VISUAL SYSTEM: STATE OF THE ART 75 



goldfish by Wagner et al. (1960) and by Daw (1968) but these refer to an 

 organization based on chromatic stimuli. With the low number of cones 

 found in the retina of Mustelus (Stell and Witkovsky 19736), the existence 

 of such a chromatic coding system seems doubtful. Thus, from the descrip- 

 tion of Stell et al., the responses seem typical of on-off cells. 



Large receptive field sizes seem to predominate in the elasmobranchs. 

 Dowling and Ripps (1970) reported a center diameter of 1.5 mm, while the 

 surround comprised an annular region extending at least 3 mm from the 

 border of the center. In the dogfish Mustelus, Stell et al. (1975) reported 

 ganglion cell receptive field sizes of 1-3 mm. They suggested that the recep- 

 tive field centers are determined by spread of the ganglion cell dendrites, 

 since the diameters of the receptive field centers approximated dendritic 

 field diameters measured by Stell and Witkovsky (1973a). 



Barlow et al. (1957) found that the level of light adaptation influenced 

 receptive field size in the cat retina. Similar results were reported by Stell et 

 al. (1975), who showed that ganglion cell receptive fields shrink during dark 

 adaptation . 



Although it is known that the receptive fields of vertebrates, including 

 elasmobranchs, are organized into an antagonistic center surround system, 

 exactly what part the various distal neurons play in producing this system is 

 unknown. To help clarify the role played by the horizontal cells in the 

 organization of the receptive field, Naka and Witkovsky (1972) undertook a 

 series of studies using the dogfish Mustelus. 



By injecting current directly into dogfish horizontal cells and recording 

 from the ganglion cells, they classified the responses into two types, A and 

 B. Type A cells gave an on response to depolarizing current, an on-off 

 response to hyperpolarizing current, and an on response to a spot of light in 

 the center of the receptive field. Type B ganglion cells responded in the 

 opposite manner; that is, they gave an off response to a depolarizing current, 

 an on response to hyperpolarizing current, and an on response to an annulus 

 of light. Polarization of a horizontal cell evoked responses from both type A 

 and B ganglion cells, thus showing that these cells are not driven by any 

 particular horizontal cells. 



It was also found that a subthreshold depolarizing current, which cannot 

 evoke a response by itself, enhances an on discharge evoked by a light 

 stimulus. Hyperpolarizing current caused a depression of the on responses to 

 light stimuli. 



Since all electrically stimulated horizontal cells evoked responses in gan- 

 glion cells, it was concluded that all ganglion cells in the dogfish retina receive 

 signals from the horizontal cells. These signals are mediated by bipolar cells 

 and probably reflect the activity of bipolar cell membranes (Naka and 

 Witkovsky 1972). 



Flicker— The ability to resolve intermittent stimuli has been shown to 

 be a property of the photoreceptors (Rodieck 1973). When critical fre- 

 quency of flicker fusion (cff) is measured in animals with duplex retinas the 

 resultant cff-vs-intensity curve always shows a "kink," marking a changeover 



