Effects of spatial separation and retinal eccentricity on two-dot vernier acuity.

Experiments investigated how vernier acuity for dot targets is affected by dot spatial separation and retinal eccentricity. Beck and Schwartz [Vision Res. 19, 313-319, 1979] found that the vernier thresholds increased linearly with interdot separation from 7.5 to 30 min arc. Experiment 1 showed that the vernier thresholds increased linearly with interdot separation from 0.5 to 8 deg arc. Experiments 2 and 3 showed that moving the dots into the periphery while keeping the interdot separation constant increased the threshold little if at all. The results are interpreted as supporting the hypothesis that there is (1) an encoding of dot positions in retinal coordinates, and (2) that the relative positions of the dots are made explicit in terms of the slope of the virtual line joining them. The increase in threshold with dot separation indicates that the visual system is unable to directly access and compare the retinal positions of the dots. The constancy of the threshold with eccentricity indicates that the visual system encodes the retinal positions of two well-separated dots as accurately in the periphery as in the fovea.