Third-person perspective full-body illusions (3PP-FBI) enable the manipulation, through multisensory stimulation, of perceived self-location. Perceived self-location is classically measured by a locomotion task. Yet, as locomotion modulates various sensory signals, we developed in immersive virtual reality a measure of self-location without locomotion. Tactile stimulation was applied on the back of twentyfive participants and displayed synchronously or asynchronously on an avatar's back seen from behind. Participants completed the locomotion task and a novel mental imagery task, in which they self-located in relation to a virtual ball approaching them. Participants self-identified with the avatar more during synchronous than asynchronous visuo-tactile stimulation in both tasks. This was accentuated for the mental imagery task, showing a larger self-relocation toward the avatar, together with higher reports of presence, bi-location and disembodiment in the synchronous condition only for the mental imagery task. In conclusion, the results suggest that avoiding multisensory updating during walking, and using a perceptual rather than a motor task, can improve measures of illusory self-location. open Scientific RepoRtS | (2020) 10:6802 | https://doi.org/10.1038/s41598-020-63643-y www.nature.com/scientificreports www.nature.com/scientificreports/ An often-reported limitation of the LT is that locomotion updates somatosensory, vestibular and interoceptive signals. Measures of self-location based on perceptual judgements rather than action-based judgements could minimize self-motion, reduce confounding sensory stimulation, and maintain illusory self-location. There is evidence to suggest that action-based judgments are less sensitive to visual illusions, as they rely on different neural pathways, than more perceptual judgments 37-39 . To measure self-location in non-moving participants, a "mental ball dropping task" has been used during 3PP-FBIs using video presentations of a human body 16,17,19 . Participants tested lying supine held a ball and had to indicate, with button presses, when they imagined dropping the ball and when the ball would hit the ground. After synchronous visuo-tactile stimulation, participants' evaluation of the time needed for the ball to reach the ground increased or decreased with regard to their illusion of being located above or below their body. Similar results were reported for a mental ball dropping task in VR 40 . However, the mental ball dropping task needs to be adapted to a more ecological 3PP-FBI in which participants are tested standing upright. Body positioning is important as lying supine modifies corporeal and extracorporeal space perception 41 . To our knowledge, there is still a lack of a measure of self-location which does not require changes in perceived self-location with respect to gravity, but rather with respect to the more ecological front-back or left-right axes. Second, participants exposed to VR are thought to distribute their self between the real and VR environments 33...
