Here, we describe virtual-hand and virtual-face illusion paradigms that can be used to study body-related self-perception/-representation. They have already been used in various studies to demonstrate that, under specific conditions, a virtual hand or face can be incorporated into one's body representation, suggesting that body representations are rather flexible.
The overall goal of this procedure is to induce the illusion of embodiment of a virtual-hand or an avatar. This method can help answering questions in the field of cogno-psychology, such as to reveal the mechanisms underlying the construction of the mental self. The advantage of this method is that it facilitates the manipulation of factors that are likely to influence perceived body ownership or perceived agency over virtual effectors or other body parts.
Note that we demonstrate only the generic parts of the procedure. Other conditions can be found in the published paper. Generally, individuals might struggle with this technique due to the high need for equipment and programming skills.
Therefore, we recommend those people to use a less technical version of our study, such as the rubber hand illusion. Begin by counterbalancing and running the experimental conditions, synchrony versus asynchrony between real hand movements and virtual hand movements. Configure the system to a specific time delay or virtual object based on the condition.
Connect the cables of the data glove and orientation tracker to the computer, and start the virtual reality programming environment by clicking the run button in the virtual reality environment interface to start the pre-written command script. Then, escort the participant into the testing room and ask them to put the data glove on his or her right hand and the orientation tracker on the right wrist. Next, seat the participant in front of the desk where the box with the computer screen on top is placed.
Ask the participant to put his or her right hand into the box along the depth axis to shield it from their view. Then, put a cape over the participant's right shoulder and cover the space between screen and participant. Have the participant rest his or her left hand on an empty part of the desk.
Monitor the participant during the experiment to ensure they follow the instructions shown on the computer screen. Ask the participant to freely move or rotate their real right hand, including opening and closing, rotating, and moving each finger individually. Finally, have the participant watch the corresponding movements of the virtual hand on the computer screen.
Note in this synchronous condition, the delay between the real hand's movements and the virtual hand's movements is close to zero. While in asynchronous condition, the delay between movements is three seconds. Begin by counterbalancing and running two experimental conditions with differing time delays between the real and virtual events, synchrony and asynchrony.
Then, acquire a head position tracking system with the corresponding hardware and software, and a three degree of freedom, or DOF, orientation tracker attached to the top of a hat. Connect the position tracking system and orientation tracker to the computer. Click the run button to execute the pre-written command script in the computer interface to start the virtual reality environment.
Next, ask the participant to sit on the chair two meters in front of the computer screen. Have the participant put on the cap with the attached orientation tracker. finally, monitor the participant during the experiment to ensure they follow the instructions shown on the computer screen.
Note in the synchronous condition, the delay between the participant's real head movements and the avatar's head is close to zero. While in the asynchronous condition, the delay between the real and avatar movements is three seconds. In this experiment investigating how people represent their hands, perceived ownership and agency were stronger if the real and virtual hands moved in synchrony, if the virtual effector was a hand opposed to a rectangle, and if the participant was active rather than passive.
Both ownership and agency showed a significant interaction between activity and synchrony, but not between appearance and synchrony. Furthermore, in the study investigating how people represent their face, results indicated that synchrony between one's own facial movements and the movements of the virtual face increased perceived ownership and agency. Interactions between synchrony and expression are driven by more positive going mood, and particularly, good flexibility in performance for the combination of synchrony and happy virtual face.
Once mastered, this technique can be completed in about 15 minutes. Following this procedure, the virtual effector can be replaced by any other static or dynamic event, so to investigate the limitations of perceived body ownership and agency. After this video, you should have a good understanding of how to induce the illusion of embodying an avator or virtual event.
