Aging and the saturation of colors. 1. Colorimetric purity discrimination

Colorimetric purity (Pc) discrimination functions were measured for 21 colo r-normal observers (11 younger and 10 older observers with mean ages of 30 and 74 years, respectively). On each two-alternative-forced-choice trial, o bservers saw two flashes of light, a broadband white light [CIE(x, y) = (0. 33, 0.35)] and a mixture of broadband and monochromatic Light (420-680 nm). The observer's task was to choose the flash that had a chromatic component . Foveally viewed, circular, 1.2 degrees-diameter stimuli were presented as 1.5-s flashes with 3-s interstimulus intervals in Maxwellian view. Stimuli [250 trolands (td) and 10 td] were equated on the basis of individual hete rochromatic flicker photometry functions. Measured Pc discrimination sensit ivity was lower in the older group than in the younger group at both light levels, and the performance difference between the age groups was approxima tely constant across the spectrum. The difference between discrimination at 10 and 250 td was relatively small for the younger group but larger for th e older group, indicating a selective performance decrement for older obser vers at low light levels. The data were modeled as a sum of differential re sponses from S-cone and L/M-cone chromatic channels. The model and the data indicate similar age-related losses of sensitivity in the two channels, pe rhaps secondary to receptoral sensitivity losses. (C) 1999 Optical Society of America [S0740-3232(99)02102-X].