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The color appearance mechanisms are to large extent unaffected by the known age-related changes in the optical media or yellowing of the lens whereas the ability to discriminate between small color differences is compromised with an increase in age. The approximate hue constancy across the life span could be explained by a concurrent parallel decline in cone signals. A mechanism that takes the difference between the L and M cone signals, for example, will not be greatly affected by a decline in both the L and M cone outputs. Hue constancy across the life span is not a simple consequence of the differential cone combinations of the higher-order chromatic mechanisms.
The human visual system can adjust the cone weightings of the chromatic mechanisms over the lifespan and thereby compensate for a decline in peripheral cone signals. This dissociation between discrimination and appearance mechanisms is supported by Neitz and colleagues who showed that shifts in unique yellow induced by long-term changes in the chromatic environment are not due to receptoral or sub-cortical changes, but must be of cortical origin, probably after chromatic information from both eyes has been integrated. The question remains how these higher order color mechanisms receive feedback on the strength of their cone inputs. The gains of the L and M cones are adjusted such that the red-green opponent mechanism is at equilibrium for the average daylight, but this recalibration is by no means complete. The brain uses information about the statistical properties of our chromatic environment to adjust the weighting of the receptor signals to achieve hue constancy across the life span.
The mean unique hue settings are not affected by the illumination conditions. There is a differential effect of adaptation on hue constancy only under daylight adaptation do the age-related changes in the green settings. What appears uniquely green for young observers appears more yellowish for older observers. Older observers require more S cone input to achieve unique green when the settings are obtained under simulated daylight, but still much less than predicted by the lens model. The yellow-blue mechanism; which is silenced by the unique green setting is most affected by the yellowing of the lens.
If the visual system were able to fully compensate for the changes in the optical media, the observed cone weightings should not vary with age. It is found that under most of the tested conditions, this is the case; only under adaptation to daylight, green hues changes slightly with age.
In summary, there are compensatory mechanisms operating on higher-order color functions and thereby ensuring that hue remains approximately constant despite the known age-related changes in the lens. The concurrent age-related decline in the chromatic discrimination sensitivity suggests that the neural site of these compensatory mechanisms is probably cortical; the underlying mechanism is still poorly understood, but is consistent with the idea that it is based on invariant sources in our visual environment.