Research reportA brief taxonomy of tactile illusions and demonstrations that can be done in a hardware store
Introduction
Perceptual illusions have furnished considerable material for study and amusement, probably much before Aristotle discussed them. Tactile and haptic illusions are frequently discussed in terms of similitude with visual counterparts, if one excludes, of course, those resulting from cross-modal interactions. An example of the parallels that are often drawn between tactile and visual illusions is found in the comments of Rivers regarding the “reverse Aristotle illusion” [67]. This illusion is when people feel only one surface (sometimes three) when two surfaces are in contact with the outer sides of the crossed fingers. In the classic Aristotle illusion most people feel two objects when only one is touched with crossed fingers (see Fig. 1). Rivers regarded this as an analog to stereopsis, to use a modern terminology. A further example, more recently discussed by Benedetti is diplesthesia that was viewed as a tactile equivalent to diplopia, or double vision [5]. Pushing gently on the corner of one eye causes diplopia, while pressing two fingers touching the same object can cause a doubling of the sensation, or diplesthesia. The analogy was thus motivated by the observation that in the two cases the perceptual disjunction results from external mechanical perturbation.
Generally, however, far fewer tactile illusions than visual or even auditory illusions have been described. A likely contributing factor to the relatively small number of known tactile illusions is not that there are few of them, but that many visual illusions can be created out of simple materials such as pencil and paper, or even simply by looking at a natural scene under the right conditions. Moreover, with the development of computers and of the Internet, uncountable websites provide a huge repository of visual illusions which are easily accessible. To a lesser extent, this is also true of auditory illusions. But for touch, there are very few that are easily accessible with the exception of an amusing version of what could be said to be a case of diplesthesia occurring when holding a pen between the lips while pulling the mouth corners diagonally [56]. To demonstrate and study tactile illusions, one is often required to set up equipment that can create the proper conditions, and rarely do they arise naturally in an obvious manner.
A first aim of this paper is to describe a taxonomy of tactile illusions, concluding that certain are quite specifically tactile and do not have clear visual analogs. As we will proceed in listing these known tactile illusions, we will indicate the ease with which they can be demonstrated. In a second part of this article, the means for producing some of these effects with commonly available supplies will be described. For more systematic studies, the generation of computer-controlled stimuli can be accomplished with special purpose, or robotic-like electromechanical equipment.
Several definitions exist for what is an illusion. These definitions can include or exclude different types of phenomena. An extreme view is that all perceptual processes are illusions to some extent. Another view is that an illusion is a wrong perception. What constitutes a wrong perception is nevertheless a debatable notion. The idea of illusions being when the senses are deceived is not satisfactory, even if illusions can be employed for deception, as in camouflage. Different stimuli can produce the same percept. For example, in color perception it is known that different spectra can elicit the same color [14]. Thus, a color percept is always wrong given a class of equivalent stimuli, yet it is not an illusion. On the other hand, equivalent stimuli form a rich source of illusions.
In the view of the author, an illusion is a percept arising from a specific stimulus delivered under specific conditions that gives a different conscious experience when the conditions are changed. In other words, the information available to the perceiver should be separable into a constant part and a variable part, and the percept produced by the constant part should be contingent on the variable part which can include endogenous neural states. In addition, the change should be surprising, unexpected, even amusing when the perceiver becomes aware of it. Consider for example the “moon illusion” [41]. The moon disk creates the same retinal image whether it is near the horizon or up in the sky, yet its perceptual size changes. With the “Necker cube illusion”, prolonged viewing can produce perceptual switches even when the viewing conditions do not vary [44]. In the former case, the visual context modifies the perception of the same disk. In the latter case, the conditions refer to endogenous neural states since the whole of the visual input is essentially invariant.
There are many motivations for the study of illusions in general, and of tactile illusions in particular. Some of these motivations are practical. Illusions are at the basis of virtually all technological displays, from cinematography, to computer screens, to audio reproduction systems. This is also true of haptic interfaces. More generally, they provide important clues regarding the processes at all levels that allow the brain to perceive and to be conscious. They have theoretical value since they suggest methods to test models, but also have clinical applications to detect sensory dysfunction.
Section snippets
A brief taxonomy
We now attempt to catalog tactile illusions as they are reported in the literature. In the interest of brevity, were excluded those illusions related to the awareness of one's own body, such as phantom limb phenomena or limb position-sense effects. Those could be the topic of a separate survey. Here we collect phenomena related to the perception of the world outside the body. Table 1 can be consulted for a summary.
Demonstrations of modest complexity
Some of the cases we have surveyed (see Table 1), can readily be demonstrated with ready-made household items such as salad bowls or chopsticks. Some require specific electromechanical equipment. In this section we look at a third category that can be demonstrated with moderately complex equipment realizable with supplies found in hardware stores.
These illusions are astonishing, robust, easy to demonstrate almost immediately, and in each case can be created in more elaborate and controlled
Discussion
Demonstrations of illusions are valuable because many people can experience them, not just a few subjects. They are portable and communicable. The spontaneous comments of many observers may sometimes be illuminating. But of course, demonstrations do not replace systematic scientific investigations where the stimuli and the conditions under which they are delivered are rigorously controlled.
It is usually a significant electromechanical challenge to design machines that can deliver, under
Acknowledgments
This research was supported by a discovery grant from the Natural Sciences and Engineering Council of Canada. The author would like to thank McGill University for a sabbatical leave. Edouard Gentaz, Jim Clark, Anatole Lécuyer, Hanifa Dostmohamed, Maarten Wijntjes, Mounia Ziat, Masashi Nakaniti, Vincent Levesque, Gabriel Robles-De-La-Torre, C. Elaine Chapman, and Allan M. Smith provided helpful comments.
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