The Certainty of Ambiguity in Visual Neural Representations

Abstract

Some images evoke bistable percepts: two different visual experiences seen in alternation while continuously viewing an unchanged stimulus. The Necker Cube and Rubin's Vase are classic examples, each of which gives alternating percepts of different shapes. Other bistable percepts are alternating colors or directions of motion. Although stimuli that result in salient bistability are rare and sometimes cleverly constructed to emphasize ambiguity, they have been influential for over 150 years, since the work of von Helmholtz, who considered them to be evidence for perceptual visual processes that interpret retinal stimuli. While bistability in natural viewing is uncommon, the main point of this review is that implicit ambiguity in visual neural representations is pervasive. Resolving ambiguity, therefore, is a fundamental and ubiquitous process of vision that routinely affects what we see, not an oddity arising from cleverly crafted images. This review focuses on the causes of widespread ambiguity, historical perspectives on it, and modern knowledge and theory about resolving it.

Keywords: ambiguity; bistable perception; contextual influences; unconscious inference; visual perception.

Figure 1

(a) (i) A Necker cube. (ii, iii) The elements of the Necker cube separated into parts that include (ii) a truncated pyramid or (iii) seven planar shapes. (b) The blue edges of the left box appear longer than the magenta ones, while the blue edges of the right box appear shorter than the magenta ones. In fact, all blue edges and blue arrows are equal in length, and all magenta edges and magenta arrows are equal in length. The blue arrow at the far right reveals that magenta edges are actually longer than blue ones. Panel adapted with permission, © 2009 Lydia Maniatis; also appears in Maniatis (2017).

Figure 2

(a) Jeune homme à sa fenêtre (Young Man at His Window) by Gustave Caillebotte (painted in 1875) (public domain). (b) (Top) The top-left and bottom-right corners appear to be under lower illumination because of the diagonal edges interpreted as shadow boundaries. This causes area A to appear lighter than area C although they in fact reflect the identical amount of light to the eye. (Bottom) The same image as above but with no shadow boundaries. Areas A and C appear to have the same lightness. Figure courtesy of P. Cavanagh, adapted with permission.

Figure 3

(a) A folded V-shaped concave card standing on end, with a magenta surface on the left and white surface on the right. (b) Card viewed directly. Illumination from the light source (green arrows) is reflected off each surface on to the other surface. Light reflected off the magenta surface has a spectrum biased toward longer wavelengths. (c) When the card is viewed through a pseudoscope, the physics of illumination and interreflection is still as in panel b, but the percept becomes a roof-shaped convex card, which is physically incompatible with interreflection between the two surfaces. Panel adapted with permission from Bloj et al. (1999).

Figure 4

(a) A shadow depends on properties of the object, the illuminant, and the surface on which the shadow is cast. (b) The car appears to float above the floor because the darker floor area is interpreted as a shadow cast by the car. In fact, the darker area is a feature of the floor itself, as appreciated by viewing the figure upside down. Figure courtesy of P. Cavanagh; image credit Honda.

Figure 5

Schematic depiction of positive and negative history effects in perception. (a) Exposure to an initial stimulus is followed by a subsequent stimulus. (b) The prior exposure to an initial stimulus can have the positive effect (center column) of making that same stimulus more easily detectable or clear when presented again (top row); of making subsequent stimuli appear more like the initial stimulus than they actually are (center row); or, in cases where the subsequent stimulus is bistable, of prompting perceptual dominance of the interpretation that matches the prior stimulus (bottom row). However, prior exposure to an initial stimulus can also have negative effects (right column), which are the opposite of the positive effects.

Figure 6

When multiple similar bistable stimuli are shown side by side, perception is often correlated across stimuli, consistent with the visual system’s use of spatial context for perceptual disambiguation. In this example the observer may perceive either a flock of waterfowl lifting off or four raptors in a hunting stoop but rarely a group made up of raptors and waterfowl going in opposite directions.

Figure 7

(Top) Rivalrous chromatic objects. In each eye, eight objects have one chromaticity, and the other eight have another chromaticity (patchwork rivalry). Chromatic rivalry is maintained at all 16 retinotopic locations. (Bottom) The resulting percept often consists of all 16 objects appearing the same color, sometimes all red and sometimes all green.