Visual feedback from a virtual body modulates motor illusion induced by tendon vibration.

Frequency-specific tendon vibration (TV) elicits illusory kinesthetic sensations around the vibrated body parts. Studies indicate that vision plays a fundamental role in modulating such illusions. In our current study, we used immersive virtual reality (IVR) to investigate the role of body-related visual feedback in modulating illusory sensation of movement in the left arm. Thirty healthy participants were asked to evaluate the onset of motor illusion and four illusion-related features (vividness, duration, extension and aftereffect), in the presence and absence of real and virtual visual feedback. Additionally, subjective reports of the embodiment illusion (the sense of embodying a virtual surrogate) were collected in virtual conditions. Results showed a progressive decrease in the perception of the motor illusion along a continuum ranging from the absence of visual feedback (maximal illusory perception) to the observation of one's own real arm (minimal illusory perception). Interestingly, the appearance of the virtual limbs affected the movement illusion differently. Specifically, TV evoked a stronger kinesthetic illusion when observing the virtual hand detached from the limb than during the observation of the virtual full limb and virtual object. This suggests that a closer visual resemblance between the virtual and real limb results in a greater effect on proprioceptive processing. However, no significant correlation was found between the illusion of arm movement and the illusion of embodiment, indicating that the two phenomena may not be directly related. These findings provide new insight into the role of body-related visual feedback in modulating motor illusions.