VISUAL SYSTEM: STATE OF THE ART 69 



duration of the first flash was decreased from 1500 ms, the size of the 

 response to the second flash increased, i.e., suppression was reduced. State of 

 adaptation also influenced suppression. With a light-adapted animal sup- 

 pression could not be evoked until after 5-10 min in the dark. Complete 

 suppression did not occur until after 40 min of dark adaptation. As the 

 interval between the two flashes was increased, the suppression was 

 diminished. However, decreasing the interval to less than 5 s increased the 

 suppression. Hamasaki and Bridges confirmed the presence of suppression 

 in the retinas of Negaprion, Ginglymostoma, and Dasyatis. Similar findings 

 were made by Dowling and Ripps (1970) for the skate Raja. 



To determine whether the suppression is a local retinal event, Hamasaki and 

 Bridges made simultaneous ERG and tectal recordings. Even though the 

 ERG was completely suppressed with a second flash, a near-normal discharge 

 was detected from the tectum, indicating that signals were being transmitted 

 from the retina to the optic tectum. This conflicts with the results of 

 Dowling and Ripps (1970), who showed a close correspondence of b-wave 

 and ganglion cell sensitivity in the skate. Since the ganglion cells represent 

 the final output of the retina, it follows that tectal responses should reflect 

 any suppression in the ERG after a second flash. Dowling and Ripps sug- 

 gested that since the ERG is a massed response, enough of the retina might 

 remain responsive enough to direct or stray light to evoke a response in the 

 tectum. 



Receptor Potentials— Direct recordings from elasmobranch photo- 

 receptors have not yet been reported. However, the receptor potential of the 

 skate Raja has been isolated by application of sodium aspartate to the eye- 

 cup preparation (Dowling and Ripps 1971a, 1972). This technique was first 

 used to evoke receptor potentials in the toad retina (Furukawa and Hanawa 

 1955). 



After immersion of the eyecup in Ringer's solution containing aspartate, 

 the b-wave of the ERG is lost, leaving the a- and c-waves (Figure 17). Visuali- 

 zation of the a-wave is increased in the presence of aspartate, due to the 

 absence of the b-wave. With removal of the pigment epithelium, from which 

 the c-wave originates (Steinberg et al. 1970), only the a-wave remains. Cur- 

 rent evidence points to the distal edge of the negative a-wave as originating 

 from the photoreceptors (Sillman et al. 1969), while the proximal part is 

 probably derived from glial cells (Witkovsky et al. 1975). 



Cohen et al. (1977) used the receptor potential of Negaprion to meas- 

 ure the spectral sensitivity of its retina. 



Horizontal Cell Responses— Horizontal cells are characterized by 

 having all of their processes end in or about the outer plexiform layer, where 

 they contact the photoreceptors (Rodieck 1973). In fishes, including elasmo- 

 branchs, they are very large. It is because of their size that they have been 

 much studied by the method of intracellular recording. Yet, in spite of this 

 wealth of information, the exact functional role of horizontal cells is not 

 known. 



