During the last years, various theories from psychological, neuroscientific, philosophical, and interdisciplinary perspectives have claimed the importance of multisensory signals and neural body representations for general theories of self-consciousness. Influential theories stated that very basic, and largely implicit and pre-reflective bodily processes crucially underlie the self (Alsmith 2012; e.g., Blanke & Metzinger 2009; Blanke 2012; Gallagher 2005; Legrand 2007). Such theories fueled experimental investigations on multisensory integration and its influence on various aspects of the self. Yet, similarly to Aristotle, who claimed that “there is no sixth sense in addition to the five enumerated—sight, hearing, smell, taste and touch”—this line of research has largely neglected the vestibular sense of balance. This is particularly surprising as a recent theory has claimed the importance of more global aspects of the bodily self (Blanke & Metzinger 2009), most importantly probably the sense of immersion or location in a spatiotemporal frame of reference (Windt 2010). This process, as we speculated above, should fundamentally rely on vestibular cues, plausibly among others coded by specific cells in the hippocampus. The vestibular system is activated by gravity, the constant force under which we have evolved, and also during all sorts of passive and active head and whole body movements. Moving in an environment is necessary for the development of a sense of bodily self, and the vestibular system is thus likely to contribute not only to the most basic (or minimal) aspects of the self but also to the different fine-graded implicit and explicit aspects of the experience of our bodily self in daily life such as body perception, body ownership, agency, and self-other distinction. It is thus not surprising that the vestibular system is intrinsically, highly linked to other sensory systems such as touch, pain, interoception, and proprioception. While some of the links between the vestibular system and the bodily self are rather well-established and the underlying neurophysiological processes known from both non-human animal and human research, several of the relations presented here are still largely speculative. Yet, we believe that the specific and testable hypotheses we have given here—once they are tested and possibly confirmed by experimental studies—might enable us to better describe neural and physiological mechanisms underlying minimal phenomenal selfhood (Blanke & Metzinger 2009) as well as refine current models of the multisensory mechanisms underlying the various aspects of the bodily self.
Acknowledgements
We thank Gianluca Macauda for his help with figures 1 and 2, as well as Dr. Jane Aspell for proofreading and her valuable comments. BL was funded by the Swiss National Science Foundation (grant #142601). CL is supported by the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7/2007-2013) under REA grant agreement number 333607 (“BODILYSELF, vestibular and multisensory investigations of bodily self-consciousness”).