Abstract
Reviewed by: Lothar Spillmann, The Laboratory of Visual Perceptual Mechanisms, Institute of Neuroscience, Chinese Academy of Sciences, Shanghai, China
The study of visual illusions has a long tradition in vision research. Wade (1998, 2005) quotes examples from antiquity. Vicario (2011) lists many examples of 19th-century psychologists remembered for the illusions that bear their names: Just think of Poggendorf, Ebbinghaus, Schumann, Münsterberg, Ponzo, Delboef, and others. Susana Martinez-Conde and Stephen Macknik, both professors at the State University of New York (Downstate Medical Center), take up the tradition of examining illusions in their most recent book, which should be of interest to general readers and academics alike. Champions of Illusion: The Science Behind Mind-boggling Images and Mystifying Puzzles draws from the vast collection of new and fascinating illusory effects presented during the first decade of the Best Illusion of the Year Contest, which started at the European Conference on Visual Perception in A Coruña (Spain) in 2005. The authors deserve the gratitude of the vision community for having provided an interactive forum for hundreds of enthusiastic vision and perception scientists (in addition to artists and lay persons), who eagerly contribute their most compelling illusion discoveries to the contest, now in its 14th edition.
Champions of Illusion presents a large and brilliantly written selection of outstanding entries on brightness and contrast, color, size, shape, ambiguity, perspective and depth, motion illusions, impossible percepts, in the nonvisual domain, and attention illusions. An appendix on dynamic illusions available on the Best Illusion of the Year Contest website, along with short biographies of the contributors, is also included. The quality of the reproductions is excellent; the capsules describing the visual effects are succinct and scholarly, frequently embedding the illusions in a greater, even historical, context. Chapter introductions guide the reader along. From reading the accompanying legends one can learn a lot not just about illusions but also about sensory psychology and neuroscience in general.
In the “Introduction” section, the authors point out that often one sees something that is not there, or fails to see something that is there, or sees something in a way that does not reflect its physical properties. This is how they define an illusion and in doing so, follow the Gestalt psychologist Wolfgang Metzger (1936/2006), who wrote similarly in “Laws of Seeing.” The authors further point out that we see depth not just from a two-dimensional drawing on canvas but also from the two-dimensional representations on our retinae. This is attributed to the creative works of our brain circuits and referred to as illusory or subjective.
Not every reader may agree on this last point. Subjective depth reflects objective depth and therefore is verifiable as well as universal across the population, so one may consider it as nonillusory. Although it is true that we never experience the real world veridically (Kant’s Ding an sich), we succeed remarkably well in navigating, finding food, escaping from enemies, and finding mates. Despite the limitations of our sense organs and the specific processing of sensory signals in our brain, we are adapted sufficiently well within our environment. Small mismatches between percept and physical reality do not usually matter. For this reason, our perceived world is not altogether illusory, even if it is partially incorrect or incomplete.
The authors propose that illusions are the product of evolution, which raises the question of their potential benefits. Clear adaptive advantages may be found in a small number of illusory percepts, such as Goethe’s colored shadows, Mach’s bands, Hering’s simultaneous contrast, and the Kanizsa illusion. Others may be a by-product or consequence of neural computations gone wrong and not serve a biological purpose. This is borne out by the fact that many known illusions are laboratory artifacts and—unlike camouflage—hardly ever seen in nature; this is why they are so unexpected, intriguing, undeniable, puzzling, and—in the words of the authors—mind-boggling and mystifying. Can we gain a better understanding of the neural mechanisms of perception and cognition from such accidental observations?
The late Whitman Richards (MIT) once told me that illusions are of interest to the vision scientist only if they can be used as a tool for the analysis of the principles and mechanisms underlying neural computation. Ratliff’s (1965) treatment of Mach bands is an excellent example. Illusions that do not lend themselves to this task may be curious and aesthetically pleasing but must be regarded mere oddities of vision. Yet, what may appear an oddity today may turn out to be profound tomorrow (e.g., the Ouchi-Spillmann illusion; Spillmann, 2013). Spurred by the Best Illusion of the Year Contest, there has been a renaissance of illusion research as testified by the appearance of the Oxford Compendium of Visual Illusions (Shapiro & Todorovic, 2017). Commercial entrepreneurs as well as newspapers are also exploiting this field, as seen by the vast number of books on Christopher Tyler’s “magic eye” patterns.
