In this last part we elaborate on the experimental suggestions provided by by Alsmith, proposed in order to investigate more fine-grained forms of perspectival perceptions and their interaction with vestibular processes. In the target article we used the term first-person perspective (mainly in the context of mental perspective taking and out-of-body experiences) to refer to an egocentric visuo-spatial perspective. Alsmith proposes a subdivision of this perspective into three forms of perspectival structures: “origin,” “egocentric frame of reference,” and “focal point of sensory flow (egomotion),” which might be differentially influenced by vestibular signals. While we do not necessarily agree on the importance and justification of these (and exactly these) components, we appreciate the experimental suggestions, on which we will briefly comment below.
A common approach to testing the influence of the vestibular system on high-level cognition is to alter vestibular information during a specific task—for example a perspective-taking task. This can be done either by applying galvanic (Lenggenhager et al. 2008) or caloric (Falconer & Mast 2012) vestibular stimulation, by natural vestibular stimulation (Van Elk & Blanke 2014), by exposing participants to microgravity (Grabherr et al. 2007), by changing the body orientation relative to gravity (Arzy et al. 2006), or by testing patients with vestibular dysfunction (Grabherr et al. 2011). What Alsmith describes[5] in the first two experiments mentioned in the commentary is the opposite approach, namely assessing vestibular processing during specific tasks, or bodily states, respectively.[6] We believe that this is a potentially powerful way to better understand vestibular implication in fine-grained aspects of the bodily self and their interrelation—both in experimental work and research in patients with bodily-self disturbances (see e.g., Brugger & Lenggenhager 2014 for a recent review). We would like, however, to point out a few important issues that should be considered.
Alsmith suggests that we measure time-locked vestibular-evoked potentials without stating more precisely what vestibular stimulation to use. However, this is crucial, since there are various ways to test vestibular processing, mostly by stimulating a specific part of the vestibular system (see e.g., Palla & Lenggenhager 2014 for a recent review). One possibility (in the suggested experiment) could be to use sound-induced vestibular-evoked potentials. The advantage of these is that they can be recorded in a static condition, unlike other forms of vestibular stimulation (e.g., rotatory evoked cortical potentials; Keck 1990), which is important for the suggested full-body illusion paradigms. When designing experiments along these lines, it is indispensable to know what part of the vestibular system is stimulated by the used technique. Sound-induced cortical vestibular potentials, for example, represent cortical processing of otolith signals, mainly from the saccule, thus coding preferentially linear movements in the vertical plane (i.e., up and down movements in a standing position). If we rather expect a difference in coding the front-back movement, as proposed in Experiments 1 and 2, a vestibular stimulation of the utricule might be more appropriate (e.g., Todd et al. 2014, using evoked-potentials by impulsive accelerations). Since testing all different aspects in all the proposed conditions is technically impossible, the specific vestibular stimulation should be carefully chosen based on the hypothesis. Alternatively, more indirect measures could be used to test a vestibular implication, such as changes in posture or stability during various experimentally-induced alterations in the bodily self, e.g., via dynamic posturography using a moving platform, as it is commonly used in clinical settings (e.g., Ghulyan et al. 2005).
In the third proposed experiment, Alsmith considers which (bodily) reference (e.g., eye, head or body centered) is taken as the egocentric reference frame. The fact that there are multiple bodily frames of reference has been nicely shown in a classical task where ambiguous letters (e.g., d/p) are written on the skin. They are typically perceived differently depending on the bodily location on which they are written (Sekiyama 1991); and interestingly the perspective can be modified by vestibular stimulation (Ferrè et al. 2014). Alsmith here suggests that there is a need to investigate the egocentric perspective both with implicit and explicit measures in a situation where body and head[7] are misaligned, as previously done to test spatial cognition (Schindler 1997) and heading direction during passive motion (Ni et al. 2013). This is a very interesting suggestion; however from the experimental description it is not entirely clear how Alsmith thinks that the vestibular contribution should be investigated. Furthermore, his hypothesis only concerns the respective contribution of head and torso position, but not its vestibular contribution. He suggests that participants might receive galvanic vestibular stimulation or tendon vibration stimulation to investigate “the relative contribution of vestibular processes to egocentric perspective.” One way to test this could be to align the participant’s head and torso, but use tendon vibration or galvanic vestibular stimulation in order to induce an illusory tilt or turn the participant’s head, thus inducing an illusory misalignment of the head and body. By doing the suggested task in such a condition, vestibular or proprioceptive contribution could be isolated. While this is theoretically very interesting, there might be practical difficulties: vestibular and proprioceptive illusions are usually susceptible to huge individual differences, and inducing illusory shift of ±15% could be difficult. Furthermore, in the proposed experiment that misaligns body and head around the yaw axis, gravitational cues do not differ between the position of the torso and the head in the misaligned condition. Adapting the experiment to a lying-down position,[8] where body and head would be at different angles with respect to gravity, could help investigating the otolithic influence on perspective.