Introduction to the special issue on visual illusions

Our sensory systems are useful biological mechanisms that provide us with indispensable and usually highly reliable information about our immediate environment and the state of our own body. However, in some cases we experience sensory illusions, when the deliverances of our senses fail to agree with physical measurements. Such phenomena have been known since antiquity and have always aroused considerable interest and surprise. Sensory illusions do not appear in a random, haphazard manner, nor only to a select set of individuals. Rather, it is ‘normal’ to have illusions, and they have regular, lawful features. Therefore the study of illusions can provide important insights into the functioning of our perceptual mechanisms. Facts and theories about illusions are potentially relevant for a diverse set of disciplines, ranging from industrial psychology and design to philosophical issues about the knowledge of the external world. Their continuing general popularity is attested by numerous books and web pages devoted to illusions. The scientific study of illusions was part of experimental psychology ever since its inception by Wundt in the 19th century and they are mentioned in most every introductory textbook of general psychology. However, despite thousands of research papers and a great amount of accumulated knowledge, many illusions still have not received a generally accepted explanation, which shows that our knowledge of sensory processes is still far from complete. This Special Issue on visual illusions is a small part of a continuing effort to cast more light on these intriguing phenomena. Two of the papers report studies of perception of direction of gaze in portraits. Faces are perhaps the most frequent visual stimuli we encounter, and