Human short-wavelength-sensitive cone light adaptation.

The cone-driven visual system is able to regulate its sensitivity effectively from twilight to bright sunlight. On the basis of a novel combination of short-wavelength-sensitive (S-) cone measurements of temporal sensitivity and temporal delay, we show that S-cone light adaptation is achieved not only by trading unwanted sensitivity for speed but also by an additional process that counterintuitively increases the overall sensitivity as the light level rises. Our results are consistent with comparable middle-wavelength-sensitive (M-) cone measurements made in protanopic observers and can be accounted for by the same two-parameter model developed to account for the M-cone data (A. Stockman, M. Langendörfer, H. E. Smithson, & L. T. Sharpe, 2006). Each stage of the model can be linked to molecular mechanisms occurring within the photoreceptor: the speeding up to increases in the rates of decay of active and messenger molecules, the unexpected sensitivity increases to increased rates of molecular resynthesis and changes in channel sensitivity, and the sensitivity decreases to bleaching. Together, these mechanisms act to maintain vision in an optimal operating range and to protect it from overload.