Visual resolution in humans fluctuates over the 24h period

The present experiment was designed to assess daily fluctuations of visual discriminability, a function reflecting the resolution power of the visual sensitivity by measure of a differential threshold. Sixteen subjects underw ent a visual discrimination threshold task (using the constant method) in a protocol allowing one point every 2h over the 24h period. The results show that the visual discrimination threshold is low in the morning and increas es progressively over the day, reaching a first peak at 22:00. During the n ight, the same pattern occurs, with low threshold levels at the beginning o f the night and high levels at the end. This profile is quite different fro m that of detection threshold variations, suggesting that the two visual fu nctions are under the control of different underlying mechanisms. Two inter pretations could account for this discrepancy. The first relates to differe nt oscillators in the eye for detection and discrimination. The second refe rs to a possible linkage of visual discriminability with the sleep-wake cyc le since threshold measures were systematically low (i.e., high resolution power) after long sleep periods.