"Mine Works Better": Examining the Influence of Embodiment in Virtual Reality on the Sense of Agency During a Binary Motor Imagery Task With a Brain-Computer Interface

Abstract

Motor imagery-based brain-computer interfaces (MI-BCI) have been proposed as a means for stroke rehabilitation, which combined with virtual reality allows for introducing game-based interactions into rehabilitation. However, the control of the MI-BCI may be difficult to obtain and users may face poor performance which frustrates them and potentially affects their motivation to use the technology. Decreases in motivation could be reduced by increasing the users' sense of agency over the system. The aim of this study was to understand whether embodiment (ownership) of a hand depicted in virtual reality can enhance the sense of agency to reduce frustration in an MI-BCI task. Twenty-two healthy participants participated in a within-subject study where their sense of agency was compared in two different embodiment experiences: 1) avatar hand (with body), or 2) abstract blocks. Both representations closed with a similar motion for spatial congruency and popped a balloon as a result. The hand/blocks were controlled through an online MI-BCI. Each condition consisted of 30 trials of MI-activation of the avatar hand/blocks. After each condition a questionnaire probed the participants' sense of agency, ownership, and frustration. Afterwards, a semi-structured interview was performed where the participants elaborated on their ratings. Both conditions supported similar levels of MI-BCI performance. A significant correlation between ownership and agency was observed (r = 0.47, p = 0.001). As intended, the avatar hand yielded much higher ownership than the blocks. When controlling for performance, ownership increased sense of agency. In conclusion, designers of BCI-based rehabilitation applications can draw on anthropomorphic avatars for the visual mapping of the trained limb to improve ownership. While not While not reducing frustration ownership can improve perceived agency given sufficient BCI performance. In future studies the findings should be validated in stroke patients since they may perceive agency and ownership differently than able-bodied users.

Keywords: agency; brain-computer interface (BCI); embodiment; frustration; interaction paradigm; virtual reality.

Figure 1

The different embodiment conditions in the design. In the avatar hand condition a hand held the balloon while two semi-transparent blocks surrounded the balloon in the blocks condition.

Figure 2

The visuals of the balloon that changed color during the five interaction phases in a traffic light style and then to white for the inter-trial period.

Figure 3

The procedure for the experiment started with the Mood questionnaire (Skola and Liarokapis, 2018), and equipping the EEG and VR headset. Afterwards the participants were allocated by order into each level and given a questionnaire after each condition followed by a debrief interview.

Figure 4

Perceived agency by ownership (A) and performance (B), the blue lines represent the single factor linear fit. Categorical responses (ownership and agency) are jittered for better visibility.

Figure 5

Perceived agency depending on performance and ownership by condition.