Amputees misperceive the size of artificial limbs
Ritika M. Mazumder & Jason M. Haberman
In creating prosthetic devices for lower-limb amputees, prosthetists make the limbs smaller than the corresponding intact limbs (along the width dimension) by about three percent (Mazumder & Haberman, VSS, 2015). This is in response to patients’ report that the limb appears too ‘bulky’ if exactly matched in size to the intact limb. In a prior experiment, we explored whether such a perceptual bias manifested in non-amputee observers. Surprisingly, the results suggested that observers perceived the size of a prosthetic device as too small, and this was most evident in the context of whole, upright bodies. The direction of this bias was unexpected given clinical practice. The critical question, explored in the current set of experiments, is whether amputees, who may have experience and/or cortical reorganization due to their amputation, exhibit any perceptual bias when viewing prosthetic devices. Participants, all lower-limb amputees, viewed images of other amputees and, using the mouse, adjusted the width of the prostheses until it looked ‘right.’ Unexpectedly, amputees showed the same bias as non-amputees, adjusting the prosthetic limbs to be larger than the intact limbs by almost seven percent (i.e., it only looked ‘right’ when its width was greater than that of the intact limb). Although the bias persisted when the images were inverted, it was significantly reduced, suggesting configural processing enhanced the size misperception. While this bias contrasts with clinical practice, we hesitate to suggest the practice is misguided, as amputees’ report of the limb being ‘too big or bulky’ might be driven by somatosensory representations rather than visual ones.