Table 1 Common subjective and objective measures used for various embodiment illusions
Measures | Details |
|---|---|
Rubber hand illusion | |
Embodiment questionnaires [subjective] | Consist of between 2 and 10 items scored with Likert scales (7 or 10-points). These questions assess embodiment experiences including the perceived location, agency (control), and ownership over the hand Higher scores indicate a stronger experience of the embodiment illusion |
Hand size estimation task [objective] [27] | For the rubber hand and their own hand, participants estimate (i) the width of the wrist, (ii) the width and the length of the hand. This is done before and after the RHI induction. For each size estimation, a calliper is used to let participants indicate when the hand fits exactly in between the two pointers of the calliper. This is done while the experimenter is moving the two pointers (1) away from each other and (2) towards each other along the back of the RHI set-up The estimation made after the RHI is subtracted from the one made before it; positive values indicate an increased size estimation after the RHI induction (i.e., greater embodiment) |
Proprioceptive drift [objective] | Participants indicate the location of their finger/hand before and after the RHI induction. For example, participants say ‘stop’ as soon as they think a vertical metal bar installed at the back of the RHI set-up matches the location of the middle part of their own finger [27]. During the task, their hands and the rubber hand are occluded from view Proprioceptive drift is calculated as the difference between the estimated location of the participants own finger/hand before and after the RHI. A positive value, or a larger bias in proprioceptive judgment towards the rubber hand is interpreted as the participant experiencing a larger visual dominance of the rubber hand over proprioception of their own unseen hand (i.e., greater embodiment) |
Reaching task [objective] [31] | Participants view the upper half of a computer screen upon which a white target line appears in one of 15 locations after each RHI induction. Each line is equally spaced between the visible rubber hand and the participant’s own occluded hand, both of which are placed beneath the screen. The participants are instructed to use their own unseen hand to make an immediate response towards the target location that appears on the screen by touching the screen collinear with the target line below the viewing field ‘Reach endpoint errors’ are scored by calculating the difference between their movement endpoints and actual target locations (i.e., greater embodiment) |
Full body illusion | |
Body size estimation [objective] | Participants are instructed to place two adhesive markers on the wall to estimate the width of their body (parts) before and after the FBI induction. Each marker represents the left and right side of the body (parts). Participants also estimate their circumference by using a piece of string/cotton rope Decrease in body (part) size overestimation indicates a stronger experience of the FBI |
Embodiment questionnaires [subjective] | Consist of between 6 and 20 items scored using a Likert scale (between 5 and 10). These questions assess embodiment experiences including the perceived location, agency (control), and ownership over the targeted body(part/s) Higher scores indicate a stronger experience of an embodiment illusion |
Visual analogue scales | Participants estimate the intensity of the illusion from 0 to 100 on a visual analogue scale. Items range from 1 to 9 (although with the latter these items formed 3 subscales—ownership, agency, and referred touch—which were each scored separately, rather than as a total) Higher scores indicate a stronger experience of the FBI |
Enfacement illusion | |
Enfacement questionnaires [subjective] | Consist of between 9 and 13 items scored with either Likert scales (between 7 and 10). The questions assess enfacement experiences including perceived agency (control) and ownership over the other’s (real or virtual) face Higher scores indicate a stronger experience of the enfacement illusion |
Including Other in the Self scale [subjective] [23] | A single-item scale where self and other are represented by seven Venn-diagram-like pairs of circles. Participants choose the overlap that they think best represents the level to which the avatar’s face looks like their own Higher Venn values show a higher perceived self-other integration (i.e., higher enfacement) |
Self-other discrimination morphing task [objective] [21] | Participants are shown morphing sequences (e.g., videos) which typically begin with the enfacement model’s face, and then gradually changes to the participant’s own face. The task involves participants to indicate when they feel that the face looks more like their own face rather than the model’s face. This task is usually done at baseline (before any enfacement procedure) and after experiencing synchronous or asynchronous enfacement Comparisons of participant’s choices between each condition with baseline indicates how much enfacement has occurred (i.e., they choose an image for self that contains higher levels of the model’s face compared to baseline), with larger changes indicating more enfacement |
Self-face recognition morphing task [objective] [22] | Participants view a series of 9 static images that gradually transition from the enfacement model’s face to the participant’s face; each image has a different level of morphing between the participant’s and the model’s face (e.g., 80% participant vs 20% avatar). Participants are asked to indicate whether the image is the avatar’s or their own face by a key-press Comparisons of participant’s choices between each condition with baseline indicates how much enfacement has occurred (i.e., they choose an image for self that contains higher levels of the model’s face compared to baseline), with larger changes indicating more enfacement |