Reachability and the sense of embodiment in amputees using prostheses

Abstract

Amputated patients are hardly satisfied with upper limb prostheses, and tend to favour the use of their contralateral arm to partially compensate their disability. This may seem surprising in light of recent evidences that external objects (rubber hand or tool) can easily be embodied, namely incorporated in the body representation. We investigated both implicit body representations (by evaluating the peripersonal space using a reachability judgement task) and the quality of bodily integration of the patient's prosthesis (assessed via questionnaires). As expected, the patients estimated that they could reach further while wearing their prosthesis, showing an embodiment of their prosthesis in their judgement. Yet, the real reaching space was found to be smaller with their prosthesis than with their healthy limb, showing a large error between reachability judgement and actual capacity. An overestimation was also found on the healthy side (comparatively to healthy subjects) suggesting a bilateral modification of body representation in amputated patients. Finally, a correlation was found between the quality of integration of the prosthesis and the way the body representation changed. This study therefore illustrates the multifaceted nature of the phenomenon of prosthesis integration, which involves its incorporation as a tool, but also various specific subjective aspects.

Figure 1

Maximal reachable distance (MRD) in cm for each condition HS = Healthy Subject, HA = Healthy Arm of Amputated Patient, PA = Prosthetic Arm of Amputated Patient *indicate a significant difference between the two groups of participants with p< 0.05.

Figure 2

(a) MRD (arrow), RJ (perpendicular line) and error, in cm, in healthy subjects and the healthy and prosthetic arms of the patients. Values indicated are the mean obtained over groups and conditions. (b) Error of RJ as a % of the MRD, in healthy subjects and the healthy and prosthetic limbs of the patients.

Figure 3

(A) Reachability judgement (in cm) for the healthy and prosthetic arms of the patients as a function of the integration score. (B) Reachability judgement error (in%) for the healthy and prosthetic arms of the patients as a function of the integration score. Reachability judgement error of healthy subjects (and its confidence interval) is also shown on the plot. (C) Difference of the reachability judgement error between the healthy and prosthetic arms as a function of the incorporation score, for 11 patients.

Figure 4

Experimental set-up The positions of the targets and associated responses were recorded by a computer.