When Scenes Look Like Materials: René Magritte’s Reversible Figure–Ground Motif

We draw attention to a frequent motif in the work of the Belgian surrealist René Magritte (1898–1967). In the motif, a scene is depicted that contains a silhouette, which itself contains another depicted scene. The silhouette is bistable, appearing either as a figural region whose positive space is covered, or filled, with the interior scene texture, or as a ground region providing a window onto a more distant scene. We call this the ‘reversible figure–ground motif’. Because the stimulus does not change when our percept changes, the motif’s appearance at any particular moment cannot be explained by its local or global image statistics. Instead principles of perceptual organization, and in particular image segmentation and figure–ground assignment, appear crucial for determining whether the interior of the silhouette is processed as a material vs. a scene — which in turn reflects the fundamental role of visual segmentation in material and scene perception more generally.

The Three-Dimensional Necker Cube

The Necker cube is a widely known example of a reversible figure. Perceptual reversals were first observed in engravings of crystals by the Swiss geologist Louis Albert Necker in 1832. Although Necker’s engravings were not exactly of regular cubes, the figure as it is used now can be perceived in two alternative arrangements of a three-dimensional (3D) cube. Although less widely known than the popular two-dimensional version, the 3D Necker cube is a surprisingly rich model for psychophysical investigation. This chapter summarizes relevant main results and their implications for diverse theoretical issues such as the definition of visual illusions, the role of global three-dimensional interpretations in the integration of local sensory signals, and the exploratory and multisensory nature of perceptual processes.

Blur and Contrast as Pictorial Depth Cues

Studies have shown that blur can act as a pictorial cue to depth perception. But blurring a stimulus reduces its contrast, and studies have also shown that contrast can act as a pictorial cue to depth perception. To determine whether blur and contrast have separate influences on depth perception, each variable was independently manipulated in two experiments. Observers reported depth alternations in a simple reversible figure. Both contrast and blur were found to influence depth perception, but blur had its greatest effect at moderate contrasts. When blurred and sharp stimuli were equated on either Michelson or RMS contrast, blur continued to affect depth perception. Hence blur can act as a depth cue independently of contrast. It is speculated that blur is effective as a pictorial cue because of its usual association with other depth cues, particularly in pictures and photographs.