Shape thresholds of geometrical figures have been found equal in cases when both the shortest and the longest linear dimensions of different stimuli were the same (Gutauskas et al, 1994 Perception 23 Supplement, 43). The regularity depended neither on colour, nor on size, nor on spatial orientation of the stimuli. In the present study we have checked this result with five different stimuli, which had a central symmetry. Circles, squares, equilateral triangles, pentagons, and hexagons were taken for the psychophysical measurements of the increment shape thresholds. The stimuli, as bright filled objects, were shown on the dark screen of the optical stimulator. Monochromatic light (wavelength 552 nm) was used for the stimuli. As the area of the stimuli increased [from 8 to 1157 (min arc)2], the threshold decreased gradually. Five curves, running in parallel, were obtained in a certain order: squares, hexagons, circles, pentagons, and triangles, if seen from top to bottom. The data obtained demonstrate that the thresholds for identification of shape are different if various figures are of equal area, and they are the same if the figures are of equal height. This supports the idea that certain spatial frequencies of the two-dimensional spectra of the stimuli and the shape thresholds may be interrelated variables.
