1. Introduction
1.1 Multistable Figures: Definitions, Properties, Examples, and Categories
1.1 Multistable Figures: Definitions, Properties, Examples, and Categories
A multistable figure is an ambiguous visual stimulus that can form at least two markedly different perceptual interpretations. When looking at a multistable figure, observers can perceive just one interpretation at one time. Some people have called this characteristic of multistable figures the "property of exclusivity” (Leopold &
Logothetis, 1999). In other words, observers’ perceptual systems seem unable to fixate on a single stable interpretation of a multistable figure. Instead, their perception fluctuates, alternating between different interpretations during a period of continuous viewing (Toppino & Long, 2005; Suzuki & Peterson, 2000).
The Necker cube is one of the famous examples of multistable figures, shown in Figure 1a, which can be seen as the front face down to the left or the front face up to the right. Other well-known examples include Rubin’s faces/vase figure, which can be interpreted as a vase or two faces, and Boring’s young girl/old woman figure, which can be interpreted as a young girl or an old woman. They are shown in Figures lb and lc, respectively.
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(a) (b) (c)
(d) (e) (f)
Figure 1. This figure shows the following examples of multistable figures: (a) Necker
cube; (b) Rubin’s face/vases figure; (c) Boring’s young girl/old woman figure; (d) Schroeder staircase; (e) duck/rabbit figure; and (f) equilateral triangles (Toppino &
Long, 2005; Long & Toppino, 2004).
Garcia-Perez (1992) classified multistable figures into five types of percept-changing properties. The first type is the figure-ground reversible figure, such as the Rubin’s vase/face figure and the diamond stimulus, which will be introduced later. Reversals of this type of figure are related to figure-ground organization changes.
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and the Schroeder staircase (Figure 1d). Reversals of this type of figure are related to fluctuations in perspective (i.e., change in perceived depth).
The third type is the meaning-ambiguous figure, such as the young girl/old woman figure and the duck/rabbit figure (Figure 1e). The perceptual instability of this type of figure is associated with changes in meaning (Long & Toppino, 2004).
The fourth type is the orientation-reversible figure, such as the equilateral triangles in Figure 1f. The multistability of this type of figure involves the assignment of reference frames for the description of the shapes.
The last type is the stereokinesis moving pattern, such as a two-dimensional ellipse rotating in the frontal plane, which can be perceived as a disc oscillating in three-dimensional space or an elongated egg slanted in three-dimensional space and describing a circular trajectory in the frontal plane. It is the analysis of motion that causes the interpretation change (Garcia-Perez, 1992).
To simplify the classification of multistable figures, it is feasible to classify them in terms of whether the perception change is related to the alternation of depth perception. Based on this definition, all figure-ground reversible figures, perspective-reversible figures, and stereokinesis moving patterns belong to the depth-reversible figure category. Conversely, meaning-ambiguous figures and orientation-reversible figures do not belong to this category because their reversals
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have nothing to do with depth reversal. This study is aimed at the investigation of depth-reversible figures.
Since the multistable figure itself does not change at all, the perceptual interpretation of a multistable figure changing with time indicates that perceptual processing must be affected by some internal factors, which may be related to perceptual processing mechanisms (Pitts, Gavin, & Nerger, 2008). Therefore, many researchers use multistable figures to determine what factors affect our perceptual processing and how to affect the processing. In other words, the operation of perceptual mechanisms might be understood better by studying the nature of multistable figure perception (Toppino, 2003).
On the other hand, many researchers believe that ambiguity is the hallmark of retinal stimulus in almost all visual perception, so the visual system must solve the ambiguity to adapt to the environment effectively (Long & Toppino, 2004; Peterson
& Gibson, 1991). The ambiguity comes from the inverse problem of the visual system.
That is, the perception of three-dimensional environments is underdetermined by the two-dimensional image on the retina. In general, our visual system usually uses a heuristic process, producing the most likely interpretation to solve the inverse problem (Palmer, 1999). However, when viewing a multistable figure, the visual system produces more than one interpretation. Instead of fixating on a single, stable
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interpretation, the observers’ perception alternates between different interpretations of a multistable figure during a period of continuous viewing, called instability. Some psychologists think that the perception of multistable figures is just like the perception of other stimuli in a normal environment; they are all ambiguous (Slotnick & Yantis, 2005; Peterson & Gibson, 1991), and they all involve the observers’ perception of form, except that the ambiguity of multistable figures is available to consciousness while the ambiguity of other stimuli is not (Peterson & Gibson, 1991). In other words, the problem of ambiguity is solved in the initial period of the perception of normal stimuli, so the ambiguity does not enter the consciousness. However, in the perception of multistable figures, the ambiguity comes into the consciousness dramatically due to the instability of multistable figure perception. Nevertheless, the underlying processes are believed to be similar to the perception of normal stimuli (Long & Toppino, 2004;
Peterson & Gibson, 1991; Leopold & Logothetis, 1999).