Using the intact hand for objective assessment of phantom hand-perception

After amputation, most patients experience a phenomenon known as a phantom limb (PL). A variety of PL experiences appear to be associated with neural plasticity within the CNS. However, due to the subjective nature of PL experiences, there was no definitive way to reliably assess PL experiences other than using patients' direct reports. Here, we were able to obtain patients' indirect responses to PL experiences, for a more objective evaluation. First, we conducted a study with normals and 17 non-PL patients experiencing pathological pain in one hand. We took digital photographs of their affected and unaffected hands, altered the sizes of the images digitally, and then asked each subject to choose the image that most closely matched the actual size of their own hands (from a series of images presented on a video screen). Subjective size perceptions of the hands were homologous, regardless of the pathological condition of one hand (p<0.0001, Spearman R(2)=0.82). Next, we used the same method for total 19 patients with a phantom hand. The intact hand-size perception was linearly correlated with phantom hand-size perception (weighted linear regression analysis: p<0.0001, R(2)=0.75, adjusted R(2)=0.73, F-value=50.1, degree of freedom=18). Thus, without requiring a subjective description about PL, the patients' evaluation of the intact hand-size precisely but indirectly indicated whether the PL was perceived to be telescoped (shrunken), normal or enlarged. This more objective evaluation of PL phenomena could become a key tool for disentangling the neural mechanisms involved.