the NeuRA blog

Typically we pay little attention to the sense that our limbs are a part of our body and that we have control over them. These mind-body connections are essential for moving and interacting with our surrounds. We first learn self-awareness and to distinguish self from other when we make exploratory movements as infants. The sense of self continues to stabilise as we develop, but how is this achieved?

The sense of self

In the early 1900s, it was noticed that an artificial finger placed on top of a cloth that covered someone’s finger was often mistakenly perceived as his or her real finger. Research in the area remained relatively dormant until 1998, when Princeton University cognitive scientist Matthew Botvinick rediscovered the illusion at a Halloween party. Participants sat with one arm under a table and researchers placed a rubber arm on top of the table in alignment with the real arm below. The experimenters stroked the participant’s arm and the rubber hand simultaneously with a paintbrush. Subjects reported feeling as if the rubber hand were their own suggesting they had “embodied” it. This rubber-hand illusion (RHI) suggests that body ownership is highly malleable.

Beyond ownership, the sense of agency is the conviction that we have control over the events we initiate. We have control when we reach for a glass of water, when we kick a football, and when we put pen to paper. However, agency can be disrupted such as a brain-damaged patient who has lost control of her left hand or the defendant who insists he did not fire the gun.

Why study self?

While the RHI is admittedly a contrived scenario, the fact that people can learn to embody a new limb has major implications for amputees who often experience a painful “phantom limb”. Can we apply our knowledge about body ownership to help amputees excise their phantom limb?

We cannot use the traditional RHI as amputees obviously lack skin, muscles, and neurons in their missing limb to stimulate. As a work-around, researchers have begun to use electrical stimulation of the brain regions that represent the body, in order to mimic the effects of stroking a real limb. Using this technique researchers were able to generate ownership of a rubber hand in upper limb amputees by stimulating the region of the somatosensory cortex corresponding to one hand while touching a rubber hand visible to the participants.

Past experience of agency over an object might also contribute to embodiment. For instance a smartphone is more readily embodied than a wooden block, which people have no previous agency experience with. This finding suggests that there is a direct impact of past agency experience on ownership.

Implications for the self

Given our seemingly boundless potential to attribute agency and ownership to other objects, it is hard to predict how we might interact with our surroundings in the future. In the coming years we will increasingly be interacting with brain-machine interfaces and neuroprosthetics, which we may incorporate into the self. Further research is needed to inform the design of prosthetics to move more naturally so that they can be more easily “embodied.” A better understanding of the link between the sense of agency and actions themselves will also have implications for treating disorders of self. Further it has ethical implications for the legal treatment of those who claim at some point to have lost control of their bodies.

This post is a summary of the full version of this article, co-authored with Roman Liepelt of German Sport University in Cologne, which appears here in The Scientist.