Residual cone vision without alpha-transducin.
Behavioral experiments in humans with a rare genetic mutation that compromises the function of alpha-transducin (Galpha the alpha-subunit of the G-protein in the primary cone phototransduction cascade) reveal a residual cone response only viable at high light levels and at low temporal frequencies. It has three characteristic properties. First, it limits temporal frequency sensitivity to the equivalent of a simple first order reaction with a time constant of approximately 140 ms. Second, it delays the visual response by an amount that is also consistent with such a reaction. Third, it causes temporal acuity to be linearly related to the logarithm of the amount of bleached pigment. We suggest that these properties are consistent with the residual function depending on a sluggishly generated cone photobleaching product, which we tentatively identify as a cone metarhodopsin. By activating the transduction cascade, this bleaching product mimics the effects of real light and is therefore one of the molecular origins of "background equivalence," the long-established observation that the aftereffects of photopigment bleaches and the effects of real background lights are equivalent. Alternative explanations for the residual cone response include the possibilities that there is a secondary phototransduction mechanism that bypasses alpha-transduction, or that the truncated alpha-transduction that results from the mutation retains some minimal functionality.
Stockman
Andrew
A
Institute of Ophthalmology, University College London, London, UK. a.stockman@ucl.ac.uk
Smithson
Hannah E
HE
Michaelides
Michel
M
Moore
Anthony T
AT
Webster
Andrew R
AR
Sharpe
Lindsay T
LT
D003623:Dark Adaptation; D005425:Flicker Fusion; D006801:Humans; D008027:Light; D008297:Male; D008954:Models, Biological; D009154:Mutation; D038761:Photobleaching; D011930:Reaction Time; D017949:Retinal Cone Photoreceptor Cells / Q000503:physiopathology*; D012168:Retinal Pigments / Q000528:radiation effects; D015293:Transducin / Q000172:deficiency* / Q000235:genetics; D014785:Vision, Ocular; D014792:Visual Acuity
D012168:Retinal Pigments; D015293:Transducin