Within psychology and cognitive neuroscience, the notion of “sense of self” is used more and more often (see fig. 1), although it is rarely explicitly defined. However, the influential distinction between the “narrative self” and the “minimal/core/embodied self” helps fleshing out a reductionist account of the sense of self. The narrative self can be defined as the open-ended construction of one’s own personal identity shaped by social and cultural factors (Gallagher, 2000; Zahavi, 2010), and is associated with high-level cognitive functions such as self-related attitudes/beliefs and autobiographic memory retrieval. The minimal/core/embodied self refers instead to the basic experience of “being a self” rooted in bodily and multisensory processes (Christoff, Cosmelli, Legrand, & Thompson, 2011; Damasio, 1999; Legrand & Ruby, 2009).
Fig. 1 Occurrences of “sense of self” on Pubmed per year
In recent years, empirical data from clinical cases and experimental paradigms have motivated the hypothesis that low-level multisensory processes, rather than high-level cognitive processes (such as thinking about oneself), underlie the elusive “sense of self” supposedly found in normal conscious experience. Multisensory integration refers the way in which the processing of stimuli from one sensory modality is sensitive to the information provided by stimuli from another. Information coming from several modalities can thus be integrated to improve perception and solve crossmodal conflicts (Spence & Driver, 2004). Multisensory integration of body-related stimuli has been recently investigated with bodily illusions induced by experimental paradigms, such as rubber hand illusion (Botvinick & Cohen, 1998) and full-body illusions using head-mounted displays (Ehrsson, 2007; Ionta et al., 2011; Lenggenhager, Mouthon, & Blanke, 2009; Lenggenhager, Tadi, Metzinger, & Blanke, 2007). Along with autoscopic phenomena of neurological origin, full-body illusion induced in experimental settings have been able to dissociate three self-referential components of ordinary conscious experience, namely self-identification to a body or body ownership (“what is my body?”), self-location in space (“where am I?”) and the experienced direction of the visuospatial perspective (“from where do I experience the world?”) (Blanke & Metzinger, 2009). There is good evidence that these components rely on the multisensory integration of spatiotemporally congruent exteroceptive (mainly visual), somatosensory (tactile and proprioceptive), interoceptive and vestibular signals (Blanke, 2012; Blanke, Slater, & Serino, 2015).
These experimental findings have paved the way for a reductionist account of the “sense of self” (assimilated to the “minimal/core/embodied self”). According to such an account, multisensory integration of self-related signals across visual, auditory, somatosensory, vestibular and interoceptive domains would underlie the elusive feeling of “being a self” in normal conscious experience. At the personal level, this “sense of self” may be constituted by several more primitive aspects, such as the sense of body ownership, the sense of agency, spatiotemporal self-location and the experienced origin and direction of the visuospatial perspective. Specifically, there in an ongoing discussion in philosophy and psychology about the contribution made by body ownership to the ordinary sense of self (de Vignemont, forthcoming; Tsakiris, 2017).
Some recent publications have also developed a computational model of the sense of self within a predictive coding framework (Apps & Tsakiris, 2014; Carhart-Harris & Friston, 2010; Limanowski & Blankenburg, 2013). According to the predictive coding hypothesis, the brain generates predictive hierarchical models of the world that are constantly updated on the basis of prediction error signals generated by unexpected sensory information. In a predictive model of perception, bottom-up sensory error signals are explained away by top-down processes to minimize surprise in multimodal areas further up the hierarchy. It has been suggested that the sense of self ultimately results from the integration of congruent predictions regarding self-related and self-generated (endogenous) multisensory stimuli in higher-level supramodal regions (Apps & Tsakiris, 2014). In other words, if the brain predicts the worldly causes of incoming sensory inputs, it also has to predict the organism’s own contribution to those inputs; thus an approximate Bayesian model of the world must include a self-model, since this is most probable explanation of congruent sensorimotor stimuli (Hohwy & Michael, forthcoming; Moutoussis, Fearon, El-Deredy, Dolan, & Friston, 2014). In the terms of Friston’s free energy principle, the hypothesis is that in normal conditions, the model which has the lowest amount of free energy is one in which the subject itself is predicted to be the “hidden cause” of congruent somatosensory and homeostatic signals (Limanowski & Blankenburg, 2013). It is worth noting that the role of interoceptive signaling in the maintenance of a sense of self has been recently emphasized and described within a predictive coding framework (Seth, 2013). Thus, in normal conditions, the sense of self might result from the generation of a supramodal model of the single cause of congruent multisensory inputs which are “most likely to be me” across exteroceptive, somatosensory, interoceptive and vestibular domains (Apps & Tsakiris, 2014).
Current questions related to this avenue of research include the following:
What roles do body ownership and agency have (if any) in the maintenance of a sense of self in ordinary conscious experience?
What roles do spatiotemporal self-location and the experienced direction of the visuospatial perspective have (if any) in the maintenance of a sense of self in ordinary conscious experience? How do spatiotemporal self-location and the experienced direction of the visuospatial perspective come apart?
What is the relationship between multisensory processes underlying a “self-model” at the subpersonal level, and the putative sense of self in conscious experience (at the personal level)?
Is there a mental/cognitive component in the sense of self which supposedly pervades ordinary conscious experience, or is it strictly constituted by multisensory and bodily components?