Combining Tendon Vibration and Visual Stimulation Enhances Kinesthetic Illusions

In recent years, human augmentation has attracted much attention. One type of human augmentation, motion augmentation makes perceived motion larger than in reality, and it can be used for a variety of applications such as rehabilitation of motor functions of stroke patients and a more realistic experience in virtual reality (VR) such as redirected walking (RDW). However, as augmented motion becomes larger than the real motion, a variety of senses that accompany will be more inconsistent with those perceived from somatic sensations, which will cause a severe sense of discomfort. To address the problem, we focus on kinesthetic illusions that are psychological phenomena where a person feels as if his or her own body is moving. Kinesthetic illusions are expected to fill the gap between the intended augmented motion and perceived physical motion. However, it has not been explored if and how large kinesthetic illusions are produced while a user is moving their limbs voluntarily in VR. To expand the knowledge on kinesthetic illusions, we have conducted two user studies on the impact of tendon vibration and visual stimuli on kinesthetic illusions. First experiment confirmed that the perceived elbow angle becomes larger than the actual angle when presented with tendon vibration. Second experiment revealed that the increase of the perceived elbow angle was about 20 degrees when both tendon vibration and visual stimuli were presented whereas it was about 10 degrees when only visual stimuli were presented. Through these experiments, it has been confirmed that combining tendon vibration and visual stimulation enhances kinesthetic illusions.