Generalised Model of Illusory Grouping Accounts for Collinear, Orthogonal, and Vertex Grouping Percepts

Abstract

A number of illusory phenomena, for example the Kanizsa illusion, exhibit boundary completion by collinearity between visible inducing edges. These phenomena have been addressed by models such as Grossberg's boundary contour system (BCS) (1985 Psychological Review92 173 – 211), which incorporates collinearity cells with receptive fields specialised to detect and enhance collinearity. Other illusory phenomena like the Ehrenstein illusion exhibit boundary completion orthogonal to the oriented inducers. The BCS model explains such orthogonal grouping by disinhibition, due to competition between collinearity cells of orthogonal orientations. There are many illusory grouping phenomena, however, which exhibit boundary completion through sharp corners in a variety of configurations, producing illusory ‘V’ or ‘Y’ vertices. Examples are seen in the diamond percept of the four-line Ehrenstein illusion, the triangular grouping percept of three dots arranged in a triangular configuration, as well as in the hexagonal percept of a grid of dots in a honeycomb pattern. These completions cannot be explained by models based on collinearity. Lehar's orientational harmonic model (1994, PhD thesis, Boston University) offers a single generalised grouping mechanism capable of collinear, orthogonal, and sharp vertex grouping. The proposed mechanism is a harmonic resonance, or pattern of standing waves in the orientational representation which promotes orientational periodicity. Computer simulations show that the model can account for a large number of diverse illusory phenomena using a single simple mechanism, and predicts the specific conditions under which a grouping of one type yields to a different grouping type.