ABSTRACT
It is well known that the orientation of an optical pattern relative to egocentric or extraneous references affects its figural quality, that is, alters its perceived form and concomitantly delays or quickens its identification (Rock, 1973). A square presented in the frontal plane to an upright person (S), for instance, changes from a “box” to a “diamond” when it is rotated with respect to the S’s median plane by 45°. This angle, that is, the angle between the orientations of the pattern in which the two apparent figures (“Gestalten”) attain a summit of purity and distinctness, will be called the “figural disparity” of the pattern. If, as in this case, the S is upright, the retinal meridian and the subjective vertical (SV) are both in the viewer’s median plane. The question arises with respect to which of these orientation references the two figures are identified. The answer may be found when the pattern and the S are oriented in such a way that the projections of the retinal meridian and the SV into the plane of the pattern diverge by the pattern’s figural disparity or its periodic multiples; that is, in the case of a square by 45° or 135°, respectively. Similarly, which reference determines whether an equilateral triangle is seen as a “pyramid” or a “traffic warning sign” may be revealed at a divergence of SV and retinal meridian of 60° or 180°, respectively. It is generally found that for head roll tilts (ρ) and figural disparities of up to 90°, the figure whose axis coincides with the SV is seen. At head tilts of ρ=180°, however, the retinal reference dominates, as a rule independently of the figural disparity (for reviews see Rock, 1973 and Howard, 1982).
