What is embodiment? A psychometric approach
Introduction
What is it like to have a body? The sense of one’s own body, variously termed “embodiment” (Arzy, Overney, Landis, & Blanke, 2006), “coenaesthesia” (Critchley, 1953), “bodily self-consciousness” (Bermúdez, 1998, Legrand, 2006), or “corporeal awareness” (Berlucchi and Aglioti, 1997, Critchley, 1979), has often been described as a non-conceptual, somatic, form of knowledge, different in kind from other types of knowledge (e.g., Kant, 1781/2003; Bermúdez). In addition, many authors have suggested embodiment is a necessary prerequisite for other types of sensation and knowledge (Kant, 2003, Johnson, 1987, Lakoff, 1987, Merleau-Ponty, 1962, Piaget, 1954). On that view, embodiment would be the cornerstone of mental life, the “storm-center” of experience as James (1905) put it. The sense of one’s own body is also intimately related to the sense of self, and is often taken as the starting point of individual psychological identity (Cassam, 1997, Edelman, 2004). However, recognition of the importance of embodiment has not been matched by theoretical clarity about what embodiment is or involves. Neurological and neuropsychological investigations have generally provided a framework for embodiment by proposing dissociations between different subcomponents of body representation, such as body image and body schema (e.g., Gallagher & Cole, 1995; see also Head & Holmes, 1911/1912). Use of these terms, however, has been plagued by confusion, disagreement, and inconsistent usage (cf. Gallagher, 2005, Poeck and Orgass, 1971). This confusion arises in part because the sense of embodiment is both rich and complex on the one hand, and elusive and hard to describe on the other (Gallagher, 2005, Haggard and Wolpert, 2005).
The phenomenological tradition, has provided rich descriptive characterizations of embodiment, and has used it as a starting point for theories of the self (e.g., Merleau-Ponty, 1945/1962). However, it has not offered the operational working definitions and measures needed for rigorous empirical research. What is needed is a more systematic, and principled approach to decomposing the bodily self. Such a project should have two aims. First, it should produce theoretically useful and clearly dissociable subcomponents of embodiment. Second, it should generate testable predictions about human experience which can be directly measured. The present study provides an initial step towards these goals, by applying psychometric methods to structured introspective reports of a conscious experience of embodiment. If embodiment is a coherent psychological construct, rigorous measurement and analysis should clarify what it is, and what its subcomponents are.
Embodiment is clearly a kind of experience, but psychology’s traditional methods of studying experience have difficulty in capturing its nature. On the one hand, the introspectionist approach seems unsuitable because one’s body so often forms the background of mental life rather than the foreground. In addition, the verbal labels that people most readily use when describing the body enumerate the different physical parts of the body, but not the experience that those parts jointly constitute the self (de Vignemont, Tsakiris, & Haggard, 2006). The objective methods of psychophysics successfully capture the occurrence and magnitude of a single identifiable experience or quale (e.g., whether a stimulus is red or green in colour), but do not easily capture more complex experiences such as the sense of one’s own body.
An ideal experimental approach to embodiment would involve comparing one condition in which a participant has a body, and another in which they do not. But such ‘brain in a vat’ experiments are confined to philosophy (Putnam, 1982), because the body is “always there” (James, 1890). Nevertheless, it is possible to manipulate the perceived incorporation of an external object into the representation of the body. In the so-called rubber hand illusion, for example, a prosthetic hand brushed synchronously with a participant’s own hand is perceived as actually being part of the participant’s own body (Botvinick and Cohen, 1998, Tsakiris and Haggard, 2005). The same visual and tactile stimulation delivered asynchronously has a quite different phenomenology. The rubber hand illusion provides one of the few means of manipulating embodiment, and has been so used in a number of recent studies (Armel and Ramachandran, 2003, Austen et al., 2004, Costantini and Haggard, 2007, Durgin et al., 2007, Ehrsson et al., 2005, Ehrsson et al., 2004, Ehrsson et al., 2007, Farnè et al., 2000, Holmes et al., 2006, Kanayama et al., 2007, Pavani et al., 2000, Press et al., 2008, Rorden et al., 1999, Schaefer et al., 2006, Tsakiris et al., 2007a, Tsakiris et al., 2006, Walton and Spence, 2004). However, most studies simply report the occurrence of the illusion, or a behavioural or neural proxy of it such as a change in the perceived position of the participant’s own hand, without systematic description or quantitative measurement of the changed sense of embodiment.
In this study, we investigate the structure of embodiment by taking a psychometric approach to introspective reports of the rubber hand illusion. Participants observed a rubber hand that was stroked either synchronously or asynchronously with their own hand and then made proprioceptive judgments of the location of their own hand and used a Likert scale to rate their agreement or disagreement with 27 statements relating to their subjective experience of the illusion. We used a classic factor analytic approach, based on principal components analysis (PCA), to investigate the latent structure of participants’ experience, and to quantify the complex experience of embodiment.
Section snippets
Participants
One hundred and thirty one current and prospective students (75 female) at University College London participated with local ethical approval. All but six were right handed, as assessed by the Edinburgh Inventory (Oldfield, 1971), M: 71.62, range: −90.91 to 100. Participants were recruited at open days offered by the University and volunteered to participate. There were no restrictions on participation.
Apparatus and materials
Participants sat at a table across from the experimenter, with their stimulated hand placed
Results
The mean and standard deviation for the raw item scores in each condition are given in Supplementary Table 1.
Discussion
To our knowledge, the present findings represent the first systematic attempt to measure embodiment. By combining an experimental manipulation of the experience of one’s own body, and a structured psychometric approach to measuring that experience, we were able to characterize what sort of experience embodiment is, and decompose it into sub-aspects. These results suggest that psychometric methods can be useful tools in elucidating structure underlying complex conscious experience. Specifically,
Acknowledgments
This research was supported by Biotechnology and Biological Sciences Research Council (BBSRC) Grant BB/D009529/1. Additional funding was provided by the Bial Foundation. We wish to thank Eva Baugh, Mirandola Gonzaga, and Jason Musil for assistance with testing, Salvatore Aglioti, Mark Haggard, and two anonymous reviewers for helpful comments and/or discussion.
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