Effectiveness of supplemental grasp-force feedback in the presence of vision

Previous studies have shown that supplemental grasp-force feedback can impr ove control for users of a hand prosthesis or neuroprosthesis under conditi ons where vision provides little force information. Visual cues of force ar e widely available in everyday use, however, and may obviate the utility of supplemental force information. The purpose of the present study was to us e a video-based hand neuroprosthesis simulator to determine whether grasp-f orce feedback can improve control in the presence of realistic visual infor mation. Seven able-bodied subjects used the simulator to complete a simple grasp-and-hold task while controlling and viewing pre-recorded, digitised v ideo clips of a neuroprosthesis user's hand squeezing a compliant object Th e task was performed with and without supplemental force feedback presented via electrocutaneous stimulation. Subjects had to achieve and maintain the (simulated) grasp force within a target window of variable size (+/- 10-40 % of full scale). Force feedback improved the success rate significantly fo r all target window sites (8-16%, on average), and improved the success rat e at all window sizes for six of the seven subjects. Overall, the improveme nt was equivalent functionally to a 35% increase in the window size. Feedba ck also allowed subjects to identify the direction of grasp errors more acc urately, on average by 10-15%. In some cases, feedback improved the failure identification rate even if success rates were unchanged. It is thus concl uded that supplemental grasp-force feedback can improve grasp control even with access to rich visual information from the hand and object.