Reversible figure

Ambiguous images or reversible figures are optical illusion images which exploit graphical similarities and other properties of visual system interpretation between two or more distinct image forms. These are famous for inducing the phenomenon of multistable perception. Multistable perception is the occurrence of an image being able to provide multiple, although stable, perceptions. Classic examples of this are the rabbit-duck and the Rubin vase. Ambiguous images are important to the field of psychology because they are often research tools used in experiments. There is varying evidence on whether ambiguous images can be represented mentally, but a majority of research has theorized that they cannot be properly represented mentally. The rabbit-duck image seems to be one of the earliest of this type; first published in Fliegende Blätter, a German humor magazine (Oct. 23, 1892, p. 147).

Middle vision is the stage in visual processing that combines all the basic features in the scene into distinct, recognizable object groups. This stage of vision comes before high-level vision (understanding the scene) and after early vision (determining the basic features of an image). When perceiving and recognizing images, mid-level vision comes into use when we need to classify the object we are seeing. Higher-level vision is used when the object classified must now be recognized as a specific member of its group. For example, through mid-level vision we perceive a face, then through high-level vision we recognize a face of a familiar person. Mid-level vision and high-level vision are crucial for understanding a reality that is filled with ambiguous perceptual inputs.

When we see an image, the first thing we do is attempt to organize all the parts of the scene into different groups. To do this, one of the most basic methods used is finding the edges. Edges can include obvious perceptions such as the edge of a house, and can include other perceptions that the brain needs to process deeper, such as the edges of a person's facial features. When finding edges, the brain's visual system detects a point on the image with a sharp contrast of lighting. Being able to detect the location of the edge of an object aids in recognizing the object. In ambiguous images, detecting edges still seems natural to the person perceiving the image. However, the brain undergoes deeper processing to resolve the ambiguity. For example, consider an image that involves an opposite change in magnitude of luminance between the object and the background (e.g. From the top, the background shifts from black to white, and the object shifts from white to black). The opposing gradients will eventually come to a point where there is an equal degree of luminance of the object and the background. At this point, there is no edge to be perceived. To counter this, the visual system connects the image as a whole rather than a set of edges, allowing one to see an object rather than edges and non-edges. Although there is no complete image to be seen, the brain is able to accomplish this because of its understanding of the physical world and real incidents of ambiguous lighting.

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