Investigation of conditions generating velvet hand illusion toward tactile displays

We have investigated several characteristics of one kind of tactile illusion, called the Velvet Hand Illusion (VHI), to utilize the experimental results to generate virtual feeling of a material. In VHI, a human subject gently rubs his/her hands on both sides of a wire grid strung through a frame. The sensation produced on his/her hands is very smooth and slippery, like velvet. We focused on the VHI mechanism for new tactile displays in the virtual reality field because such tactile illusions play a useful role in deceiving the brain so that operators believe a virtual sensation is real. VHI characteristics are obtained from accomplishing a series of psychophysical experiments using Thurstone's method of paired comparison. In the experiment, the stroke movement distance of wires, r, is varied under constant wire spacing, D; the velocity of wire movement is varied, with both wire spacing and stroke movement distance of the wires held constant. It is found that the strongest VHI was obtained at r/D and that the strongest VHI occurred at a specific velocity generating tangential vibration of around 50 Hz. Since VHI requires both compressive stress and tangential stimulus, it is caused by not just one of the four varieties of mechanoreceptive units, but two or more of them. This finding confirms that VHI does not occur in the mechanoreceptive units themselves but in the brain.