78 VISION 



sharks, nurse sharks, and the stingray Dasyatis sayi. The curves were 

 characterized by an initial sudden drop in threshold, followed by a slower 

 return to the original dark-adapted thresholds 70 min later. They also 

 reported the absence of a rod-cone break in the dark-adaptation curves in 

 spite of the presence of cones in all three species examined. Although 

 Hamasaki and his coworkers used the b-wave as a measure of sensitivity, they 

 did not really know what the relationship of this transient to the actual 

 sensitivity of the retina was. 



To determine this, Dowling and Ripps (1970) compared the adaptational 

 properties of the b-wave to ganglion cell discharges, since the ganglion cells 

 represent the final output of the retina and thus reflect its final sensitivity. 

 They demonstrated that the slopes of increment thresholds were the same 

 for both the ERG (b-wave) and ganglion cell discharges over a 6 log unit 

 range of stimuli. These increment thresholds required long periods to reach a 

 stable value (up to 40 min) and did not seem to saturate, a result unlike that 

 obtained for man (Aguilar and Stiles 1954). In addition, strong light adapta- 

 tion, which bleached about 80% of the photopigment, suppressed both gang- 

 lion cell spikes and b-wave responses for 10 to 15 min. Thereafter, thresholds 

 for both fell rapidly to within 3 log units of the dark-adapted values. Both 

 types of responses then followed the same course of recovery, which lasted 

 2 h. After 20 min in the dark the time course of these responses was almost 

 identical to that of rhodopsin regeneration. These results were comparable to 

 those from similar experiments on other vertebrates and demonstrated that 

 the early part of adaptation is mediated by neural mechanisms while later 

 adaptation is photochemically controlled (Dowling 1963). 



Thus, Dowling and Ripps (1970) concluded that the b-wave of the ERG 

 and the ganglion cell spikes show adaptation properties that are similar and 

 therefore that amplitude of the "b-wave provides a reliable measure of 

 retinal sensitivity in the skate" (p. 512). 



During these experiments it was shown that an ERG suppressed by 

 moderate light adaptation will reappear after 10 min in the dark. As stated, 

 very bright adapting stimuli inhibited responses for 15 min, and stable 

 response levels were not reached for an additional 20 min. A similar "silent 

 period" was observed for the ganglion cells. Because the a-wave, which orig- 

 inates in the photoreceptors, also disappears, Dowling and Ripps suggested 

 that the mechanism behind the silent period occurs in the receptors. As 

 expected, a similar silent period occurs for the S-potential. When light is first 

 turned on, the horizontal cell hyperpolarizes to a fixed level (i.e., saturates) 

 and remains at this level for many minutes, during which further responses 

 cannot be evoked. After 5 min the membrane potential starts depolarizing 

 towards its dark-adapted level. Responses of increasing amplitude can be 

 evoked as the membrane potential becomes more positive. Increment thresh- 

 olds were obtained even upon backgrounds that bleached more than 95% of 

 the available rhodopsin. In fact, increment thresholds for horizontal cells 

 could be measured on background fields 10 000 times (or 4 log units) more 

 intense than those needed to saturate the S-potential. There seemed to be no 

 correlation between the level of the membrane potential and sensitivity. This 



