VIEWPOINT-INVARIANT WEBER FRACTIONS AND STANDARD CONTOUR-CURVATURE DISCRIMINATION

It is proposed that any cue for the visual discrimination of shape, in particular the discrimination of curved contours, should be such that the perceptual relationships defined by the cue are invariant under c hanges in observer viewpoint. Such relationships may be quantified by the Weber fraction; that is, the ratio DELTAc/c, where, for any partic ular value c of the cue, DELTAc is the smallest difference in c that c an be detected. Eight geometric attributes of curved contours having o ne symmetry axis and parallel chords (a standard stimulus configuratio n) were examined for invariance of the Weber fraction under symmetry-p reserving affine transformations of the image plane (changes in viewpo int are well approximated by affine transformations when depth is smal l relative to viewing distance). The attributes, each investigated in previous psychophysical studies, were equivalent-curvature, radius-of- curvature, turning-angle, arc-length-divided-by-chord-length, arc-leng th, maximum-deviation (sag), area, and mean-deviation. Three of the at tributes, namely sag, area, and mean-deviation, satisfied the viewpoin t-invariance condition; the remainder failed. These results are consid ered in relation to previously published empirical data on the Weber f raction for contour-curvature discrimination.