Interdependence of movement and anatomy persists when amputees learn a physiologically impossible movement of their phantom limb
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Edited by Dale Purves, Duke University Medical Center, Durham, NC, and approved September 16, 2009 (received for review June 28, 2009)

Abstract
The feeling we have of our own body, sometimes called “body image,” is fundamental to self-awareness. However, by altering sensory input, the body image can be modified into impossible configurations. Can impossible movements of the body image be conjured solely via internally generated mechanisms, and, if so, do the structural characteristics of the body image modify to accommodate the new movements? We encouraged seven amputees with a vivid phantom arm to learn to perform a phantom wrist movement that defied normal anatomical constraints. Four reported success. Learning the impossible movement coincided in time with a profound change in the body image of the arm, including a sense of ownership and agency over a modified wrist joint. Remarkably, some previous movements and functional tasks involving the phantom arm became more difficult once the shift in body image had occurred. Crucially, these introspective reports were corroborated by robust empirical data from motor imagery tasks, about which amputees were naïve and to which assessors were blind. These results provide evidence that: a completely novel body image can be constructed solely by internally generated mechanisms; that the interdependence between movement repertoire and structural constraints of the body persists even when the structural constraints imparted by the body do not—the body image we construct still constrains imagined movements; and that motor learning does not necessarily need sensory feedback from the body or external feedback about task performance.
Footnotes
- 1To whom correspondence should be addressed at: Prince of Wales Medical Research Institute, Corner of Easy and Barker Streets, Randwick 2031, Australia. E-mail: lorimer.moseley{at}gmail.com
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Author contributions: G.L.M. and P.B. designed research; G.L.M. performed research; G.L.M. and P.B. analyzed data; and G.L.M. and P.B. wrote the paper.
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The authors declare no conflict of interest.
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This article is a PNAS Direct Submission.
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This article contains supporting information online at www.pnas.org/cgi/content/full/0907151106/DCSupplemental.
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