Human cone light adaptation: from behavioral measurements to molecular mechanisms

The ability of the cone visual system to regulate its sensitivity from twilight to bright sunlight is an extraordinary feat of biology. Here, we investigate the changes in visual processing that accompany cone light adaptation over a 5 log10 unit intensity range by combining measures of temporal sensitivity made in one eye with measures of the temporal delay between the two eyes in different states of adaptation. This combination of techniques, which provides more complete information than has been available before, leads to a simple model of steady-state light adaptation. At high light levels, visual sensitivity is maintained mainly by photopigment bleaching. At low-to-moderate light levels, it is maintained by trading unwanted sensitivity for speed and by an additional process that paradoxically increases the overall sensitivity as the light level rises. Each stage of the model can be linked to molecular mechanisms within the photoreceptor: The speeding up can be linked to faster rates of decay of activated molecules; the paradoxical sensitivity increases can be linked to faster rates of molecular resynthesis and to changes in channel sensitivity; and the sensitivity decreases can be linked to bleaching. Together, these mechanisms act to maintain the cone visual system in an optimal operating range and to protect it from overload.