Recent work in our laboratory has used a simulated upper extremity, body-powered prosthesis to study the early development of prosthetic coordination and control. The simulator closely resembles the cosmetic and functional features of a standard prosthesis worn by people with a below elbow amputation. In a preliminary study, we have already demonstrated the difficulty with which participants perform a functional reaching and grasping task using the prosthetic simulator (Wallace et al., 1999).
In the present study, we investigated the perceptual limitations of the prosthetic simulator by comparing the ability to discriminate weight with the prosthesis versus the normal, anatomical hand. Using the psychophysical method of constant stimuli (Gescheider, 1976), ten right handed persons without amputation (3 male and 7 female) were asked to compare 75-, 100-, 125-, 150- and 175 g weights against a standard weight of 125 g. All weights were identical canisters made of capped cylinders (55 mm high x 35 mm diameter) filled with lead shot. Each participant completed 40 randomly presented comparisons against the standard weight (8 for each of the weights used) with the prosthetic arm and 40 presentations with the anatomical arm. The order of type of effector (prosthesis or anatomical) was counter-balanced across participants.
Surprisingly, while the just noticeable difference (JND) was higher for the prosthetic (13.9 g) compared to the anatomical hand (7 g), the difference was not significant. However, the point of subjective equality (PSE), which indicates the value of the comparison weight that is subjectively equal to the standard weight, was significantly less for the prosthetic (113 g) compared to the anatomical arm (125 g), F(1,6) = 12.16, p<.01, suggesting an underestimation of weight in the former case.
These preliminary results suggest that weight discrimination may not be as effective using a prosthetic arm, at least with inexperienced users. However, further research, using finer discriminations, is required to find the level of discrimination at which the prosthesis is clearly and robustly inferior to the anatomical hand and arm.