Abstract
In 100 years since the figure-ground phenomenon was first discussed by Rubin (1921), very little attention has been paid to the importance of task set and instructions on the outcome of figure-ground organization. Here we test whether changes in task set and instructions can alter the outcome of the cross-border competition between figural-priors that underlies figure-assignment. In figure-ground organization, the figure is defined as a region that is both “shaped” and “nearer”. Extremal edge (EE), a relative distance prior, has been established as a strong figural prior when the task is to report “which side is nearer?” In a within subject experiment using bipartite stimuli, EEs competed and cooperated with familiar-configuration, a shape prior for figure assignment. The experiment consisted of two blocks marked by the presence (Block-A) vs. absence (Block-B) of instructions emphasizing that familiar objects might be present. Within each block, two sub-blocks, one for each task set: “which side is shaped?” and “which side is nearer?” were counterbalanced across participants. Data from 24 participants for Block-A showed small but significant effects of familiar-configuration for displays sketching upright familiar objects, although “shaped-side” responses were predominantly determined by EEs. In Block-B, instructions regarding the possibility of perceiving familiar shapes were added. Now, although EE remained the dominant prior, the figure was perceived on the familiar-configuration side of the border on a significantly larger percentage of trials across all display types. With familiarity thus “primed”, effects of task set emerged when EE and familiar-configuration favored opposite sides as figure. Thus, changing instructions can modulate the weighing of figural priors for shape versus distance in figure assignment in a manner that interacts with task set. Moreover, we show that the influence of familiar parts emerges in participants without MTL/PRC brain damage when instructions emphasize that familiar objects might be present.