In much recent philosophical and neuroscientific research on self-consciousness, the experienced first-person perspective is treated as a simple phenomenon identified with the experienced origin of an egocentric frame of reference centred upon an individual’s own body (Blanke & Metzinger 2009; Vogeley & Fink 2003). But egocentric perspective, despite being an apparently simple phenomenon, is in fact as potentially complex as the macroscopic structure of the body itself (Smith 2010). Human bodies are composed of a number of parts that are to some degree independently mobile, any of which may serve to centre a distinct egocentric frame of reference. As this observation is well known, we may presume that theorists who treat egocentric perspective as simple are assuming that locations in these various egocentric frames of reference are translated into a single, ultimate egocentric frame reference which itself determines egocentric perspectival phenomena.
However, neurophysiological and neuropsychological research on spatial representation suggests independent motivation for this ultimate frame being centred upon the head (e.g., Avillac et al. 2005) or the torso (e.g., Karnath et al. 1991). By rotating head and torso in opposite directions, an egocentric frame of reference centred upon the head can be misaligned with another frame centred upon the torso. In such a “misalignment” situation, a single object may be “to the right” with respect to the head and “to the left” with respect to the torso (Longo & Alsmith 2013). Following Christopher Peacocke’s (1992) description of the phenomenology of experienced direction, one would hypothesise that differences in experienced posture would determine differences in egocentric perspectival experience.[7] One could thus use misalignment situations to determine the respective contributions of the head and the torso to the organisation of egocentric perspectival experience at a given point in time in the following experiment:
Experiment 3: Standing with their head and torso aligned or misaligned ±15°, participants perform a task that involves either an explicit or only an implicit egocentric perspective (see below). The angular deviation of the stimulus in relation to the head and/or torso is recorded, such that one would be able assess the respective contributions of each body-part's orientation to the participants egocentric perspectival judgments. Participants would receive either galvanic vestibular stimulation (GVS) or tendon vibration stimulation to precisely assess the relative contribution of vestibular processes to egocentric perspective.
In more detail, the suggestions are these. For an explicit task, stimuli could be presented across the entire visual field in regular intervals, varying in distance and elevation, and participants would judge whether a stimulus presented looks “to their left or to their right”. A potential limitation of the explicit task is that in using overt left/right judgements, participants’ responses may reflect a stipulated meaning of these terms that is independent of the egocentric perspectival structure of their experience. However, a recent study using a covert attentional cuing paradigm found that rotation of the torso primes participants to respond more quickly to visual stimuli appearing on the side of a computer screen congruent to the direction of rotation (Grubb & Reed 2002).[8] One could adapt this paradigm to directly compare the respective influences of head and torso by rotating the head and/or the torso ±15° relative to the screen where stimuli would be presented. Target and cueing visual stimuli would appear on either congruent or incongruent sides of the screen and participants would make speed responses to indicate whether the target appears to the left or the right on each trial. Again, as the angular deviation of the stimulus in relation to the head and/or torso would be known, one would be able assess the respective contributions of each body-part's orientation to the participant’s egocentric perspectival judgments.
Based on previous work, I would expect participants’ judgements to implicate both their head and torso as determining their egocentric perspectival experience (Alsmith & Longo 2014). More specifically, I would expect that both head- and torso-centred reference frames would influence explicit and implicit egocentric perspectival phenomena (Longo & Alsmith 2013), though the exact weighting will be unequal at lateral extremes of each body part and will differ between individuals (Alsmith et al. in preparation). The further prediction would be that manipulating vestibular and proprioceptive processing will modulate felt postural misalignment and thereby systematically influence performance on explicit and implicit egocentric perspectival tasks.