Routine Clinical Electroretinography in Dogs
The well established procedure for clinical electroretinography in man (Finkelstein et al., Invest. Ophtal. 7, 214–218, 1968) was adapted as a routine screening procedure in dogs (Schaeppi and Liverani, Agents and Actions 7, 347–351, 1977).
Following pupillary dilatation and immobilization of the dog with a cataleptic drug (1-Polamivet, Hoechst) the electroretinogram (ERG) was recorded with the above technique of Ganzfeld stimulation. The head of the dog was kept within a sphere of 60 cm diameter, the white inner surface of which could be illuminated with light flashes of 10 μsec duration. The source for light flashes, a Grass stroboscope was located in a way that no flashes directly reached the eye fundus. The ERG was recorded by means of modified contact lenses. Testing was performed in dark adapted dogs. Luminance curves were carried out with white, blue and red stimuli. Flicker fusion frequency in response to repetitive maximum photic stimulation served for determination of cone function.
Studies with known retinotoxic agents demonstrated that the ERG was essential for determination of minute derangements of vision in dogs. The studies also gave further information on the process of vision in dogs. Observations by other investigators that the dog has a relatively small number of cones, a high threshold luminance for cone activation and a lack for being conditioned to red versus grey stimuli, together with our observation that white, blue and red light flashes caused a qualitatively similar ERG is consistent with a limited capability of this species for color discrimination.