The ‘chopstick’ illusion: A simply demonstrated tactile illusion

This paper introduces and analyzes a simply demonstrated tactile illusion. The tactile illusion described here is named the “chopstick” illusion since it can be easily demonstrated using a pair of wooden chopsticks. This illusion is demonstrated by moving one´s fingers up a broken-off wooden chopstick with the fingertips of the thumb and index finger. As two chopsticks are separated, an angled edge is often created at the chopstick head as one tears away a piece of the other. This edge has a gradual constant increase in the surface dimension. A tactile illusion is experienced by moving the midpoints of the fingertips of the thumb and index finger along the flat portion of the chopstick. The observer of the illusion feels as if the thickness of the material at the edge of the chopstick increases in the form of a bump, but in reality does not and the stick is only getting wider. To investigate this illusion, two experiments were conducted: One with commercially available broken-off wooden chopsticks and one with fabricated plastic chopsticks. The first experiment showed that the chopstick illusion was perceived 72% of the time, while the second experiment showed a 63% perception rate. It was also found that the chopstick groove, which initially separates the two chopsticks played a key role in perceiving this tactile illusion.