Sense of body and sense of action both contribute to self-recognition
Introduction
How do we recognize ourselves? Although this seems a bizarre question, it needs to be considered seriously. There are circumstances in everyday life where self-recognition may not be straightforward. Consider, for example, a situation like seeing one's face in a mirror at the far wall of a restaurant while looking for an empty table, or seeing one's hand through a hole. Because there is discontinuity between the body part we see and the rest of the body, an active process must take place in order to refer the body part to a representation of the whole body, what Gallagher calls our body image. According to this author (Gallagher, 1995, Gallagher, 2000), the body image is a representation (sometimes conscious, sometimes not) of an owned body, one that belongs to the experiencing self.
Developmental studies provide evidence that self-recognition appears early in life. At the preconceptual stage, infants at 5 months of age are able to discriminate their own leg movements displayed in a mirror from those of another infant, presumably by making use of a perceived contingency between their own behavior and its effects (Bahrick & Watson, 1985). As they grow older the infants' behavior will increasingly testify to their development of a conscious self-representation. Infants of 15–20 months of age, for example, will typically resolve the task of wiping a red spot stuck on their face, when they see themselves in a mirror (see Bahrick, 1995 for review).
In the present study, we were interested in identifying the constituents of self-recognition in normal adult subjects. We focused on several potential sources of information indicated by the available literature to contribute to self-recognition. First, the matching of visual, tactile and proprioceptive signals originating from the same body parts contributes to an intermodal sensory image of the body. Second, the matching of one's intentions and the bodily effects of self-generated actions contributes to a sense of the self as an agent.
Concerning the contribution of sensory cues, many experimental results stress a prevalent role of vision over other senses in self-recognition: we feel our hand where we see it, not the converse. Optical distortion of the visually perceived position of a limb with respect to its felt position (e.g. by displacing prisms) produces no alteration of the sense of ownership: the position sense is actually recalibrated to conform with the visual information (Harris, 1965). This prevalence of vision was confirmed in experiments using a rubber hand. Botvinick and Cohen (1998) positioned a realistic rubber arm in front of subjects, while their real arm, hidden by a screen, was placed aside: tactile stimulation was applied simultaneously to the real and the rubber arms. After some time, the subjects experienced an illusion in which they felt the touch at the locus of the rubber arm (that they could see), not of their real (hidden) arm. In other words, the tactile stimulus was felt where it was seen, at the expense of a distortion of the felt position of the real arm. In addition, subjects spontaneously reported experiencing a clear sense of ownership for the rubber arm. According to other authors who replicated this experiment, the displacement of tactile sensations and the illusion of ownership disappear if the rubber arm is not properly aligned with the subject's body (Farné, Pavani, Meneghello, & Ladavas, 2000).
Body ownership, however, is only part of the problem of self-recognition. The self is most of the time an acting self. Body parts are moving with respect to one another and with respect to external objects as the result of intentional actions. It is common experience that our actions are readily self-attributed as a consequence of a normally perfect correlation between their expected effects and the flow of resulting (visual and proprioceptive) stimulation. This matching process provides the agent of an action with the sense that he is causing that action (the sense of agency). Again, however, situations may arise where this attribution becomes less than obvious. In social interactions (like playing a ball game) several people might participate in the same action and interact rapidly on the same object. In this situation an active identification process is needed to disentangle one's hand among other hands. Both the sense of ownership and the sense of agency concur to self-identification: it is as essential to recognize oneself as the owner of one's body as it is to recognize oneself as the agent of one's actions. Indeed, the role of movement is implicit in many descriptions of self-recognition (e.g. in the developmental context). This point was the subject of experiments where subjects had to identify their own hand from an alien hand appearing at the same locus (Nielsen, 1963). In the experiment of Daprati et al. (1997), subjects were instructed to perform simple finger movements on command: when the movement performed by the subjects with their (unseen) hand departed from the movements made by the alien hand, no attribution errors were found. By contrast, when the alien hand performed the same movement as the subject, the error rate amounted to 30%: subjects tended to over-attribute to themselves movements that were grossly compatible with their own movements. In other words, subjects used movement as a cue for self-recognition when the movement they saw matched more or less closely the representation they had of their own movement.
In the present paper, we used a situation which combined uncertainty about the ownership of the subject's hand and uncertainty about the agency of the movements performed with that hand. This situation, designed by Farrer et al. (in press), involved simultaneous presentation of two hands, one of which was the subject's hand, the other being an alien hand. This situation is more realistic than the one used in previous experiments, since it involves ‘social’ interaction between two people, in which problems of self-recognition are most likely to arise. In social interactions people will generally be able to see both their own hand and the other's. The question in self-recognition in this situation was therefore not whether an observed action corresponded to the action one had performed, but rather which of several observed actions was the one corresponding to the action performed by the self.
Section snippets
Participants
Sixteen normal subjects (six males and ten females, mean age 28.6 years) volunteered to participate in the experiment. All but two were right handed. One male subject was later excluded, because he did not comply with the instructions.
Apparatus
The experiment took place in a dimly lit room. The subject and the experimenter sat at the opposite sides of a table. The subject was facing an LCD screen positioned at 20 cm above the table at an inclination of 45° from the horizontal plane. The experimenter sat
Results
A total of 1.4% of trials were considered invalid, because the subject executed a movement different from the required one, performed additional movements during or immediately after the required one, missed the beep or started the movement before the beep. Invalid trials were rejected and repeated.
The mean proportion of erroneous responses over all conditions was 0.17 (SD=0.06). There was no significant correlation of Age and proportion of errors (Pearson's r=0.33, P>0.05). The ANOVA revealed
Discussion
The present experiment reveals that self-recognition is a highly consistent process which depends on available cues. Our main finding was that, in the absence of morphological characteristics which were controlled by the presence of gloves, two main cues influenced self-recognition. First, the visual position of the hand with respect to the body. Second, the presence of movements, which was found to override other cues.
Acknowledgements
We thank Marc Thevenet for his technical support and Chlöé Farrer for her comments on the experiment.
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