Discrimination of the center position of a circle exceeds discrimination of the center position between two dots for stimuli beyond a critical size

Abstract

Spatial discriminations that are finer than the center-to-center spacing of foveal photoreceptors (hyperacuity) are readily observed for 1-D spatial tasks such as Vernier offset, spatial frequency and line separation discriminations, and bisection tasks. We introduce a new task where the just- noticeable difference in offset from the center of circle Δc is measured. Circle center discriminations were measured over a range of circle radii and compared to just-noticeable differences in bisection between two dots Δb with separations equal to the circle diameters. The data show that Δb and Δc were equivalent for small stimuli (radii or bisection distance were approximately ≤0.025° of visual angle). For stimuli larger than this critical size, performance on the 2-D circle task Δc progressively exceeds performance on the 1-D bisection task Δb. These results demonstrate that the human visual system is able to exploit a 2-D spatial parameter (area) to achieve a hyperacuity advantage.