2 The differentiation of perspectival phenomena
The notion of a subjective perspective (sometimes described as a first-person perspective) is at the core of contemporary research on bodily self-consciousness (Blanke & Metzinger 2009; Metzinger 2003, 2009). However, its role has often been merely facilitative, serving as a means to study other components of bodily self-consciousness, such as the experience of bodily agency, ownership, and self-location (Ehrsson 2007; Lenggenhager et al. 2007; Petkova et al. 2011a, see Serino et al. 2013 for review). Consequently, the fact that the very notion of perspective covers a range of distinct phenomena has tended to be overlooked.[1] Referring to someone’s perceptual experience as having a perspectival structure may mean any one of several distinct things. It may mean that there is an origin to her sensory field, relative to which certain things (or parts of things) are perceptible and perceived from a particular direction and relative to which certain other things (or parts of things) are not perceptible or noticeably occluded.[2] Alternatively, it may mean that her experience is organised according to an egocentric frame of reference centred upon her body, according to which she experiences locations as situated relative to a particular point at the intersection of three orthogonal axes. Or it may be that, thanks to egomotion, the flow of her sensory experience is such that she can see where she is headed as she moves. Taking another individual’s perspective into account in social interactions can involve either of the first two forms of perspective (Moll & Meltzoff 2011). Moreover, the perspective of the subject need not figure explicitly in the experience for it to be perspectival; perspective can structure perceptual experience implicitly, by determining the way in which objects are experienced, without itself being part of the content of the experience (Campbell 1994; Merleau-Ponty 2002; Perry 1993; Zahavi 2005).
We can summarise these remarks by saying that perspectival phenomena in spatial experience vary along three dimensions.[3] First, perspectival structure can take at least three forms: [4]
Origin of a sensory field (origin)
Centre of an egocentric frame of reference (egocentric)
Focal point of a sensory flow field in action (egomotion)
Figure 1: Three forms of perspectival structure. A. An artistic rendition of a human monocular visual field. After Mach 1959, p.18. B. An egocentric frame of reference centred upon a human torso. C. The directions of deformations in the visual field specifying egomotion. Cf. Gibson 1950, p. 123.
Perspectival phenomena that exhibit any of these forms of perspectival structure can vary along two further dimensions: the perspective of a given perspectival experience may be either implicit or explicit, and may be attributed to the subject or to another individual. A perspective is explicit in a perspectival experience if the subject is consciously aware of the location of the origin, centre, or focal point in question; it is implicit if the subject is not.[5] The perspective in question may belong to the subject, a first-person perspective, or it may belong to another individual, a third-person perspective.
This simple framework enables one to study perspectival phenomena selectively, rather than studying an undifferentiated cluster of perspectival phenomena simultaneously. In the sections that follow, I shall suggest a number of ways in which one might engage in such a selective study of perspectival phenomena by intervening upon and registering the activation of vestibular processes.