VISUAL SYSTEM: STATE OF THE ART 



67 



properties and to correlate the spectral sensitivity of the animal with its 

 environment. 



The electroretinogram can be observed by recording the electrical poten- 

 tial difference between a wick electrode placed on the surface of the cornea 

 and an indifferent electrode situated either behind the eye or subcutaneously 

 in the head. Alternatively, a fine chlorided silver wire may be placed in the 

 vitreous humor through a puncture in the sclera. The ERG may also be 

 recorded from what is termed an "eyecup preparation." With this method 

 the eye is removed from the animal and the anterior section, including 

 cornea, lens, and humors, is removed. The eye is then placed in a chamber 

 through which oxygen or another gas mixture flows, to keep the retina alive. 

 A wick electrode is used for recording. 



In general the ERG consists of a series of negative and positive waves 

 (Figure 16). The a- wave or PHI is a negative deflection whose distal or leading 

 edge is thought to arise from the photoreceptors. Recent evidence shows 

 that the proximal part of the ERG originates from glia (Witkovsky et al. 

 1975). The positive wave following the a-wave is termed the b-wave (PII) 

 which apparently originates in the Muller fibers (Miller and Dowling 1970). 

 A slow positive wave termed the c-wave (PI) follows and this originates in 

 the pigment epithelium (Steinberg et al. 1970). A positive off effect, termed 

 the d-wave, is sometimes seen. 



Kobayashi (1962) recorded two types of ERG's in the eyecup preparation 

 (i.e., cornea, lens, and vitreous removed) of the dogfish Mustelus manazo: a 

 fast diphasic and a slow, negative monophasic form. He noted that the fast 

 type tended to be recorded from the ventral region, while the slow form 

 apparently originated from the dorsal side of the retina. After partial dark 

 adaptation, weak light stimuli of short duration evoked a positive monophasic 

 wave. In the completely dark-adapted eye, the ERG was followed by a slow 

 negative wave. Long-duration stimuli did not evoke an off response. With 

 more intense light, a fast negative potential preceded the positive one. The 

 slow response was a negative deflection that increased in amplitude: an off 







Negapnon 



Dasyatis 



Ginglymostoma 



light adapted 



dark adapted 



I sec 



Figure 16 Comparison of light- and dark-adapted ERG waveforms in three elasmobranch 

 species. Calibration 2.5 s; 250 juV. (Taken from Hamasaki and Bridges 1965 with kind 

 permission of the authors and Vision Research © 1965 Pergamon Press Limited.) 



