The spectral ON-OFF ERG and its use for studying blue-yellow color vision circuitry

Abstract

The electroretinogram (ERG) waveform reflects signals originating from all retinal neural cell types, rod photoreceptors, multiple spectral types of cone photoreceptors, bipolar cells, amacrine cells and ganglion cells. By manipulating the ERG stimulus conditions it is possible to preferentially advantage, and in some cases, isolate responses from particular cell types. One such stimulus paradigm that has been particularly useful for studying cone pathways is the light-adapted, long-flash (or ON-OFF) ERG. This method uses a background photopic light to saturate the rods, effectively isolating cone photoreceptor pathways; and long duration flashes are used to separate responses from downstream ON- and OFF- biopolar cells with ON-bipolars being favored at flash onset and OFF-bipolar cells at offset. We have extended this technique to further isolate responses that originate from subclasses of cone photopigment by adding a silent substitution technique. Specifically, to isolate responses that originate from short wavelength sensitive (S) photopigment ca. 500 msec flashes of short- and long-wavelength light are alternated and the intensities of the two lights are adjusted so that quantal catches of the longer wavelength cones are constant when the long wavelength light is substituted for the short wavelength one. Similarly, responses generated by middle-to-long wavelength photopigments are isolated by adjusting the lights so that S cone quantal catches are held constant when the lights are exchanged. We validated the technique by testing it in mice whose S cones were knocked out genetically. We are using this technique to explore mechanisms of blue-yellow color vision in rodents.