Abstract

We used stimuli consisting of two parallel, briefly flashed lines to measure two different aspects of spatial vision. In the first experiment, we determined the luminance threshold for the two-line stimulus as a function of line separation. From these data we determined the area of spatial summation. Using the same stimulus, at 3× threshold, we measured acuity as the separation required to resolve the two lines. In the fovea, the spatial extent of these two measures is comparable, suggesting that both may be tapping the same underlying spatial mechanism. When the experiments are repeated outside the fovea, however, the two measures diverge grossly. At 7° eccentricity, acuity has fallen by about a log unit, while spatial summation decreases little or not at all. It is difficult to devise a plausible model in which spatial summation (as defined here) and acuity reflect the same underlying process. We consider ways to correlate these findings with retinal physiology.

© 1985 Optical Society of America