Contour curvature analysis: hyperacuities in the discrimination of detailed shape

Curvature discrimination thresholds were measured for a wide range of stimulus curvatures and sizes. Results are compared with an ideal processor of curvature to provide relative efficiencies. The results lead to three major findings. (i) Curved lines may be processed with the same precision as straight lines for decisions of shape, demonstrating a new class of hyperacuity. (ii) High relative efficiencies are obtained for all curved lines with an orientation range of less than 40 deg. (iii) This performance is not however consistent with common processing for straight and curved lines. The results are discussed, and two or possibly three separate processes are suggested as the basis for detailed shape perception.