Psychophysical measure of the impact of healthy aging on rods and cones of the retina

Previous studies found that the efficiency of cones to detect light declines with healthy aging. These psychophysical studies used an equivalent input noise (EIN) paradigm based on the measurement of contrast thresholds with and without visual noise under specific conditions in which contrast threshold is limited by the variability in the amount of light detected by cones (i.e., photon noise). We recently adapted this paradigm to measure the efficiency of rods, which are the photoreceptors used under scotopic conditions (i.e., night vision). To assess the impact of healthy aging on the efficiency of rods and cones to detect light, the current study measured contrast thresholds in 22 young [20-30 years old, mean=25.7] and 22 older [65-77 years old, mean=70.9] healthy adults using a blue background under scotopic condition, and a red background under low photopic condition, respectively. The task consisted in discriminating the motion direction of an annulus grating centered on fixation with a radius of 10° of eccentricity in presence and absence of visual noise. A 2x2 mixed factorial ANOVA showed that older adults had significantly higher EIN, which suggests a lower amount of light detected by photoreceptors (rods and cones) compared to young adults (F(1,42)=29.7, p<.001). No significant interaction was found between the group and the display color, which suggests a similar age-related decline when using a blue and red display. We conclude that the amount of light detected by rods and cones declines similarly with healthy aging. These similar age-related declines in the amount of light detected by both rods and cones can be explained by similar age-related physiological alterations for the two types of photoreceptors, or a common cause affecting both types of photoreceptors such as a change in the orientation of the photoreceptors.