Ultrasound Haptics For XR and Rehabilitation: Influence of Arm and Hand Muscle Stiffness on User Perception

Background: Ultrasound phased arrays are being increasingly used in extended reality (XR) since they enable contactless haptic feedback generation, inducing tactile sensation on a user's hand. Objective: Studies have been conducted to evaluate the tactile detection threshold when using ultrasound, with the hand usually positioned on an armrest or a foam plastic, so that the palm is facing downwards in a fixed position. Moreover, typical applications of ultrasound haptics always show the hand palm suspended in air and facing downwards. Methods: This paper presents a preliminary proof of concept by conducting a psychophysical experiment on three subjects, to analyze the influence of arm and hand stiffness on tactile perception when the palm is facing upwards. Two conditions are examined, i.e., with the arm and palm placed on an armrest and suspended in air, respectively. For each condition, two experiments are designed to determine the absolute detection threshold, i.e., starting with highest and lowest pressure stimuli, respectively. Results: Higher sensitivity is obtained when the experiment started with lower pressure values. Moreover, when the hand is in a resting position, the detection threshold of different participants presents low variance (standard deviation lower than 75 Pa, three times lower than suspended position). Conclusions: This preliminary study paves the way for further studies about the effect of muscle stiffness and palm orientation on tactile perception, laying the foundation for potential applications in virtual reality and in rehabilitation.