Abstract
BackgroundA prosthetic system should ideally reinstate the bidirectional communication between the user’s brain and its end effector by restoring both motor and sensory functions lost after an amputation. However, current commercial prostheses generally do not incorporate somatosensory feedback. Even without explicit feedback, grasping using a prosthesis partly relies on sensory information. Indeed, the prosthesis operation is characterized by visual and sound cues that could be exploited by the user to estimate the prosthesis state. However, the quality of this incidental feedback has not been objectively evaluated.MethodsIn this study, the psychometric properties of the auditory and visual feedback of prosthesis motion were assessed and compared to that of a vibro-tactile interface. Twelve able-bodied subjects passively observed prosthesis closing and grasping an object, and they were asked to discriminate (experiment I) or estimate (experiment II) the closing velocity of the prosthesis using visual (VIS), acoustic (SND), or combined (VIS + SND) feedback. In experiment II, the subjects performed the task also with a vibrotactile stimulus (VIB) delivered using a single tactor. The outcome measures for the discrimination and estimation experiments were just noticeable difference (JND) and median absolute estimation error (MAE), respectively.ResultsThe results demonstrated that the incidental sources provided a remarkably good discrimination and estimation of the closing velocity, significantly outperforming the vibrotactile feedback. Using incidental sources, the subjects could discriminate almost the minimum possible increment/decrement in velocity that could be commanded to the prosthesis (median JND < 2% for SND and VIS + SND). Similarly, the median MAE in estimating the prosthesis velocity randomly commanded from the full working range was also low, i.e., approximately 5% in SND and VIS + SND.ConclusionsSince the closing velocity is proportional to grasping force in state-of-the-art myoelectric prostheses, the results of the present study imply that the incidental feedback, when available, could be usefully exploited for grasping force control. Therefore, the impact of incidental feedback needs to be considered when designing a feedback interface in prosthetics, especially since the quality of estimation using supplemental sources (e.g., vibration) can be worse compared to that of the intrinsic cues.
Highlights
A prosthetic system should ideally reinstate the bidirectional communication between the user’s brain and its end effector by restoring both motor and sensory functions lost after an amputation
“saturated” at the difference of 1% in 9 and 10 out of the 12 subjects, respectively. This did not happen for any of the subjects in the VIS condition, and it was uncommon at the low level of the standard stimulus regardless of the feedback condition
All feedback conditions resulted in a small just noticeable difference (JND) with a median value of less than 4%
Summary
A prosthetic system should ideally reinstate the bidirectional communication between the user’s brain and its end effector by restoring both motor and sensory functions lost after an amputation. Humans can effortlessly grasp and manipulate objects of very different properties, from heavy and robust to delicate and fragile. This is possible thanks to a sophisticated musculoskeletal structure innervated by a network of sensorimotor nerves, providing advanced motor commands and a comprehensive multimodal feedback (e.g., touch, proprioception, force). The lost motor functions can be restored to a certain degree using myoelectric prostheses These systems are controlled by recording the electrical activity of the user’s muscles to estimate the motion intention, which is translated into prosthesis commands [4]. Current prosthetic systems are controlled in open loop, without explicitly providing any somatosensory feedback on the prosthesis state (e.g., hand aperture or grasping force) to the user. In order to truly compensate for the missing biological limb, it is commonly assumed that a prosthetic system needs to establish a bilateral communication to the user’s brain, by restoring both motor and sensory functions [6, 7]
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