A few years ago we had the opportunity to use a robot at Johnson Space Center, Houston, which allowed us to realise just how plastic and fragile a sense of ownership of our bodies is. The robot's arms have joints that move like those of human arms and three fingers on each hand. The arms are viewed by the human subject through a virtual reality set placed over the eyes, with the robot cameras set in the robot's "head"‚ so that the subject views the robot arms from a similar view point as one views one's own arms. No direct vision of one's own body is possible, while one sits across the room from the robot. A series of sensors are placed on one's own arms which in turn control the robot's arms. Then when one moves, the robot's arm move similarly, after a short delay.

Thus, one sees and controls the robot's moving arms, without receiving any peripheral feedback from them, (but having one's own peripheral proprioceptive feedback from one's unseen arms). In this situation we transferred tools from one hand to another, picked up an egg, and tied knots. After a few minutes we all became at ease with the feeling of being in the robot. Making a movement and seeing it effective successfully led to a strong sense of embodiment within the robot arms and body. This was manifest in one particular example when one of us thought that he had better be careful for if he dropped a wrench it would land on his leg! Only the robot arms had been seen and moved, but the perception was that one's body was in the robot. This feeling was present both in able bodied people who tried the robot and in a subject, IW, who has a large fibre sensory neuropathy that led to the loss of the sensations of movement/position sense and light touch, below the neck (Cole, 1995).

In this case one's sense of agency and ownership of action are intact but there is a misidentification of the sense of ownership of one's own body, this being transferred into a set of steel rods and stubby robotic hands with little visual similarity to human arms. One might suggest that in this case senses of agency and ownership of action redefine any sense of ownership of body. That this sense is so plastic and fragile was a surprise. It may arise because during our lives our bodies may alter considerably in size, shape and motor skill. As children we learn to impose our senses of agency, ownership of action and of body onto a corporeal body which grows and changes. If we injure ourselves, (our embodied corporeal selves), then we have to maintain a sense of agency and ownership with the injured and altered body. The mere act of limping, for instance, must profoundly alter peripheral feedback and the programs required to move. As we age similar corporeal changes must also be assimilated. If we did not have this ability to alter our mapping of a sense of ownership and agency onto altered bodies we might be at risk of alienation from them.

Lastly Gallagher suggests that while a sense of ownership for motor action can be explained in terms of ecological self awareness, the sense of agency may be based on a process within the brain which precedes action and translates intention into action, (based on evidence from the elegant work of, amongst others, Anthony Marcel [in press] and Patrick Haggard [1999]). The example of IW might be relevant here. As a result of his illness, IW was completely unable to make any controlled movement for several weeks. Slowly he learnt to make movements by conscious effort and with visual feedback. Initially, when without voluntary control, he had little sense of agency and felt most disembodied. Regaining movement control required a redevelopment of the focus of motor command and with this came a sense of agency. Both however seemed to require peripheral verification or "reality testing"‚ from visual feedback. Although the sense of agency does seem to reside at a level preceding action in control subjects, one would be wary of completely excluding a role for verification, peripherally, of the success of movement. Peripheral feedback is not sufficient for a sense of agency (clearly passive movement does not lead to it), but without such feedback, in the unperceived background ( at the level of the motor schema), the senses of agency and ownership of action may be altered and even fail to be maintained.

References

Cole, J. (1995), Pride and a Daily Marathon. The MIT Press.

Gallagher, S. (2000). Philosophical concepts of the self: implications for cognitive science Trends in Cognitive Science, 4, 14-21.

Haggard, P. and Magno, E. (1999) On the relation between brain potentials and awareness of voluntary movements. Exp Brain Res, 126, 128-133.

Marcel, A. J. (in press). The sense of agency: Awareness and ownership of actions and intentions. In Agency and Self-Awareness (Roessler, J. and Eilan, N., eds.), Oxford University Press.