How to tell circles from ellipses: perceiving the regularity of simple shapes

Human observers achieve a surprising precision in many visual judgements, such as estimating relative position, colinearity and the regularity of shape. We measured the sensitivity in detecting shape deformations by presenting a square simultaneously with a rectangle of variable aspect ratio, or a circle with an ellipsoid. Weber fractions approach 3-5% and improve to approximately 1% when subjects are asked to tell which of the two objects was 'oriented more vertically', instead of identifying the square or circle. Contour position can be judged with a precision of 10-20 arc s, clearly in the hyperacuity range and also beyond the thresholds known for detecting differences in the curvature of comparable line segments. Our results suggest that detecting deformation in rectangles seems to rely on aspect ratio, whereas performance is improved for ellipsoids by a high sensitivity for changes in local curvature.