Editorial: The Future of Perceptual Illusions: From Phenomenology to Neuroscience

We have accepted a total of 42 articles for this special issue, which together have earned 80,000 views as of June 2, 2016, showing the popularity of illusions as a subject for both authors and readers. The papers address conceptual and perceptual illusions, typically using psychophysical, computational, or phenonomological methods, or some mixture of these. Narrowly defined, an illusion is a mismatch between the physical and the phenomenal world, and this definition encompasses the great majority of the articles. Somewhat more broadly defined, though, illusory perceptions may be based on ambiguous stimulus information, which may or may not count as mismatches. Examples are the famous duck/rabbit image in which perceptual selection of a part (and hence a " negative hallucination " —a destruction of the rest) occurs, rather than misperception of the whole; or in displays in which lightness, brightness, and illumination are potentially confounded, as considered by Rudd; or when local and global motions are intertwined, as in the paper by Erlikhman et al. The papers fall into 5 rather broad categories: models and theory, illusions of geometry and position, illusions of motion, illusions of brightness and color, and multisensory and other illusions. We list them by author in this order to facilitate comparison on like themes, but here we wish to emphasize the broader point that the authors find that the study of illusions can be useful in most, perhaps all, of vision science. Indeed, taken together, the papers support what we claim to be the fundamental principle of perceptual science, namely, that illusions (especially of the mismatch variety) illustrate basic processes in perception; they should not be disregarded as unecological, accidental, or due to random failures of processing. The papers by Kingdom, and by Zeman et al., illustrate this point; Allard and Faubert specifically rely on basic visual mechanisms (not accidental effects) to explain particular illusions. We do have two caveats; first, as pointed out by Gibson, the emphasis on " what is inside the head " may ignore what is really in the world; and second, an over-reliance on the fundamental principle may limit research to variations on known process–illusion relations, restricting exploration of new phenomena—who could have predicted, for example, the now well-known rubber-hand illusion (c.f. Limanowski's study)? Nevertheless, the current papers, and the vast stream of background studies that they refer to, make a good case for the fundamental principle. A consequence is that …