[1]
Jennifer Windt (2010) suggested, based on dream research, an even more basic form of minimal phenomenal selfhood, which she defined as a “sense of immersion or of (unstable) location in a spatiotemporal frame of reference”, thus not needing a global full-body representation (see also Metzinger 2013, 2014 for an interesting discussion of this view). We believe that for this more basic sense of a self especially, the vestibular system should be of importance, as a vestibular signal unambiguously tells us that our self was moving (i.e., change in self-location and perspective) without an actual sensation from the body (i.e., a specific body location as it is the case in touch, proprioception, or pain).
[2]
It is interesting to note for the frame of this chapter that these authors describe the importance of the detection of coherence of all self-motion specific information (including the vestibular system), despite the fact that their experimental setup involved only proprioceptive and visual information (leg movements in a sitting position).
[3]
This component is in such context usually termed self-location, but a more accurate formulation is “body part location with respect to the self” (Blanke & Metzinger 2009; Lenggenhager et al. 2007).
[4]
Proprioception classically refers to information about the position of body segments originating from muscle spindles, articular receptors, and Golgi tendon organs, while interoception refers to information originating from internal organs such as the heart, gastrointestinal tract, and bladder.
[5]
The sense of agency has not yet been investigated using neuroimaging studies in the context of the rubber hand illusion.
[6]
While these experiments are targeting illusory full-body ownership, it has recently been criticized (Smith 2010; see also Metzinger 2013) that it has not empirically been shown that it really affects the full body (as opposed to just certain body parts). We agree that this argument is justified and that further experiments are needed to address this issue (see also Lenggenhager et al. 2009).
[7]
Similarly to the rubber hand illusion, changes in self-location and self-identification have been associated with physiological changes such as increased pain thresholds, decreased electrodermal response to pain (Romano et al. 2014), and decreased body temperature (Salomon et al. 2013).
[8]
As we will see below, the neural signal provided by the peripheral vestibular organs does not allow us to distinguish whether the self is (active motion) or is not (passive motion) the agent of the action. Therefore, peripheral vestibular signals are ambiguous regarding the sense of agency. Yet, comparisons with motor efference copy in several vestibular neural structures allow such distinction and provide a sense of agency.
[9]
Olfactory processing in the thalamus seems also to be different from processing of the main senses as there is no direct relay between sensory neurons and primary cortex, and olfactory thalamic nuclei have been identified only recently (Courtiol & Wilson 2014).
[10]
Caloric and galvanic vestibular stimulations are the two most common techniques to artificially (i.e., without any head or full-body movements) stimulate the vestibular receptors. Caloric vestibular stimulation was developed by Robert Bárány and consists of irrigating the auditory canal with warm (e.g., 45°C) or cold (e.g., 20°C) water (or air), creating convective movements of the endolymphatic fluid mainly in the horizontal semicircular canals. This stimulation evokes a vestibular signal close to that produced during head rotations. Galvanic vestibular stimulation consists of the application of a transcutaneous electrical current through electrodes placed on the skin over the mastoid processes (i.e., behind the ears). Galvanic vestibular stimulation is often applied binaurally, with the anode fixed behind one ear, and the cathode on the opposite side. The cathodal current increases the firing rate in the ipsilateral vestibular afferents.
[11]
A recent study investigating pain thresholds during the rubber hand illusion did not show any change in pain threshold or perception (Mohan et al. 2012), suggesting that pain perception is linked more to global aspects of the bodily self, e.g., self-location.
[12]
Although the coding of movements by the peripheral vestibular organs is ambiguous regarding the sense of agency, the coding is not ambiguous regarding the sense of ownership for the movements and self-other distinction. Indeed, because vestibular sensors are inertial sensors, vestibular signals are necessarily related to one’s own motion and are the basis of the perception that I have (been) moved, irrespective of whether the “self” is or is not the agent of this movement.
[13]
Depending on the stimulation parameters and method, participants describe various sensations of movements and change in position.
[14]
Visuo-spatial perspective-taking has not only been used in the field of spatial cognition but also in the field of social cognition. Perspective taking is a very crucial aspect of human cognition, which allows us to understand other people’s actions and emotions. The fact that the vestibular system is importantly involved in such simulations might further suggest that the vestibular system is important for social cognition (see also section 5 and Deroualle & Lopez 2014).
[15]
The research on bodily illusions has recently extended to social neuroscience by investigating how sensorimotor self-other confusion (during the rubber hand, full-body, and enfacement illusions) affects the perception of another person and, vice versa, how the perception of another person influences illusory self-other confusion (e.g., Bufalari et al. 2014; Paladino et al. 2010; Tajadura-Jiménez et al. 2012).