Abstract
Threshold vs. intensity (tvi) functions were measured under conditions in which the slope of the rising branch approximated the deVries-Rose law in order to evaluate the contribution of intrinsic visual noise (dark light, Eigengrau) to age-related elevations in threshold under photopic conditions. Data were obtained from 48 observers (20-88 years) using a temporal 2AFC procedure. The stimulus was centered at 8° nasal retinal eccentricity and consisted of a 560 nm, 14.4' test flash (10 ms) concentric with a steady 500 nm (12.9°) adapting field (13 intensity levels ranging from 0-9 log quanta · sec(-1) · deg(-2)), which resulted in clear scotopic and photopic branches. Photopic thresholds increased linearly with age at a rate of 0.08 log unit per decade at the cornea. The mean slope of the rising portion of the tvi functions (in log-log coordinates) was 0.62, and not correlated with age. Dark light values increased with age, but not significantly. Dark light was a statistically significant predictor of individual differences in absolute photopic threshold, but it is not responsible for age-related increases in threshold under photopic conditions.