Differential ambiguity reduces grouping of metastable objects

Abstract

Multiple copies of an ambiguous figure are often perceived to share a common interpretation and to switch interpretations synchronously. Examples include Attneave's triangles and the jumping dot quartets of Ramachandran and Anstis. It remains uncertain whether this phenomenon is to be understood as the outcome of a network with local cooperative and competitive interactions, or of a top-down system in which constraints based on object and spatial properties conspire to impose a global interpretation on the ambiguous figures. We decided to investigate this issue using pairs of reversible rotating objects (defined by kinetic dots or wire frames). We compared the degree of rotational coupling of pairs of coaxial ambiguous objects to pairs of coaxial objects where one object had an unambiguous direction of rotation. When both objects were ambiguous, rotational direction was strongly coupled (80–95% in different conditions). Surprisingly, when an ambiguous object was paired with an unambiguous object, they rotated together with little better than chance probability (typically 50–60% coupling). We evaluated the hypothesis that rotational coupling depends on stimulus similarity by measuring the coupling between different ambiguous shapes, between a kinetic dot cube and a Necker cube, and between objects rendered with opposite contrast dots. Rotational coupling remained high in all of these cases, implying that the reduction of coupling between objects differing in ambiguity is not attributable to their lack of similarity as such. It is difficult to see how these results could be predicted by a model in which grouping arises from local cooperative and competitive interactions. They are equally difficult to reconcile with an account based on shared object or reference frame constraints. Objects with different degrees of ambiguity appear to pose a challenge both for our perceptual grouping mechanisms, and for our understanding of those mechanisms.