Sensory Feedback for Upper-Limb Amputees by Noninvasive Electrical Stimulation and Its Performance Assessment

Most commercial prostheses lack a function of natural and intuitive sensory feedback, which is one of the reasons for their high rate of abandonment. Transcutaneous Electrical Nerve Stimulation (TENS) has been proved as an effective approach to evoke sensations for limb amputees. This paper aims to explore the impact of TENS parameters on sensation evoking and evaluate its performance based on behavioral response and EEG data, involving three transradial amputees and seven able-bodied subjects. Experimental results show that the sensation thresholds were predominantly influenced by stimulus amplitude and width, and the sensation intensity increased with the increase of either amplitude or width. Variation of stimulus frequency caused transitions between sensation types, where stimulating at 10 and 100 Hz could achieve stable vibration and pressure sensations for all subjects, respectively. A stimulation encoding strategy was thereupon proposed, where a pressure sensation was to simulate grasp force and the sensation intensity to encode force amplitude. The amputees could achieve a high accuracy rate above 94.4 and 77.8% for sensation type discrimination and intensity grading, respectively, with slightly longer response time than the able-bodied. The obvious cortical activation and clear ERP components demonstrated the reliability of TENS-based sensory feedback, where the N1 component could distinguish different sensation types and intensities (p ≤ 0.05), and the amputees had slower discriminatory responses and weaker activation in sensorimotor cortices than the able-bodied (p ≤ 0.05). This study promisingly confirmed TENS for restoring sensory feedback in limb-amputees, providing a support for closed-loop interactions in amputee-prosthesis systems and even bionic robots.