Creating an Illusion of Movement between the Hands Using Mid-Air Touch.

Apparent tactile motion has been shown to occur on many contiguous parts of the body, such as the fingers, forearms, and back. More recently, the illusion has also been elicited on non-contiguous parts of the body, such as between one hand and the other, either when the hands are interconnected or not interconnected by an object (e.g., when holding a tablet or not). Here, we explore the reproducibility of the intermanual tactile illusion of movement between two free hands by employing mid-air tactile stimulation. We investigate the optimal parameters to generate a continuous and smooth motion using two arrays of ultrasound speakers and two stimulation techniques (i.e., static versus dynamic focal point). In the first experiment, we investigate the occurrence of the illusion when using a static focal point, and we define a perceptive model. In the second experiment, we examine the illusion using a dynamic focal point, defining a second perceptive model. Finally, we discuss the differences between the two techniques.