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Andrew Stockman, Lindsay T. Sharpe, Michel Michaelides, Anthony T. Moore, Andrew R. Webster, Hannah E. Smithson; Second sight: Vision sustained by a secondary activation of the phototransduction cascade. Journal of Vision 2006;6(6):120. doi: https://doi.org/10.1167/6.6.120.
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© ARVO (1962-2015); The Authors (2016-present)
Introduction: A father and son are homozygous for a rare frameshift mutation (M280fsX291) in the gene encoding the a-subunit of cone transducin (GNAT2), the G-protein in the cone phototransduction cascade. Although this mutation should entirely abolish their cone vision, it does not. Here, we investigate the properties of their residual cone vision.
Methods: We measured cone-mediated critical-flicker-fusion as a function of the radiance of a 650 nm red target, and temporal modulation sensitivity as a function of temporal frequency at a fixed time-averaged 650 nm radiance (of 10.68 log quanta s−1 deg−2). Targets were superimposed in the centre of a 480 nm, rod-saturating background. Additional spectral sensitivity and bleaching controls were carried out to ensure that the responses were cone responses.
Results: Both father and son retain a rudimentary cone-driven response restricted to high light levels and low temporal frequencies, which has two singular properties. First, temporal frequency sensitivity is limited by the equivalent of a simple 1st order reaction with a time constant of ∼140 ms. Second, their temporal acuity is linearly related to the logarithm of the amount of bleached cone pigment.
Conclusion: The results are consistent with the residual cone response being dependent on the secondary activation of the phototransduction cascade by a bleaching photoproduct produced by a first-order reaction, such as a metarhodopsin derivative or the freed opsin, which bypasses the defective α-transducin. Such a secondary activation has long been hypothesized to explain the equivalence between the visual effects of bleaches and those of steady background lights.
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