The use of beyond-real interactions (BRI) expands the possibilities of virtual reality (VR) experiences by enabling novel capabilities, such as extending the range of manipulation or employing unusual body configurations. However, such interactions require users to adapt to novel sensorimotor mappings, which can lead to decreased performance and a poorer quality of user experience. Recently, proprioceptive suppression with noisy tendon electrical stimulation (n-TES) has been proposed to promote adaptation to BRI, though its effects have been reported to be limited. This study investigated the effects of proprioceptive suppression via n-TES on the adaptation process to BRI and the consequent changes in body schema, which have remained unexplored in prior research. Using a between-subjects design with 24 participants, we examined the effects of n-TES on a reaching task with Go-Go interaction, which extends virtual hands to enable users to reach farther than physically possible. Body schema changes were measured using a pointing task without visual feedback of the hand after adaptation to the Go-Go interaction. The results revealed that n-TES significantly altered the adaptation process by facilitating early learning and also enhanced body schema changes and aftereffects of adaptation under certain conditions. However, parameter fitting of the learning curves also suggested that n-TES might lead to a deterioration in the final level of performance achieved through adaptation. Our findings provide initial evidence that proprioceptive suppression by n-TES can contribute to facilitating sensorimotor adaptation within VR applications, while its practical utility may require adjusting stimulation to prevent potential side effects in the later stages of adaptation, specifically in trajectory smoothness.

Virtual reality (VR) enables beyond-real interactions (BRI) that transcend physical constraints, offering effective user experiences like extending a hand to grasp distant objects. However, adapting to novel mappings of BRI often reduces performance and the sense of embodiment. To address this, we propose using noisy tendon electrical stimulation (n-TES) to decrease proprioceptive precision. Previous studies have suggested that attenuating proprioceptive precision is crucial for sensory-motor adaptations. Thus, we hypothesize that n-TES, which has been shown to reduce proprioceptive precision and induce visual-dependent perception in VR, can enhance user adaptation to BRI. We conducted a user study using go-go interaction, a BRI technique for interacting with distant objects, to assess the effects of n-TES. Given the individual variability in n-TES response, participants first underwent a proprioceptive precision test to determine the optimal stimulation intensity to lower the proprioceptive precision from 5 levels $(\sigma=0.25-125\text{mA})$. Reaching tasks using a 2x2 within-participants design evaluated the effects of go-go interaction and n-TES on performance, subjective task load, and embodiment. Results from 24 participants showed that go-go interaction increased reaching time and task load while decreasing the sense of embodiment. Contrary to our hypothesis, n-TES did not significantly mitigate most of these negative effects of go-go interaction, except that perceived agency was higher with n-TES during go-go interaction. The limited effectiveness of n-TES may be due to participants' habituation or sensory adaptation during the tasks. Future research should consider the adaptation process to BRI and investigate different BRI scenarios.

To increase the flexibility of haptic feedback in virtual reality (VR), hand redirection (HR) has been proposed to shift the hand’s virtual position from its actual position. To expand the range of HR applications, a method to broaden the detection threshold (DT), which is the maximum amount of shift that can be applied without the user noticing, is required. Multisensory integration studies have revealed that the reliability of senses affects the weight of integration. To expand the DTs of HR, we propose a method to increase visual dominance in the integration of vision and proprioception by introducing noise to the latter, thereby decreasing its reliability through weak Gaussian white noise electrical stimulation ($\sigma$=0.5mA). The results of a user study comprising 22 participants (11 women and 11 men) confirm that noisy electrical stimulation significantly expands the DTs of HR with the mean range of DTs ($R_{DT}$) was 20.48° (SD =7.90) with electrical stimulation and 19.15° (SD =7.11) without electrical stimulation. Interestingly, this effect was only observed in women. The average $R_{DT}$ for men was 15.36° (SD =6.13) and 15.18°(SD=5.58), whereas that for women was 25. 61°(SD=5.89) and 23.12°(SD=6.21), with and without electrical stimulation, respectively. Electrical stimulation was mostly tolerable for the participants and did not affect embodiment or presence ratings. These results suggest that expansion of the DT without disturbing the user’s VR experience is feasible.