What can body ownership illusions tell us about minimal phenomenal selfhood?

Abstract

Illusions have become an invaluable tool for investigating how the sense of a body as one's own is constructed and maintained: During the rubber hand illusion (RHI, Botvinick and Cohen, 1998), congruent touch to one's hidden hand and a fake counterpart produces an illusion of feeling the touch on the fake hand and, more strikingly, an experience of the fake hand as part of one's body (Ehrsson et al., 2004; Tsakiris, 2010). The principles of the RHI paradigm have been extended to various body parts (Petkova et al., 2011), including even the face (Tsakiris, 2008; Apps et al., 2013); most notably, the RHI has been induced for the entire body (full body illusion, FBI), producing similar behavioral and neural responses (anxiety responses, ownership of the fake body, misperception of one's physical location; Ehrsson, 2007; Lenggenhager et al., 2007; Maselli and Slater, 2013). Such body ownership illusions (BOIs) have generated a substantial amount of research and invaluable insights into the mechanisms of body ownership (Tsakiris, 2010; Blanke, 2012; Moseley et al., 2012). The special importance of these illusions lies in the fact that what they manipulate—the sense of having a body—is one of the enabling conditions of minimal phenomenal selfhood (MPS, Gallagher, 2000; Blanke and Metzinger, 2009; Metzinger, 2013a). MPS is defined as the most basic possible kind of self-consciousness or self-awareness (Blanke and Metzinger, 2009; Gallagher and Zahavi, 2010), and investigating its enabling conditions may help us understand what it takes for an organism to have the experience of being a self. Nevertheless, in this paper I will argue that it is still unclear what exactly the mechanisms revealed by BOIs tell us about MPS, and that this needs to be clarified via a joint effort of phenomenological analysis and formal accounts of self-modeling. BOIs rest on the induction of some crossmodal conflict (e.g., touch seen on a fake hand but felt on one's real hand), which violates the predictions of one's body-model about the unity of one's body (Hohwy, 2007). This conflict is resolved by the brain by remapping modality-specific body part-centered reference frames onto each other (e.g., proprioception onto vision), so that the multimodal representation of the body and the space surrounding it remains coherent (Holmes and Spence, 2004; Makin et al., 2008; Tsakiris, 2010; Blanke, 2012). Thereby the spatio-temporal and anatomical constraints of BOIs (touch needs to occur simultaneously at corresponding locations and on a congruent body part in an anatomically plausible posture) suggest that multisensory input has to be compatible with a prior representation of the body (Tsakiris and Haggard, 2005; De Preester and Tsakiris, 2009; Tsakiris, 2010; Moseley et al., 2012). The brain seems to make a probabilistic either-or decision based on current sensory input under a prior body model, which during BOIs results in the replacement of the real body part by the “owned” fake body part (Longo et al., 2008; Moseley et al., 2012). And indeed, the neural mechanisms integrating multisensory information during the RHI may be similarly employed for one's real body parts (Gentile et al., 2013). When the rubber hand is threatened or injured, participants show behavioral, and neural anxiety responses similar as for one's real body part (Ehrsson et al., 2007). BOIs may even affect the regulation of one's physiological states: During illusory ownership, one's real limb's temperature may be downregulated (Moseley et al., 2008), and even the immune system may decrease “protection” of the own limb (Barnsley et al., 2011; Costantini, 2014). In sum, there is compelling evidence that BOIs interfere with the representation of one's body. Upon closer inspection, however, the fact that BOIs isolate “the various components that converge in the holistic experience of our bodies” (Maselli and Slater, 2013) may be a fundamental limitation when it comes to relating them to MPS.