VISUAL SYSTEM: STATE OF THE ART 79 



suggested to Dowling and Ripps that factors controlling sensitivity occurred 

 not in the horizontal cells but in the photoreceptors. 



With present techniques, the photoreceptors of elasmobranchs are too small 

 to record from. Thus it is not possible to directly test the various hypotheses 

 on retinal sensitivity. However, the addition of Na aspartate to the retina 

 suppresses the ERG except for the a-wave, whose distal or leading edge 

 originates at the photoreceptors. 



Treating the skate retina with aspartate, Dowling and Ripps (1972) investi- 

 gated adaptation properties of the photoreceptors by recording the receptor 

 potential. The adaptation properties of the receptor potentials were similar 

 to those of b-wave, S-potential, and ganglion cell responses. These included 

 an initial silent period, induced by a fairly strong background light, which 

 eventually recovered. Thereafter responses could be elicited in the presence 

 of a background light, even though a large fraction of the visual pigment was 

 bleached. Thus, threshold responses assumed a linear relation with back- 

 ground intensity. 



In addition to the adaptation response properties of the skate retina pre- 

 viously mentioned, evidence for a gain control mechanism was found. It has 

 been suggested that a background light causes a maintained receptor poten- 

 tial (Boy ton and Whitten 1970). This potential or voltage has the effect of 

 moving the receptor towards its saturating voltage, thereby compressing its 

 dynamic range. This gain mechanism therefore opposes the so-called com- 

 pression effect by increasing the gain of the receptors. Green et al. (1975) 

 recorded receptor potential, S-potential, b-wave, and ganglion cell responses 

 under light and dark adaptation and incremental stimulation to determine 

 the exact site of adaptation in the skate retina. Two sites of adaptation were 

 found: one probably resides in the photoreceptors and operates under in- 

 tense background light; the second resides somewhere proximal to the hori- 

 zontal cells and is active during weak background light. This is the site of 

 "network adaptation" which is a loss of b-wave and ganglion cell sensitivity 

 under very dim background lights. It is not seen in increment thresholds of 

 receptors or S-potentials. The b-wave and S-potential reacted differently to a 

 weak adapting light. Therefore, it stands to reason that the site of this 

 network adaptation is not the horizontal cells since, if it were, the S- 

 potential and b-wave would behave alike. In addition, when a weak adapting 

 light was used the receptors and horizontal cells recovered their sensitivity 

 together, while the ganglion cells followed the recovery of the b-wave. 



Green et al. (1975) hypothesized that an excess of a substance, perhaps 

 potassium, was responsible for the loss of b-wave sensitivity, since the time 

 course for both b-wave and ganglion cells to reach final threshold levels was 

 several minutes, too slow to be of neural origin. 



This hypothesis was further strengthened by Dowling and Ripps (1976), 

 who applied different concentrations of external potassium in the eyecup 

 preparation of the skate. When the data were normalized, increasing the 

 amount of external potassium caused the V log I curve of the b-wave to be 

 shifted to the right on the intensity axis, indicating a loss of sensitivity. No 

 loss of sensitivity was seen for the receptor potential. 



