Exploring factors associated with upper-limb prosthesis embodiment : a mixed-methods approach

Current rejection rates among upper-limb prosthesis users are particularly high. A significant psychological factor associated with prosthesis use is the extent to which users feel their prosthesis is a natural part of them (termed Prosthesis Embodiment [PE]). Many researchers and clinicians suggest that encouraging PE should be an aim of rehabilitation. However, the factors influencing PE, how PE changes over the user’s lifetime, and the potential clinical consequences of PE, are currently unclear. In Study 1, in-depth qualitative email interviews were conducted with 10 upper-limb prosthesis users to explore both factors influencing and resulting from PE, via a qualitative Directed Content Analysis. Participants reported an unexpectedly wide range of both influences (e.g. type of prosthesis) and outcomes (e.g. better prosthesis proficiency). Temporary changes to PE were more noticeable to the user than a gradual change over time. These findings were then used to design a quantitative online questionnaire survey for upper-limb prosthesis users (Study 2) in order to follow-up findings in a larger sample (N = 34). In addition to statistically confirming most of the expected relationships, potential motivational and social aspects for PE emerged from the analysis. Further key findings are that PE is on a continuum rather than being dichotomous, and that satisfaction with aspects of the prosthesis (e.g., functionality) is also a major factor. An Immersive Virtual Reality (IVR) study was conducted with 31 anatomically-intact participants to explore experimentally how the prosthesis control method influences PE (Study 3). Body ownership techniques were utilised in a virtual-hand illusion paradigm, combined with motion tracking of a myoelectric prosthesis. This tested the relative impact of electromyographic (EMG) control (via muscle flexes) on virtual PE compared to anatomical-hand control. Results indicated the feeling of agency and skin conductance response to a virtual threat to the prosthesis were similar between the conditions. However, the feeling of ownership was significantly reduced with EMG control of the prosthesis, suggesting the influence of control method on PE, and may be a key factor for rejection. The results indicate there are a much greater range of influences and outcomes of PE than previously expected or explored, and that IVR prosthetic simulations may provide a method to test specific factors in a controlled setting. Understanding such factors could potentially inform prosthetic design, and ultimately aid in rehabilitation