Keeping in Touch with One's Self: Multisensory Mechanisms of Self-Consciousness
1 Laboratory of Cognitive Neuroscience, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland, 2 Department of Neurology, University Hospital, Geneva, Switzerland
Abstract Top
Background
The spatial unity between self and body can be disrupted by employing conflicting visual-somatosensory bodily input, thereby bringing neurological observations on bodily self-consciousness under scientific scrutiny. Here we designed a novel paradigm linking the study of bodily self-consciousness to the spatial representation of visuo-tactile stimuli by measuring crossmodal congruency effects (CCEs) for the full body.
Methodology/Principal Findings
We measured full body CCEs by attaching four vibrator-light pairs to the trunks (backs) of subjects who viewed their bodies from behind via a camera and a head mounted display (HMD). Subjects made speeded elevation (up/down) judgments of the tactile stimuli while ignoring light stimuli. To modulate self-identification for the seen body subjects were stroked on their backs with a stick and the felt stroking was either synchronous or asynchronous with the stroking that could be seen via the HMD.
We found that (1) tactile stimuli were mislocalized towards the seen body (2) CCEs were modulated systematically during visual-somatosensory conflict when subjects viewed their body but not when they viewed a body-sized object, i.e. CCEs were larger during synchronous than during asynchronous stroking of the body and (3) these changes in the mapping of tactile stimuli were induced in the same experimental condition in which predictable changes in bodily self-consciousness occurred.
Conclusions/Significance
These data reveal that systematic alterations in the mapping of tactile stimuli occur in a full body illusion and thus establish CCE magnitude as an online performance proxy for subjective changes in global bodily self-consciousness.
Citation: Aspell JE, Lenggenhager B, Blanke O (2009) Keeping in Touch with One's Self: Multisensory Mechanisms of Self-Consciousness. PLoS ONE 4(8): e6488. doi:10.1371/journal.pone.0006488
Editor: Mark A. Williams, Macquarie University, Australia
Received: April 1, 2009; Accepted: June 27, 2009; Published: August 5, 2009
Copyright: © 2009 Aspell et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This work was supported by the Swiss National Science Foundation (#3100-067874.02). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
* E-mail: jane.aspell@epfl.ch
Introduction Top
The most basic foundations of the self arguably lie in those complex brain systems that represent the body [1]–[5]. This has been explored in research investigating multisensory and sensorimotor bodily mechanisms and their relevance for conscious aspects of processing related to body and self (or bodily self-consciousness: [5]–[9]). An important line of research has studied bodily self-consciousness by investigating the sense of ownership for one's hand [3], [4], [8], [10]–[14]. These experiments manipulated the sense of hand ownership by altering the congruence between multimodal sources of hand-related signals. For example, in the ‘rubber hand illusion’ (RHI), a subject looks at a fake hand that is being stroked by a paintbrush in synchrony with stroking applied to his own (occluded) corresponding hand, positioned a small distance away from the fake hand. Synchronous stroking of the seen fake hand and one's own unseen (real) hand can induce the illusion that the fake hand ‘feels like it's my hand’ (illusory ownership or self-attribution [10], [11], [13]). In the RHI there is also a mislocalization (or drift) of the subject's hand towards the fake hand. Importantly, illusory ownership and drift are much reduced when the stroking is asynchronous [10], [11], [13], [15].
Investigations of the RHI and related studies of the conscious experience of hands and other body parts are very important, but in addition, some authors argue that to achieve a full understanding of bodily self-consciousness we must also investigate its global character [5], [16]–[18]. A fundamental aspect of bodily self-consciousness is that the bodily self is experienced as a single and coherent representation of the entire, spatially situated body, not as a collection of several different body parts [5], [16], [19]. This is also apparent in neurological observations. Although illusory ownership in the RHI and somatoparaphrenia (when neurological patients claim either that their arm belongs to another person or that another person's arm belongs to them [20], [21]) exemplify deviant forms of bodily self-consciousness, they affect body part ownership, or the attribution and localization of a hand with respect to the bodily self, i.e. they are characterised by part-to-whole relationships. This can be contrasted with neurological patients who have illusory perceptions of their full bodies such as in out-of-body experiences and heautoscopy. These states are characterized by abnormal experience with respect to the global bodily self, e.g. a mislocalization and a misidentification of the entire body [22]–[24].
Recent studies [17], [18], [25]–[27] have further demonstrated that global aspects of self-consciousness (self-location and self-identification for the full body) - which are disturbed in neurological patients with autoscopic phenomena - can also be manipulated in healthy individuals by generating multisensory conflicts. In one study [18] subjects viewed their own body from behind via a head-mounted display while their backs were stroked. When the ‘felt stroking’ on the back of the body was congruent with the ‘seen stroking’ on the ‘virtual’ body, subjects showed higher degrees of ownership (or self-identification) for the virtual body, and mislocalized their self to a position outside their bodily borders. The studies on global bodily self-consciousness quantified ownership by verbal or physiological responses [18], [25], [27], or behavioural proxies such as perceived spatial ‘drift’ [18], based on drift measures in the RHI [10]. However these measures do not reveal whether modifications in global bodily self-consciousness are associated with changes in tactile spatial representations. Investigating this aspect is important, as it will reveal whether basic sensory processing of bodily signals is involved in the representation of the bodily self. What is more, the supposed primacy of the tactile sense in self-consciousness [28], [29] generates the prediction that whenever self-location is displaced, an associated change in the mapping of tactile sensations should also occur.
Here we linked the study of global bodily self-consciousness with the measurement of the spatial representation of visuo-tactile stimuli by using the crossmodal congruency task [30]. We hypothesized that this task could function - during the ‘full body illusion’ described above - as an effective measure for probing global aspects of bodily self-consciousness (global ownership and self-location) because the crossmodal congruency effect (CCE) can function as a behavioural index of whether visual and tactile stimuli are functionally perceived to be at the same spatial location. In previous studies of the CCE [12], [30]–[33] the visual and tactile stimuli were presented on the hands (a very recent study tested CCEs with stimuli on feet [34]). Subjects performed worse when a distracting visual stimulus occurred at an incongruent elevation with respect to the tactile (target) stimulus. Importantly, the CCE (difference between performance in incongruent and congruent conditions) was larger when the visual and tactile stimuli occurred closer to each other in space [30]. The CCE has previously been used as a measure of the tactile mislocalisation of touch towards a rubber hand when a fake hand was either aligned or misaligned with subjects' own hands ([12], see also [15]). This measure has a number of advantages: its magnitude is relatively large and it is less susceptible to experimenter expectancy effects than previous behavioural proxies of bodily self-consciousness. Moreover, the congruency task enables the collection of repeated, ‘online’ measurements during manipulations of self-consciousness: this has not previously been done in studies of partial or global bodily self-consciousness.
In the present study we tested whether CCEs – studied so far only for hands – would also be found when viewing one's own body from an external perspective, from two metres behind. Firstly, we studied whether CCEs were modulated by the visual presence or absence of the subject's own body. Secondly, to investigate whether these ‘full body CCEs’ could be associated in a predictable way with changes in bodily self-consciousness, we kept the visual stimulus constant and manipulated self-identification with the virtual body and self-location by employing either synchronous or asynchronous stroking of the back.
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