Task set and instructions influence the weight of figural priors: A psychophysical study with extremal edges and familiar configuration

Abstract

In figure-ground organization, the figure is defined as a region that is both "shaped" and "nearer." 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. 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 three experiments using bipartite stimuli, EEs competed and cooperated with familiar configuration, a shape prior for figure assignment in a "which side is shaped?" task." Experiment 1 showed small but significant effects of familiar configuration for displays sketching upright familiar objects, although "shaped-side" responses were predominantly determined by EEs. In Experiment 2, 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. In Experiment 3, both task set (nearer/shaped) and the presence versus absence of instructions emphasizing that familiar objects might be present were manipulated within subjects. 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 medial temporal lobe/ perirhinal cortex brain damage when instructions emphasize that familiar objects might be present.

Keywords: Depth ordering; Extremal edges; Familiar configuration; Familiar parts; Figural priors; Figure–ground perception; Perceptual organization; Shape perception; Task instructions; Task set.

Fig. 1

Figural priors. a Figure–ground organization: The “figure” is the side of a bipartite image that appears to be “shaped by the shared contour, nearer to the observer, and thing-like.” In the image on the left, black and white equal area regions share a border. The top and bottom insets illustrate the shaped figures that would be perceived if the border were assigned to the white side (top) or the black side (bottom). The region on the opposite side of the border would simply appear to continue behind the shaped figure. b Familiar configuration. Intact upright familiar configuration is a prior for figural assignment (%figural = 76%), while inverted ones are not as strong (%figural = 60%; adapted from Gibson & Peterson, 1994). c Extremal edges (EE). EEs arise when a 3D convex surface curves around to occlude a part of itself. For the EE side (on left), the steepest part of the shading gradient is parallel to the shared border, thus specifying that the left side is closer than the right side. The side for which the shared border is an extremal edge is seen as figural 83% of the time, even when 3D convexity is the same for the two sides sharing the border, as in c (adapted from Palmer & Ghose, 2008). d Congruent versus incongruent. EE alters the probability that the figure is perceived on the familiar configuration side of a border depending on whether it is cooperating (98%) or competing (8%) with familiarity (adapted from Ghose & Palmer, 2010) in a “Which side is closer?” task

Fig. 2

Experimental design. a Congruency of familiarity and EE: Surfaces of revolution were created by rotating the shared contour around different axes to create congruent (blue) and incongruent (pink) versions from a given black-and-white image. b Factors: Orientation upright versus inverted. Configuration type: Intact familiar where the shared border elicits good match to a memory representation of a well-known object. Part rearranged: Novel configuration created by spatially rearranging parts of familiar configuration (in the example shown, the neck of the guitar separates the parts above and below its waist). Congruency: Congruent—EE and familiarity present on the same side of the shared border (blue frames). Incongruent—EE and familiarity are present on the opposite sides of the shared border (pink frames)

Fig. 3

a Experimental trial. The trial structure illustrated here was used for both Experiments 1 and 2. b Shaped versus shapeless. Shape judgment task was explained by indicating that blue and green regions appear to have definite “shape” followed by clarification that stimuli will be bipartite images. c Task instruction. Experiment 1: Shaped-side task instruction “Your task will be to judge which side of the shared border is ‘shaped.’ Please report your first impression/ gut reaction.” Experiment 2: Shaped-side task with familiarity instruction present “Your task will be to judge which side of the shared border is ‘shaped.’ Sometimes the shape can be a familiar shape. PAY ATTENTION. Please report your first impression/ gut reaction. Verbally identify the familiar shapes shown here.” Sample images from the practice session are illustrated

Fig. 4

Results. Data are plotted as percentage of trials the EE side was chosen as shaped. a Experiment 1. Familiarity instruction absent, shaped-side task. b Experiment 2. Familiarity instruction present, shaped-side task. The data show that the closer EE side also appears to be shaped unless familiarity is “primed” with instructions. Error bars correspond to the standard errors of the means

Fig. 5

Illustration of difference in design between Experiments 1, 2 (between-subject), and 3 (within-subject). Experiments 1 and 2 involved shaped-side task only, while Experiment 3 included two task sets: shaped-side task and nearer-side task, with task order counterbalanced across participants. Experiment 3b always followed Experiment 3a

Fig. 6

Results. Data are plotted as percentage of trials the EE side was chosen as nearer (top row) and shaped (bottom row) for Experiment 3a: familiarity instruction absent and Experiment 3b: familiarity instruction present. The data show that the closer EE side also appears to be shaped unless familiarity is “primed” with instructions emphasizing its presence in the stimuli. Error bars correspond to the standard errors of the means