Bistable Apparent Motion Axis Determined by Axis of Surrounding Object

Abstract

Dynamic dot quartets undergoing apparent motion appear to move along either of two orthogonal axes with perception switching unpredictably between the two interpretations. The axis of motion (horizontal or vertical) can be biased by embedding a quartet in a larger framework that is undergoing apparent motion. We were interested in whether a static spatial context could bias the perceived axis of motion. In the first experiment a real bar ran along the medial axis of the quartet either horizontally or vertically, and in the second experiment a virtual or Kanizsa bar was employed. In both cases the quartet's motion axis was biased parallel to the bar. In the third experiment the quartet was centered within a real or virtual object (ellipse, rectangle, diamond: each with 2:1 aspect ratio) such that one axis of the quartet lay along the principal medial axis of the object and the other perpendicular to it. Viewers were biased to see quartet motion along the axis of the surrounding object for both real and virtual objects. A fourth experiment found the same result with disks placed in dipole, rectangle or diamond configurations but lacking well-defined real or virtual bounding contours. This implies that perceptual grouping of the disks into geometric objects permits definition of an axis sufficient for inducing quartet alignment. In the first two experiments there was a real or virtual medial contour dividing the quartet, while in the latter two experiments there was no perceived contour, only the abstract medial axis of the object, suggesting that the medial axis representation (Blum, 1973) may have some real foundation in perception. Induction of apparent motion parallel to an object's principal axis may be less surprising if the “motion” is more properly understood as an object undergoing a transformation about an axis.