Vernier judgments in the absence of regular shape information

Vernier acuity is a form of hyperacuity in which the threshold offset between a test object and a reference object is smaller than the size of a foveal cone. Because the test and the reference objects usually have regular shapes (e.g. rectangular, triangular or circular), relatively few studies have addressed the role of shape information in determining hyperacuity thresholds. In this study, we investigated the effect of shape information on hyperacuity performance using targets of irregular shape with different skew and symmetry properties. Vernier thresholds smaller than 10 arc-sec were obtained for closely spaced asymmetric irregular-shape targets. Thresholds for dots and asymmetric irregular shapes increased with increase in center-to-center gap between the targets. Unlike dots, the thresholds for asymmetric irregular shapes also increased with target area. Although the thresholds for asymmetric irregular shapes were higher than those for dots, thresholds for symmetric irregular shapes were similar. Target skew below a certain level had a negligible effect on Vernier thresholds for asymmetric shapes. Our results suggest the existence of feature-independent neural circuitry that can support hyperacuity thresholds and are consistent with the use of the centroid as a primitive for relative localization.