Optical illusions

Summary: Optical illusions are predictable illusory phenomena that are not yet fully understood.

Optics, generally, is the science of the visible. Physical optics is the study of the nature and propagation of light. Physiological optics is the study of neurophysiological processes of light reception and image forming as conditions of vision and merges with psychology and cognitive science into a unitary “vision science.” Optical illusions likely have been observed as long as mankind has existed. Some optical illusions arise from people’s ability to see in three dimensions, even though retinal images are flat representations on a curved surface. Extracting three-dimensional information from ambiguous two-dimensional images requires interpretive rules in the brain. Many optical illusions have mathematical connections, especially in the perception of geometry within the illusion. They are popular as entertainment, and mathematics teachers sometimes use optical illusions in the classroom in order to engage students and to develop visualization skills.

Examples

Physical phenomena leading to seeing unreal things, or to seeing real things in a distorted way (for example, phenomena due to special atmospheric conditions: halos, coronas, and sightings of distant objects caused by reflections between air layers of different density) are now well understood, and not usually named “illusions.” The apparent flattening of the sun disc at the sunset is in accord with the laws of light propagation (differential refraction), but it is not an illusion. Illusions of perception are situations “when perception goes wrong” and where a central (neurophysiological or psychological) cause must be supposed—something is perceived as something else (error of identification) or is perceived differently than it is (error of quality or quantity). For example, the moon at the horizon is often reported to appear larger than if seen high in the sky, although the angular size of the moon disc is in both instances the same (approximately 30 arc minutes)—this is the famous “moon illusion.”

Illusory phenomena have been observed since ancient times, for example, the “moon illusion” was known to Ptolemy, and an illusory “motion after-effect” caused by watching a waterfall was mentioned by Aristotle. However, the proper scientific study of visual illusions began in the middle of the nineteenth century with the discovery of geometric-optical illusions, (distortions of perceived lengths, sizes, and shapes observed in simple drawings or in real-world situations). For example, a path in the visual field subdivided into a series of segments usually appears longer than the same path that is empty (see the Oppel–Kundt illusion, Figure 1). Lengths of linear segments may be overestimated or underestimated, depending on added elements (for example, the popular Müller–Lyer illusion). Geometric figures drawn over linear or curvilinear rasters often appear deformed (see the Hering, Zöllner, or Ehrenstein–Orbison illusion Figure 2). Other instances of optical illusions involve judgments of brightness (see Figure 3), and particularly illusory “contrast phenomena,” such as well-known Mach bands, or the Hermann grid (see Figure 4). More recently, dynamic phenomena, such as illusory motion seen in static pictures, or the famous “scintillating grid,” have been described.

Generally, all these phenomena demonstrate the universal principle of context-dependence in visual (and any) perception: a stimulus, S, is perceived differently if presented together with a context stimulus S′ than if presented alone. In other words, a purely attentional separation of S from S′ is impossible in spite of the observer’s effort.

What differentiates all these phenomena from incidental errors of perception is that they occur regularly and predictably in most or all observers. After more than 150 years since their discovery, there is still no satisfactory theory of these phenomena, although a great variety of explanations have been proposed.

Explanations

The two main directions of explanatory approaches have traditionally been the empiricist and the nativist theories. The empiricist theories, going back to Herman Helmholtz’s theory of unconscious inferences, emphasized the role of the subject’s past experience and of cognitive factors forming the perception. By contrast, the nativist theories searched for explanations in the structure and the functional principles of the sensory organ itself.

Empiricist theories, in spite of their speculative character, have been revitalized by cognitive psychologists and are still influential; for example, a popular theory sought to explain a group of optical illusions as results of inappropriate constancy scaling due to erroneous perspectival interpretation of the illusion-inducing figure. However, these theories ignore much of empirical counter evidence, such as tactile analogies of certain optical illusions, or geometrico-optical distortions observed in contexts not suggesting any perspectival interpretation. Neonativist theories integrating approaches of Gestalt psychology and neurophysiology and searching for interactions within higher levels of the visual system are arguably more promising, although they are usually limited to circumscribed groups of illusory phenomena. The general opinion in the early twenty-first century is that the broad variety of optical illusions cannot be explained by a single cause; therefore, a unitary theory of optical illusions is rather unlikely.

Optical illusions are neither deceptions of the eye nor errors of the cognitive processing of sensory data. They are facts of vision, presumably manifestations of the functional principles of the visual system in its entirety. The same functional principles, or the “laws of seeing,” are at work in visual arts, or in visualization technologies such as virtual reality. The study of optical illusions in laboratory as well as in natural environments importantly contributes to the understanding of the process of vision and of the nature of the visual life world.

Bibliography

Boring, E. G. Sensation and Perception in the History of the Experimental Psychology. New York: Appleton-Century-Croft, 1942.

Coren, S., and J. S. Girgus. Seeing Is Deceiving. The Psychology of Visual Illusions. Hillsdale, NJ: Lawrence Erlbaum, 1978.

Gombrich, E. H. Art and Illusion. Oxford, England: Phaidon Press, 1977.

Metzger, W. The Laws of Seeing. Cambridge, MA: MIT Press, 2006.

Robinson, J. O. The Psychology of Visual Illusion. 2nd ed. New York: Dover, 1998.

Ross, H. E., and C. Plug. The Mystery of the Moon Illusion. New York: Oxford University Press, 2002.

Seckel, Al. Masters of Deception: Escher, Dali & the Artists of Optical Illusion. New York: Sterling, 2007.

———. Optical Illusions: The Science of Visual Perception. Buffalo, NY: Firefly Books, 2009.