The Influence of Axiality on Figure/Ground Assignment

OF THE THESIS The Influence of Axiality on Figure/Ground Assignment By SEHA KIM Thesis Director: Dr. Jacob Feldman This study investigates the influence of shape on figure/ground assignment. We measure the influence of axiality – a measure of how well a shape is described by a skeletal representation – on border ownership (f/g assignment) by pitting axiality against symmetry, a known figure/ground cue. The axiality is defined via the description length of the maximum a posteriori (MAP) skeleton for a shape. We designed a display in which an axial shape shares a common boundary with a symmetric shape, and used two methods to assess perceived f/g assignment along the common boundary: a local method, the motion probe task, and a global one, an explicit verbal response. Experiment 1, using the motion probe method, found a systematic influence of axiality on border ownership assignment: the shape with stronger axiality tends more often to be perceived as figure, that is, to own the common boundary. The results from Experiment 2 confirm that such effect of axiality was consistent for global f/g assignment as well. We conclude that (a) the axiality has an influence on f/g assignment, so global (from all contour points) and local (only near the motion probe) axialties of shapes predict border ownership, and (b) the contour of the shared boundary is perceptually “owned” by the skeleton that best explains the shape.