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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.

Original publication

DOI

10.1167/7.4.8

Type

Journal article

Journal

J Vis

Publication Date

23/03/2007

Volume

7

Keywords

Dark Adaptation, Flicker Fusion, Humans, Light, Male, Models, Biological, Mutation, Photobleaching, Reaction Time, Retinal Cone Photoreceptor Cells, Retinal Pigments, Transducin, Vision, Ocular, Visual Acuity