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James A. Kuchenbecker, Scott H. Greenwald, Katherine Mancuso, Matthew C. Mauck, Maureen Neitz, Jay Neitz; ON-OFF Electroretinogram Using S-Cone Isolating Stimuli Elucidates S-Cone Circuitry in the Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3908.
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To develop a silent substitution ON-OFF electroretinogram (ERG) paradigm to isolate ON and OFF pathways carrying S-cones signals important for color vision.
A Roland Consult Q400 Ganzfeld was modified to hold 26 short wavelength LEDs (365nm peak or 405nm for human) and 26 green LEDs (520 nm). Photopic ERGs were recorded with rod suppressing backgrounds. In the ON-OFF paradigm, short and long wavelength lights were alternated at 1 Hz (50% duty cycle) and the relative intensities were adjusted via pulse width modulation to isolate S or L/M cone responses.
To validate S-cone isolation, ERGs were performed on both wildtype and S-cone knockout mice. The S-cone isolating stimulus produced robust responses in wildtype mice by no response in the S-cone knockout. S-cone ON-OFF ERGs were obtained from humans, gerbils and squirrel monkeys. For the latter two species, ERG measurements were repeated after treatment with L-AP4. ERGs in response to L/M-cone stimuli showed a large hyperpolarizing a-wave, a depolarizing b-wave, and a depolarizing d-wave. In contrast, the S-cone ERG traces have no a-wave and there is a hyperpolarizing response at S-cone stimulus offset. When L-AP4 is injected, the b-wave is abolished in the M-cone isolating trace demonstrating effective ON-pathway blockade. Under S-cone isolating conditions there is a depolarizing response to stimulus onset despite the ON blockade. Overall the ERG is sign inverted compared to the L/M isolated response.
In all species examined, there were consistent and striking differences between S-cone ERGs and their L/M counterparts. These appear to reflect differences in S vs L/M circuitry conserved across mammals. In primates, L/M cones contact ON and OFF bipolars while S-cone bipolars are only of the ON variety. This is reflected in the S-cone ERG by the absence of an a- and d-wave. Both bipolar center and surround responses are reflected in the ERG and since there is no S-cone OFF bipolar to produce a depolarizing d-wave, horizontal cell mediated S-cone input to the surrounds of L/M cones produces a hyperpolarizing response at stimulus offset. We conclude that in addition to S-cone pathways mediated by the S-cone specific ON bipolar cell there is an additional S-ON and an additional S-OFF pathway mediated by horizontal cells via the surrounds of L/M cones that has characteristics favorable for mediating color vision.
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