AMBIGUOUS PICTORIAL DEPTH CUES AND PERCEPTIONS OF NONRIGID MOTION IN THE 3-LOOP FIGURE

The three-loop figure is a two-dimensional (2-D) pattern that generate s (mis)perceptions of nonrigid three-dimensional (3-D) structure when rotated about its centre. Such observations have been described as cou nterexamples to the principle whereby a moving object is presumed to b e rigid, provided that a rigid interpretation is possible (ie the 'rig idity constraint'). In the present investigation we demonstrated that stationary three-loop figures exhibit many of the classic properties o f multistable/ambiguous figures, with any one of several possible 3-D configurations being reported at any one instant. Further investigatio n revealed that perceived nonrigidity during rotation was markedly red uced (and rigidity enhanced) when the figure was modified with static pictorial depth cues (eg shading, interposition). These cues had no ef fect on the overall proportion of time that observers reported 3-D org anisations in stationary versions of the figure, but significantly red uced the frequency of perceptual reorganisation, and increased the dur ation for reporting a particular organisation. Since each of the perce ived 3-D structures in a stationary ambiguous 2-D figure has a unique kinetic counterpart (ie rigid transformation), we attribute the nonrig id structure perceived when the figure rotates to the integration of t hese otherwise inconsistent kinetic components; and have further illus trated this with modified versions of a Penrose impossible triangle. U nder kinetic versions of the classical size/distance invariance hypoth esis, the rigidity constraint may be considered to represent a special instance of size/shape constancy, in which case counterexamples invol ving (mis)perceptions of nonrigid structure are comparable to other we ll-known exceptions to such principles of minimum object change (eg cl assical illusions).