Investigating the size weight illusion in upper limb amputees

Introduction: The size weight illusion occurs when a person underestimates the weight of a larger object in relation to a smaller object of the same mass. This is well documented in normal populations but there has only been one study looking into its presence in populations with limb loss. This project aims to expand on the work by Wallace comparing the performance of a larger sample of upper limb amputees with able-bodied persons to investigate whether the size weight illusion exists in amputees and whether it is of the same magnitude as in the normal population. There are several potential benefits to this study. Currently what causes the size weight illusion is unknown although there are several theories. Testing with active prosthetic users allow the researchers to eliminate certain variables such as sensory feedback as current prostheses used do not provide sensory feedback from the fingers to the user. The findings from this study also provide a greater understanding of what information individuals with a prosthetic limb use to judge the weight of objects, which may have consequences for the environmental ergonomics of this population. Methodology: During our investigation we compared a group of upper limb amputees using prosthetic devices to a group of people with normal upper limb function. Approval for the study was granted by the University of Strathclyde Ethical Committee. Participants were tested one at a time in separate testing sessions. Participants were asked to lift objects of varying size and weight and ask them to rate them as a number, with a larger number indicating a larger weight. This number was then used to determine if the participants were experiencing the size weight illusion. The order in which the objects were offered to the participants was random and different for each participant. The data was analysed using T-tests and ANOVA with the SPSS software package. Results: The data collection phase is due to be completed on 31/3/14. To date we have collected data on 5 amputee and 5 control subjects. Conclusion: Preliminary analysis of the data shows that the amputee group did experience the size weight illusion. This supports the previous findings by Wallace. Interestingly, the magnitude of the illusion and sensitivity to weight appear to be different according to the subject groups, although this will be confirmed upon completion of the data collectio