Senescence of the human cone photoreceptor pathways
Color mechanisms dominated by the sensitivity of short-, middle- and longwave cones have been isolated in individuals from infancy to old age. Losses in sensitivity occur in all three cone mechanisms beginning in adolescence and continuing through later life. These changes are due to both optical and neural factors. Quantitative modeling of threshold vs. intensity functions indicates that much of the neural loss occurs at early stages of retinal processing associated with senescent changes in quantal efficiency rather than increases in neural noise (dark light). Intrinsic compensation mechanisms appear to normalize the balance of cone sensitivities across the life span.