Who is pulling the strings? The self-avatar follower effect in the context of involuntary movements in immersive and embodied virtual reality.

In conditions of virtual immersion, the relationship between the user and their avatar can be bidirectional: while the user usually controls the avatar, the avatar can also, under certain circumstances, influence the user's movements, a phenomenon known as the Self-Avatar Follower Effect (SAFE). Previous research has demonstrated this effect in voluntary actions, particularly where there is a mismatch between the avatar and the user. However, it remains unclear whether the SAFE extends to involuntary movements, such as those elicited by the Kohnstamm phenomenon (an involuntary arm elevation following sustained isometric contraction). Here, we tested whether the SAFE modulates the Kohnstamm effect by immersing 24 healthy young adults in a virtual environment with a humanoid avatar that either (1) faithfully replicated their involuntary arm movements or (2) moved at a predefined slow or fast speed. Kinematic analysis revealed that the velocity of the Kohnstamm phenomenon, which occurs shortly after the cessation of sustained muscle contraction, was higher when the avatar's movements were fast compared to when they were slow. Similarly, the time required to reach the maximal amplitude was shorter when the avatar's movements were fast than when they were slow or closely mirrored the participant's movements. Crucially, since the Kohnstamm effect lacks a predefined motor goal, these adjustments cannot be attributed to an intentional imperative. Instead, they suggest an automatic sensorimotor adjustment aimed at reducing the avatar-user mismatch and maintaining bodily coherence.