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Sizar Kamar, Marcus Howlett, Maarten Kamermans; Silent Substitution Stimuli silence Cones Light Responses but not their Output. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2363. doi: https://doi.org/.
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The silent substitution concept was suggested by Rushton et al. as a way of investigating the contribution of specific photoreceptor types to vision. A silent substitution stimulus consists of two alternating stimuli, of which wavelengths and radiances are chosen such that they present steady excitation in one cone type, while others are modulated. In principle this procedure prevents modulation of the phototransduction cascade in the silenced cone type. Since, the output of cones is determined by both the modulation of the phototransduction cascade and the feedback signal from horizontal cells, we asked the question how the output of the cones is affected by the silent substitution stimulus.
Responses of cones from isolated goldfish retina were measured by whole cell voltage clamp. The retina was stimulated with either silent substitution or cone isolating stimulus, specifically tailored for each cone type. Direct light responses were measured outside the activation range of the Ca2+-current of cones (-70 mV). And the cone output, i.e. the modulation of the Ca2+-current of cones, was measured at the half activation potential of the Ca2+-current (about -40 mV).
For the cone isolating stimuli, light responses were only seen when stimuli were matched to the cone type being recorded indicating that the spectral composition of our stimuli were well matched to each cone type spectral sensitivity. No direct light responses were found in any of the cones when the appropriate silent substitution stimuli were used. However, the silent substitution stimuli for the specific cone types modulated the cone output.
Although silent substitution stimuli may prevent direct light responses in cones, they do not silences the cone output. This modulation of the cone output is generated via negative feedback from horizontal cells to cones. These results indicate that caution is needed by using silent substitution and cone isolating stimuli to study the wiring of the visual system.
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