Measuring visual form discrimination with blur thresholds

A method is described for gauging the discriminability of spatial forms. Rather than challenging form discrimination by size reduction as is done in testing visual acuity, the maximum image degradation by blurring is determined that still allows shape recognition. The procedure has the advantage that tests are substantially independent of optical (resolution) and retinal (light-processing) stages of vision and concentrate on the perceptual demands of distinguishing form. Candidate spread functions are analyzed with respect to both their spatial and spatial-frequency properties and compared with dioptric defocus. Form discrimination thresholds, in terms of the parameter of the imposed Gaussian spread, were determined for several classes of patterns and compared with contrast reduction, where target size can also be kept constant but retinal sensitivity issues predominate. The technique has utility for experiments in ordering shape difference hierarchies, in examining rules of Gestalt properties, and in identifying progress in perceptual learning. Diagnostic potential in patients with spatial visual dysfunction such as amblyopia remains to be explored.