68 VISION 



effect was never observed. Kobayashi noted that thresholds for the slow 

 response were 10 times higher than those for the fast type. 



In spite of finding the maximum sensitivity at 500 nm for both the light- 

 and dark-adapted preparations, Kobayashi nevertheless considered that the 

 two different responses were subserved by different mechanisms: the slow 

 type from a photopic mechanism, while the fast type derived from a 

 scotopic mechanism. However, this seems reversed, since intracellular record- 

 ings from the photoreceptors showed that the cone responses exhibited a 

 faster time course of decay than did the rods (Baylor and Fuortes 1970). 



In a similar series of experiments, again using the eyecup preparation, 

 Kobayashi (1962) investigated properties of the ERG in a number of rays 

 and skates, including Narke, Holorhinus, Dasyatis, Urolophus, Platyrhina, 

 and Raja. Results were similar to those of the dogfish. 



Although Kobayashi was the first to use elasmobranchs for ERG studies, 

 many of the responses he recorded were negative deflections and those that 

 were positive were small in amplitude and changed the sensitivity by only 

 2-3 log units during dark adaptation. Granit (1947) has shown that such 

 responses are typical of anoxic and deteriorating preparations. 



To overcome these difficulties, Hamasaki and Bridges (1965) and 

 Hamasaki et al. (1967) studied the ERG in the intact preparation of the 

 lemon shark Negaprion, the nurse shark Ginglymostoma, and the stingray 

 Dasyatis sayi. 



By inserting a fine chlorided wire into the vitreous of the eye of an 

 immobilized animal, they recorded ERG's with positive components whose 

 sensitivities increased by at least 6 log units (i.e., 1 million-fold) during dark 

 adaptation, thus demonstrating that their preparations were in excellent con- 

 dition. 



During these experiments it was found that in the light-adapted state the 

 predominant form of the ERG was negative; it changed to a positive 

 response during dark adaptation. Large, positive off effects were seen in the 

 light-adapted state, but little or no off effect was recorded during a dark 

 adaptation (Figure 16). No c-wave was seen in any of the elasmobranch 

 ERG's studied by Hamasaki and Bridges (1965). 



O'Gower and Mathewson (1967) also studied the ERG of the lemon 

 shark, noting only an increase in the amplitude of the ERG when stimulus 

 intensity was changed. The authors did not report a change in latent period 

 or waveform of the ERG when stimulus wavelength was changed. 



In addition to the usual waveform, Hamasaki and Bridges (1965) observed 

 a series of rhythmic potentials that appeared after 1 min of dark adaptation 

 but disappeared as dark adaptation continued. High -intensity stimuli blocked 

 these oscillations. 



In addition, depending upon intensity of the first flash, an ERG ordinarily 

 evoked by a second flash of light might be completely suppressed. At stim- 

 ulus levels 3 log units above threshold, the response amplitude to a second 

 flash is reduced. With full-intensity stimuli, the first flash completely sup- 

 pressed any ERG response to a second flash. Duration of the first flash was 

 also an important factor in the amount of suppression produced. As the 



