resolution in these creatures. Some estimates 

 have been made, based on packing density of photoreceptors (Franz, 1931) 

 but these are largely speculative. The temporal resolution of sharks is 

 similar to man's and is the type associated with a duplex retina. Thus, 

 under high illumination a shark can detect, as flashing, light which is 

 intermittently blocked 40 times per second. This has theoretical impli- 

 cations with regard to function of cone photoreceptors and practical 

 implications on how fast a shark can swim without its visual world 

 blurring. 



The large gap here is in visual acuity. This factor is amenable to 

 analysis by physiological and behavioral techniques and should be under- 

 taken. By determining parameters such as the spatial modulation transfer 

 function, practical knowledge can be gained on the ability of sharks to 

 distinguish two contours as separate. Additionally, theoretical data 

 on size tuned channels in the sharks visual system could be derived. 



Another area of importance concerns qualitative aspects of vision: 

 for example, do sharks possess trichromatic color vision? Again, for 

 both theoretical and practical reasons the nature and importance of color 

 vision must be revealed. 



Finally, as mentioned in the previous section, the role of vision in 

 the field behavior of sharks is virtually unknown. Not only does little 

 useful information exist on the behavior of sharks in general, but it is 

 also conceptually difficult to establish criteria for determining the 



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