This study assessed the feasibility to restore finger-specific sensory feedback in transradial amputees with electrical stimulation of evoked tactile sensation (ETS). Methods: Here we investigated primary somatosensory cortical (SI) responses of ETS using Magnetoencephalography. Results: SI activations revealed a causal correlation with peripheral stimulation of projected finger regions on the stump skin. Peak latency was accountable to neural transmission from periphery to SI. Peak intensity of SI response was proportional to the strength of peripheral stimulation, manifesting a direct neural pathway from skin receptors to SI neurons. Active regions in SI at the amputated side were consistent to the finger/hand map of homunculus, forming a mirror imaging to that of the contralateral hand. With sensory feedback, amputees can recognize a pressure at prosthetic fingers as that at the homonymous lost fingers. Conclusions: Results confirmed that the direct neural pathway from periphery to SI allows effective communication of finger-specific sensory information to these amputees. INDEX TERMS Evoked tactile sensation (ETS), magnetoencephalography (MEG), prosthetic hand, sensory feedback, transcutaneous electrical nerve stimulation (TENS). IMPACT STATEMENT This study substantiated the neural basis and feasibility that electrically evoked tactile sensation can afford a non-invasive neural interface capable of restoring finger-specific sensory ability to transradial amputees.
System overview of a virtual prosthetic hand with a biorealistic neuromorphic reflex controller, whose inputs are α motor command, γs and γd commands. B.Block diagram of the integrated virtual hand system. It consists of a tendon-driven virtual hand, a pair of antagonistic muscles with the biorealistic controller and a sensory feedback system. Force perturbations can be applied at the fingertip, or at the tendon of flexor/extensor muscles in perturbation experiments. C. Simulation results unveil that the virtual hand has acquired human-like ability of compliant control as follows. (1) The virtual hand can switch control modes between position and force naturally according to external loads. (2) The variability of fingertip force increases with the mean force in proportion (R 2 =0.99, p=0.0000). It implies the capability for fine force manipulation with low levels of muscle activation. (3) The virtual hand can achieve stable control of finger equilibrium positions and reflex regulation of muscle stiffness via Ia afferent, which enhances fingertip and muscle stiffness. (4) Muscle stiffness can be modulated by α command linearly and adaptively adjusted with object stiffness for a given background activation. The length-tension curve and reflex compensation underscore the neuromechanical mechanism of stiffness adaptation.
The ability to perceive prosthetic grasping may enable amputees to better interact with external objects. This may require customized coding of multiple sensory feedback for each amputee. This study developed a protocol to determine optimal modulation ranges of sensations elicited by transcutaneous electrical nerve stimulation (TENS). These sensations that were referred to the lost fingers provided the possibility for restoring multi-modalities of sensory feedback for amputees with evoked tactile sensation (ETS) non-invasively. To match the restricted projected finger map area, smaller electrodes must be used to deliver electrical stimulation for multi-channel sensory information, which resulted in fewer types of sensations. Our protocol provided comprehensive information for optimal selection of amplitude and frequency in a personalized, pulse-width encoding paradigm. The good sensitivity for vibration and buzz in both able-bodied and amputee subjects suggested that perceptual intensity can be effectively modulated to convey sensory information via either of the sensations. The efficacy of this protocol in sensory coding for forearm amputees was demonstrated in finger-specific identification experiment. This protocol may allow customization of ETS-based sensory feedback with an optimal encoding strategy for individual amputees.
The ability to perceive prosthetic grasping may enable amputees to better interact with external objects. This may require customized coding of multiple sensory feedback for each amputee. This study developed a protocol to determine optimal modulation ranges of sensations elicited by transcutaneous electrical nerve stimulation (TENS). These sensations that were referred to the lost fingers provided the possibility for restoring multi-modalities of sensory feedback for amputees with evoked tactile sensation (ETS) non-invasively. To match the restricted projected finger map area, smaller electrodes must be used to deliver electrical stimulation for multi-channel sensory information, which resulted in fewer types of sensations. Our protocol provided comprehensive information for optimal selection of amplitude and frequency in a personalized, pulse-width encoding paradigm. The good sensitivity for vibration and buzz in both able-bodied and amputee subjects suggested that perceptual intensity can be effectively modulated to convey sensory information via either of the sensations. The efficacy of this protocol in sensory coding for forearm amputees was demonstrated in finger-specific identification experiment. This protocol may allow customization of ETS-based sensory feedback with an optimal encoding strategy for individual amputees.
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