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Visual functions decline with age, but how aging degrades visual functions remains controversial. In the current study, the mechanisms underlying age-related visual declines were examined psychophysically. We developed an efficient method to quickly explore contrast sensitivity as a function of nine spatial frequencies at three levels of external noise in both young and old subjects. Fifty-two young and twenty-six old subjects have been screened for ophthalmological and mental diseases and participated in the experiment. Contrast sensitivity varied significantly with spatial frequency, age, and level of external noise. By adopting a nonlinear observer model, we decomposed contrast sensitivity into inefficiencies in internal additive noise, internal multiplicative noise, perceptual template gain, and/or system non-linearity. Model analysis revealed that aging impacts both internal additive noise and perceptual template …