Perception, 2008, volume 37, pages 1305 ^ 1307

doi:10.1068/p6016

LAST BUT NOT LEAST

Two new illusions of the tongue Marc Egeth

The Children's Hospital of Philadelphia, Department of Radiology/Neuroradiology, 3400 Civic Center Blvd, Philadelphia, PA 19104, USA e-mail: [email protected] Received 11 December 2007, in revised form 16 January 2008

Abstract. New illusions resulting from failures in proprioception are described. The Forked Tongue illusion results from weaving the tongue among the prongs of a fork, or from using the prongs of a fork to distort skin on other parts of the bodyöin these cases, the prongs feel misaligned even though the fork has not bent. The Tongue Twister illusion results from turning the tongue upside down and probing it with a finger or foreign objectöthe object can be felt to be in the wrong spatial location, opposite to where it actually is on the tongue.

Here I present two new illusions best experienced through self-manipulation of the tongue. Though neither of these illusions are necessarily tongue-specific, the consistency of the tongue happens to make for an excellent substrate. Both illusions illustrate a failure of proprioception. Forked Tongue illusion Intertwine the top of your tongue among the prongs of a fork (see figure 1). You will obtain a strong impression that the one or two middle prongs are misaligned and vertically separated. (If you use a plastic fork, of course, the prongs might actually move. Use a sturdy metal fork that you are sure will not bendöyou won't be disappointed!) Everybody I have asked to try this, usually at dinner parties, has experienced this illusion (upwards of fifteen people öfifteen very obliging people). [Note: while the tongue is the most conducive body part on which to experience this illusion, this effect can also be felt on other parts of the body, for example, on a fingerpad, when the prongs are used in a `weaving' motion to gently shift the skin so that the prongs touch three points that are not usually aligned.]

Figure 1. A tongue `woven' around the prongs of a fork.

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Tongue Twister illusion If you can turn your tongue upside down (figure 2), do so, and run your finger along the top and bottom of your tongue.(1) Alternatively, have someone else run the tip of a dining utensil along your tongue. When a foreign object is touching your tongue on the right side of space, it might feel `to your tongue' like it is on the left side of space. Your finger might feel like it is touching a numb part of your tongue while your tongue registers sensation on the side opposite to where your finger is touching. Similarly, swirling an object clockwise around your inverted tongue may result in a sensation of counterclockwise swirling.

Figure 2. An upside-down tongue.

In an e-mail to, again, very obliging friends, family, and colleagues, I asked them to turn their tongues upside down, touch their tongues, and describe any unusual sensations they experienced. Of the approximately forty people I asked to try this, only four people reported being able to turn their tongues upside down. One respondent reported experiencing a spatial reversal effect while touching his tongue with his finger, feeling his own finger in a location opposite to its actual placement. Later, when shown the above description of the Tongue Twister illusion, he agreed that the description captures his experience (2). Two respondents reported experiencing spatial confusion when using a utensil, but neither of them got the same effect with his/her own fingers. For me, the illusion is very robust, with a clear `contradiction' between the touch felt by my finger and the touch felt by my tongue. Discussion Perhaps any body part that can be stretched, twisted, or prodded in a way described above could produce the illusory experiences of the Forked Tongue and Tongue Twister illusions. Any such body part could potentially lack the mechanisms for appropriate proprioceptive feedback. Both of these illusions might fall under Hayward's (2008) taxonomy of tactile illusions: section 2.2, Mislocalization, or section 2.10, Shape from (1) The

following three steps from http://www.wikihow.com/Roll-Your-Tongue-(Upside-Down) explain how to turn your tongue upside-down: 1. Turn the tongue onto its side by placing it side-to-side between the teeth (so the taste buds face the left cheek). 2. Lift your bottom jaw slowly and press lightly with your top teeth. 3. Watch as the tongue looks like it's upside down with the taste buds at the bottom of the inside of your mouth! (2) After the current paper was accepted, but before it went to press, I found a reference to the upside-down tongue effect in Richard Powers' (1991) novel The Gold Bug Variations.

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Figure 3. Interlaced fingers with crossed arms.

Distributed Cutaneous Deformation. However, the current illusions have a different focus than those in Hayward's categories. As presented, Mislocalization includes only vibration-induced errors, while Shape from Distributed Cutaneous Deformation does not capture the essence of the Forked Tongue illusion, as cutaneous deformation per se is not essential to this illusion (the deformation of the tongue rather than just its surface being crucial here). There is also a similarity between the current illusions and Aristotle's illusion, in which a single object touched by two crossed fingers feels like two objects. (Hayward classifies Aristotle's illusion in section 2.1, Object Perceptual Disjunction and Conjunction, because of the percept-doubling effect of the crossed fingers.) There is also a similarity between the Tongue Twister illusion and the mirror reversal confusions engendered through the following examples: intertwining one's fingers and arms and then trying to move a particular finger (see figure 3), trying to identify a reversible letter (`p' versus `q') that is drawn on the front versus back of the head, and trying to draw while looking in a mirror. All of these examples highlight the fact that we are not perfect at representing ourselves during certain types of perceptual reversals. Using Hayward's categories, Mislocalization, or, more specifically, describing a new category, Mislocalization from Bodily Deformation, might provide us with a good context for both the Tongue Twister and Forked Tongue illusions. References Hayward V, 2008 ``A brief taxonomy of tactile illusions and demonstrations that can be done in a hardware store'' Brain Research Bulletin (special issue on Robotics and Neuroscience) 75 742 ^ 752 Powers R, 1991 The Gold Bug Variations (New York: William Morrow)

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