Effect of luminance on the relation between accommodation and convergence.
When luminance is lowered, both vergence and accommodation tend to shift toward different and individually characteristic resting postures, commonly referred to as dark-vergence and dark-focus. In order to determine the luminance level at which these two mechanisms correspond to a target distance, subjects viewed a light spot 0.4 mm in diameter, corresponding to 1.4 to 2.7 min arc, located at the individually determined dark-focus position. As the luminance of a binocularly viewed spot increased, accommodation did not change significantly from the target distance. However, fusional vergence gradually shifted from the dark-vergence to the target position. The critical luminance level at which the 2 responses were coupled for the 12 observers ranged from 0.01 to 0.45 cd/m2, with distinct individual differences. In a second experiment, the target was positioned at distances different from the dark-focus and luminance was set at 0.5 and 1.0 log unit higher than the subject's critical luminance level. At lower luminance levels, mononuclear focus tended to remain at the individual dark-focus, whereas the binocular focus tended to correspond to the target distance. These differences can be attributed to vergence accommodation. Previous studies on night myopia reported that a negative correction based on one-half of the individual dark focus was superior to a full dark-focus correction. The present results suggest that this difference is due to vergence accommodation and that the optimum correction for night myopia depends, in part, on the critical luminance level for activation of fusional vergence.