This paper verifies the ability of human subjects with no audiovisual clues to trace their hands along an invisible vibrotactile line in three-dimensional space created by an ultrasound Bessel beam. A narrow, long, and stationary Bessel beam that passes through a target position is generated. The beam produces midair vibrotactile stimuli on the subjects' hands. The subjects are required to perceive the beam location and direction actively to trace the presented linear trajectory. With our method, no real-time hand tracking is necessary, which guarantees no latency in presenting the vibrotactile stimuli. We experimentally verified that the subjects were able to trace the beam over 50 cm in its stretching direction with their hands. The average deviation from the beam center was less than 6 cm. Unlike conventional wearable-based motion guidance, the proposed technique requires no devices to be worn by the users in practical situations.
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