Basic investigation of velvet hand illusion using a dot matrix for haptic display development

To realize a human-centered society, it is essential to develop advanced display technology that can artificially recreate tactile sensations for users. However, because the mechanoreceptors in human skin are microscopic, it is challenging to stimulate these receptors with currently available actuators. Thus, to cover the limitations of current actuators, we focused on the tactile illusion known as the velvet hand illusion (VHI), in which a sensation of smoothness occurs when people rub two parallel wires between the palms of their hands. In addition, it is known that a smooth sensation similar to the VHI can be induced using a dot matrix display when two lines composed of pins move in an apparent reciprocating motion on the palm. It is interesting that this smooth sensation is created by the uneven stimulation of the pins. To apply this sensation, it is necessary to elucidate the mechanism behind it. Therefore, in this study, we conducted psychophysical experiments and brain function measurements to determine whether this sensation produced by a dot matrix display is the VHI, and to clarify how sensations differ between displays. First, in the psychophysical experiments, we used the semantic differential method and factor analysis to construct tactile dimensions to investigate the characteristics of the VHI sensation. As a result, it was found that the stronger the VHI, the greater the sense of smoothness and softness. This characteristic was also confirmed in the VHI induced by the dot matrix display. Next, we used functional near-infrared spectroscopy to investigate brain activation related to VHI generation. The investigation confirmed that the strength of the illusion is related to the activity level of the hand area in the left primary somatosensory cortex for both the wire display and the dot matrix display.