70 



VISION 



RETINAL ILLUMINATION 

 1 1x10' 3 /jW/cm 2 (dim) 3 48 x 10'' fjW/cm 2 (bright) 



bwave 



eyecup 



eyecup 



(Na aspartate! 



retina 



(Na aspartate) 



1 sec 



100 fjV 



stimulus 



Figure 17 Effect of sodium aspartate on the massed response 

 of the retina of Raja. Responses evoked by bright and dim 0.2 s 

 flashes of light are shown. Immersion of the preparation for 

 3 min in Na aspartate abolished the b-wave of the ERG. Re- 

 moval of the retinal epithelium eliminated the c-wave. Thus the 

 aspartate-treated retina produces a massed response consisting 

 primarily of the a-wave, most of which represents the activity of 

 the photoreceptors. (Modified from Dowling and Ripps 1972. 

 Reproduced by kind permission of the authors and the Journal 

 of General Physiology © 1972 Rockefeller University Press.) 



Horizontal cells respond to light with a graded, hyperpolarizing potential 

 (Figure 18). This response, termed the "S-potential," is divided into two 

 types. The first, referred to as the L- or luminosity type, responds to all 

 wavelengths of light with hyperpolarizing potentials. Because of this it is 

 thought to encode luminosity or brightness information. The second type of 

 S-potential is termed the C- or chromaticity type. Cells of the chromaticity 

 type respond with hyper- and /or depolarizing potentials, depending on stimu- 

 lus wavelength. New evidence has shown that in the turtle and goldfish these 

 responses are the result of feedback from receptors via horizontal cells onto 

 other receptors (Fuortes and Simon 1974, Stell et al. 19756). 



Most S-potentials recorded from elasmobranchs are of the L-type (Tamura 

 et al. 1966, Tamura and Niwa 1967, Dowling and Ripps 19716, and Niwa 

 and Tamura 1975). However, C-type potentials were recorded from the 

 retina of Dasyatis akajei (Tamura et al. 1966, Tamura and Niwa 1967, and 

 Niwa and Tamura 1975), which possesses both rods and cones (Table 5). 



Properties of elasmobranch S-potentials are similar to those recorded from 

 other animals. However, some differences exist; these will be dealt with later. 



