Phenomenological analysis has emphasized the paradox role of the body in our experience (Legrand, 2010) as both an “objective” body that is a physical thing and thus part of the world, and a “subjective” or “lived” body that is our means of experiencing and interacting with the world (this can be traced back to Husserl, Merleau-Ponty, and arguably Sartre, see Gallagher, 1986; Gallagher and Zahavi, 2010). Crucially, during interaction with the world, we usually do not experience the lived body as a thing enabling this interaction, and it may be this very “experiential absence” of the body that gives us the feeling of “being there” in the world (Gallagher, 1986; Metzinger, 2004). Such a phenomenological distinction may be of particular relevance for BOIs, which isolate and manipulate, and direct attention to perceptual features of oneself such as visual appearance or physical location. In William James' words, BOIs may interfere with the “me” (i.e., the features that one ascribes to oneself) but not the “I” (the subject of experience) (James, 1890; Christoff et al., 2011). Thus, the paradox role of the body may be part of what makes the interpretation of BOIs and their relation to MPS so difficult: When we speak of body “ownership” in these paradigms, we may only refer to self-identification with the perceived body.

Although MPS can be broken down into distinct features (e.g., self-identification, self-location in space and time, or a first-person perspective), it is still one phenomenal property that “does not have proper parts that could themselves count as a kind or type of self-consciousness” (Metzinger, 2013a, 2004). MPS thus acknowledges the “global and unitary character of self-consciousness” (Blanke and Metzinger, 2009). This unified character of MPS—characterized by the fact that we cannot introspectively access its underlying mechanisms and components—is what lends our experience its phenomenal “realness” (Metzinger, 2004, 2013a). As we have recently argued (Limanowski and Blankenburg, 2013), this MPS conceptualization is elegantly compatible with a theory of cortical information processing based on predictive coding (Friston, 2010): The free energy principle (FEP) postulates that the brain implements a hierarchical generative model of the world (including the organism itself) that constantly predicts the causes of its sensory input, and that may be updated if its predictions fail. Due to the model's constant bidirectional and hierarchical information flow with the aim of reducing overall prediction error, this model is spatially, temporally, and phenomenally centered onto the organism itself (Hohwy, 2007, 2013; Friston, 2011; Limanowski and Blankenburg, 2013; Apps and Tsakiris, 2014). The FEP account, albeit mechanistic in nature, thus acknowledges that one's self-representation is plastic (i.e., probabilistic), hierarchical, and yet unified. It builds upon existing neuropsychological models of body ownership (e.g., Makin et al., 2008; Tsakiris, 2010), and shares key assumptions about being a self with the MPS conceptualization—most importantly, that the phenomenal self is a result of probabilistic self-modeling.

Thereby the MPS and FEP accounts both emphasize that such probabilistic self-modeling is a “risky business” for the phenomenal self: When the current generative model is abandoned in favor of another model that fits the sensory data better, the agent in its present form—encoded as the model evidence of the self-centered world-model—ceases to exist (Friston, 2011). Luckily, this does not need to happen: If prediction error can be explained away at lower levels, there is no need to adjust higher-level predictions, let alone to abandon the current world-model. In the case of BOIs this means that if the probabilistic representation of one's physical features can be updated to eliminate the surprise originating from the ambiguous sensory input, there is no need to abandon the actual self-model (Hohwy, 2013; Limanowski and Blankenburg, 2013; Apps and Tsakiris, 2014). Such an updating of self-representations is demonstrated, for example, by the increased perceived similarity of the “owned” dummy hand (Longo et al., 2009), and even of another face (Tsakiris, 2008; Apps et al., 2013) to oneself, or by the fact that the mere expectation of an upcoming touch on the dummy hand may evoke a BOI (Ferri et al., 2013).

In this light, BOIs can be conceived of as targeting specific inferential mechanism employed by the organism, thus directly confirming the functional architecture suggested by probabilistic models of MPS—but this also opens up some questions. It seems that BOIs primarily affect mechanisms operating at lower levels of self-modeling, because the induced conflict can readily be resolved without inducing panic or pathological conditions, and because the recalibration of one modality onto another relies on relatively basic multisensory mechanisms (Tsakiris, 2010; Blanke, 2012; Gentile et al., 2013). On the other hand, the adjustment of physiological responses (Ehrsson et al., 2007; Moseley et al., 2008; Barnsley et al., 2011) might imply that BOIs also affect higher levels of the self-model (Seth, 2013). To understand the implications of BOIs for MPS, it is crucial to identify which levels of MPS they affect: When do these illusions merely alter perceptual features of oneself, and when—if at all—do they in fact affect MPS per se?

Metzinger (2013a,b) has recently clarified the MPS conceptualization by introducing the term phenomenal unit of identification (UI), “the phenomenal property with which we currently identify, exactly the form of currently active conscious content that generates the subjective experience of ‘I am this”’ (2013b). As I take it, the UI concept can be used in two ways: It specifies the content with which the self-model identifies—this self-identification leads to the experience of MPS (Metzinger, 2004). Formally, we may think of the UI as the evidence or the “origin” of the current generative model (the “region of maximal invariance,” Metzinger, 2013a; see also Friston, 2011; Limanowski and Blankenburg, 2013).

The important point is that the UI concept may help to answer our question: BOIs do affect MPS, just not in its entirety. The resolution of BOI-evoked conflicts by changing the representation of certain features of oneself, despite attacking fundamental conditions of MPS, may not be sufficient to change the UI. Even the extension of BOIs to the fully body—although surely a step in the right direction—does not necessarily imply a change in the UI: For example, since FBIs seem to employ the same multisensory mechanisms as the RHI (Petkova et al., 2011; Maselli and Slater, 2013), it could be that what becomes subject to the illusion is actually not the whole body, but just the stimulated part of the body (e.g., the torso in the FBI, Smith, 2010; Metzinger, 2013a; see Tsakiris et al., 2006, for such evidence). More importantly, FBIs may also manipulate only individual features of one's self-representation such as one's visual appearance or physical location.

Interestingly, however, Metzinger (2013a,b) proposes that the UI may change during mind wandering episodes, bodiless dreams, and some forms of out-of-body experiences. The resulting claim that an explicit body representation may actually not be required for MPS (Metzinger, ibd.) is in line with the proposal that a core self is present even when disconnected from exteroceptive sensory input (e.g., during sleep, Park and Tallon-Baudry, 2014). Such proposals are inspiring, but they also show that much work still needs to be done to understand the exact relation between the body, MPS, and its UI. Probabilistic models following the FEP offer a promising way of formally describing the underlying self-modeling mechanisms, but in addition, new experimental approaches addressing specific levels of MPS are needed. To avoid confusion, these approaches should acknowledge the dual role of the body as both subject and object of experience.

BOIs have proven an important tool for understanding MPS. But one should keep in mind that BOIs manipulate individual features of one's self-representation, and that this manipulation of certain dimensions of MPS not necessarily affects the UI. Nevertheless, if situated within a phenomenologically inspired probabilistic model, BOIs can contribute to our understanding of MPS by clarifying its hierarchy, and to further developing the UI concept and its relation to the body. Whether we can develop paradigms that manipulate the UI to actually create the illusion of being someone else is a different question, but there are promising new developments that encourage to pursue it.