The rules for competition in the Best Illusion of the Year Contest require that illusion entries be innovative, counterintuitive, spectacular, beautiful, and significant to the understanding of the human mind. An implicit requirement would likely have been that they enable correlations with putative neural mechanisms. The Neural Correlate Society, a nonprofit organization established by the authors in 2006, was founded in this spirit. Under its umbrella, researchers, software engineers, mathematicians, magicians, graphic designers, sculptors, and painters rally to invent ever-new phenomena to challenge our imagination and intellect. Almost from the beginning, artists and op-artists have also participated in this endeavor, with pictures, rather than words, providing a common platform (see also Gregory & Gombrich, 1973).
Do illusion discoveries reveal something about the way our brain operates in daily life? The authors believe that apparent neural failures—illusions—are crucial to the understanding of how we perceive the world. Gregory (1998) devised a scheme to categorize illusions into four groups: ambiguities (e.g., Necker cube), distortions (Poggendorf illusion), paradoxes (impossible triangle), and fictions (Kanizsa triangle). In looking at the pictures and reading the accompanying capsules, I have assigned either a plus or a minus to each illusion in the book for novelty, strength, and likelihood of a neuronal explanation.
In the order of appearance, I found the following illusions most striking: The Dress, the Primal flashlight, Weaves and the Hermann grid, Shape-specific afterimages, White’s effect, the Warped Münsterberg illusion, the Illusory pyramid, the Leaning tower illusion, the Enigma illusion, the Rotating tilted lines illusion, the Pinna-Brelstaff illusion, and the Impossible illusory triangle. Many of the dynamic illusions presented in the contest are accessible on the http://illusionoftheyear.com website are equally enjoyable and often even more compelling, such as Impossible motion on magnetic slopes, the Monkey-business illusion, and others. In a few instances, such as the Rotating snakes illusion, or the Spinning disks illusion, I never did see the effect. The Pulsating hearts (in Kitaoka’s rendition) were surprisingly weak for me, so were most of the effects listed under attention. This may be due to my age and tell us something about the interindividual variability of our perceptions (Billino, Hamburger, & Gegenfurtner, 2009).
Future work will show which and how many of the above illusions can be linked to known neurophysiological mechanisms in monkey and human observers. Here, it is important to realize that illusions are not stimuli but the result of stimulus processing. Excellent examples of in-depth studies to clarify the neuronal origin of an illusion can be found in the works of von der Heydt et al. (Kanizsa triangle, Abutting grating illusion), DeWeerd et al. and Sasaki et al. (Filling-in), Conway et al. and Kuriki et al. (Rotating snakes), Pan et al. (Pinna illusion), and others. Single-cell recording, optical imaging, and fMRI were used to pin down the neural correlates of a given phenomenon. This collective work is highly promising and may ultimately convince us that illusions have a reality of their own.
It has been a long road from my first acquaintance with visual illusions in the classical book by Robinson (1972). The exhibits in the San Francisco Exploratorium, the British Museum, the Hands-on Museum in Brighton, Hans Irtel’s and Michael Bach’s interactive illusion website (http://www.michaelbach.de/ot/) and now this scholarly and inspiring book prompt visitors and readers to think of illusory phenomena not just as curiosities, but more importantly as scientific tools for finding neurophysiological correlations.
Champions of Illusion by Martinez-Conde and Macknik belongs on the shelf of every researcher, experimental or theoretical, who seeks to discover and understand the fascinating phenomena of visual perception and the neuronal mechanisms underlying them. Psychologists, artists, neuroscientists, and others will be captivated by this book. It is warmly recommended to students who wish to enhance their knowledge of the magical and unexplained visual world. Curious lay persons will also benefit from this unusually lucid, persuasive, and admirably crafted tome.