1 Introduction
Embodiment science is the field inside virtual reality (VR) research that studies and attempts to understand the effects of self-avatars on its users (Spanlang et al., 2014). Embodied avatars are defined to be avatars that are co-located with the user’s body and seen from a first person perspective within an immersive virtual environment (VE) (Kilteni et al., 2012).
Research in this field has shown evidence of the importance of being embodied in the self-avatar. Beyond the obvious needs of being virtually represented to interact with others in social VR setups, being embodied has been shown to increase users cognitive abilities (Steed et al., 2016), improve haptic performance (Maselli et al., 2016; Gonzalez-Franco and Berger, 2019) or increase self recognition and identification through enfacement (Gonzalez-Franco et al., 2020b). However, the cognitive load impacts of self-avatars are not well understood and may affect results (Peck et al., 2018; Peck and Tutar, 2020). Lush et al. Lush et al. (2020) has raised concerns that the illusion may be a response to imaginative suggestion and is caused by suggestion. Regardless, being embodied in an avatar can dramatically change a user’s experience in VR, including reducing biases, such as racial bias (Peck et al., 2013), mitigating stereotype threat (Peck et al., 2018; Peck et al., 2020a), responding to a domestic violence scenario (Gonzalez-Liencres et al., 2020), or affecting how users move and act inside VR (Kilteni et al., 2013; Gonzalez-Franco et al., 2020a).
A standardized measurement of embodiment is needed to be able to compare and replicate experiments across embodiment science. Further, participants are unique and can have significantly different experiences and responses in the same VR setup. For example, avatars have been shown to enhance distance estimation (Ries et al., 2008; Ebrahimi et al., 2018), object size estimation (Jung et al., 2018; Ogawa et al., 2019), and the level of embodiment in an avatar may further affect distance perception (Gonzalez-Franco et al., 2019). A standardized embodiment questionnaire that is sensitive enough to detect individual embodiment differences could aid researchers in better understanding and interpreting the effects of virtual embodiment. Research using self-avatars should measure embodiment on every experiment to rule out and understand intrinsic variables that might be affecting results.
There are challenges to measuring embodiment due to the relation with our own bodies not being something we normally think about. Questions such as “I felt out of my body”, can be unrelatable to the many people who have neither experienced nor heard about an autoscopic phenomena (Blanke and Mohr, 2005). There have been attempts to measure embodiment to bridge the gap between the user’s physiological experience and reporting of the experience. The most advanced measures include electrophysiological recordings to find quantitative methods (González Franco, 2014; Alchalabi et al., 2019). Previous research has shown that highly embodied participants responded with stronger 400N amplitudes in the parietal cortex when they lost agency over their bodies (Padrao et al., 2016; Pavone et al., 2016), or had stronger P300 responses when their virtual avatar was threatened (González-Franco et al., 2014). In both experiments researchers found correlations between these numerical and physiological responses and a series of embodiment questions. This supports that subjective questionnaires are less cumbersome and yet still a valid form of embodiment evaluation. However, questions need to account for the challenge of asking someone about something they are not able to quantify. A similar challenge was introduced by Slater when addressing why questionnaires cannot fully assess presence in VEs, (Slater, 2004).
With that aim in mind a new questionnaire was introduced by (Gonzalez-Franco and Peck, 2018) “Avatar Embodiment. Toward a Standardized Questionnaire.” In this questionnaire the authors proposed 25 questions for an embodiment questionnaire, as collected from the most used embodiment questions in the literature. Starting with the original rubber hand illusion introduced by Botvinick and Cohen (Botvinick and Cohen, 1998) the authors analyzed up to 30 other experiments. The new questionnaire not only included sufficient control questions but also categorized the main questions into six different recurrent themes in embodiment science (Kilteni et al., 2012, 2015; Maselli and Slater, 2013, Maselli and Slater, 2014) such as: 1) body ownership, 2) agency and motor control, 3) tactile sensations, 4) location of the body, 5) external appearance and 6) response to external stimuli. The paper proposed a calculation for a final embodiment score that could be achieved arithmetically or through an open sourced Principal Component Analysis (PCA).
However, given the nature of the questionnaire based only on a review of previously used questions, the authors carefully titled the paper as an ongoing effort Toward a standardized questionnaire, and highlighted that there would be a need to further validate the questions by the community.
Our analysis converged into a reduced questionnaire that proposes 16 questions of the original 25, and four sub-scales instead of the original six. The revised questionnaire is compared to the original on n = 101 participants, the results of which are used in the discussion of this paper to further understand the revised embodiment questionnaire. We also discuss potential reasons why some commonly used questions rendered irrelevant, as well as highlight the importance of using a common questionnaire across the field.
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