Sensory feedback by peripheral nerve stimulation improves task performance in individuals with upper limb loss using a myoelectric prosthesis

Abstract

Objective: Tactile feedback is critical to grip and object manipulation. Its absence results in reliance on visual and auditory cues. Our objective was to assess the effect of sensory feedback on task performance in individuals with limb loss.

Approach: Stimulation of the peripheral nerves using implanted cuff electrodes provided two subjects with sensory feedback with intensity proportional to forces on the thumb, index, and middle fingers of their prosthetic hand during object manipulation. Both subjects perceived the sensation on their phantom hand at locations corresponding to the locations of the forces on the prosthetic hand. A bend sensor measured prosthetic hand span. Hand span modulated the intensity of sensory feedback perceived on the thenar eminence for subject 1 and the middle finger for subject 2. We performed three functional tests with the blindfolded subjects. First, the subject tried to determine whether or not a wooden block had been placed in his prosthetic hand. Second, the subject had to locate and remove magnetic blocks from a metal table. Third, the subject performed the Southampton Hand Assessment Procedure (SHAP). We also measured the subject's sense of embodiment with a survey and his self-confidence.

Main results: Blindfolded performance with sensory feedback was similar to sighted performance in the wooden block and magnetic block tasks. Performance on the SHAP, a measure of hand mechanical function and control, was similar with and without sensory feedback. An embodiment survey showed an improved sense of integration of the prosthesis in self body image with sensory feedback.

Significance: Sensory feedback by peripheral nerve stimulation improved object discrimination and manipulation, embodiment, and confidence. With both forms of feedback, the blindfolded subjects tended toward results obtained with visual feedback.

Figures

Figure 1

(a) S1 was implanted with 2 FINEs and 1 spiral nerve cuff. The FINEs were implanted on the median and ulnar nerves. The spiral was implanted on the radial nerve. Leads were tunneled to the lateral upper arm, where they exit as 20 helical wires. S2’s amputation is in the proximal forearm and cuffs were implanted in the arm 5–10 cm proximal to the elbow. (b) The subject’s prosthetic hand was mounted with low-profile force sensors on the pads of D1–D3 as well as a bend sensor measuring the D1–D2 angle. Both subjects used their own prosthetic hand, the Ottobock SensorHand Speed, for the tests. The prosthetic hand has 1 degree of freedom (DoF) and 1 grip pattern. The internal slip sensor was disabled. The pressure and bend sensors regulated the stimulus applied to the nerves. Photo courtesy Russell Lee/Case Western Reserve University. (c) S2 using his instrumented prosthetic to locate and remove magnetic blocks from a metal platform while blindfolded.

Figure 2

ODT accuracy for S1 and S2. Both subjects always identified the presence of a wooden block with their intact hand (not shown). Without feedback, both subjects performed slightly above chance. Addition of one of the two forms of sensory feedback, either pressure or aperture, could significantly improve accuracy but was never significantly better than the accuracy achieved using both forms of feedback simultaneously. Both forms of feedback also resulted in significantly greater performance than with the bend sensor alone in S1 or with the force sensors alone in S2. An accuracy of 50% represented chance. ***(p ≤ 0.001), **(p ≤ 0.01), *(p ≤ 0.05).

Figure 3

Results from mBB tests in S1 (left column) and S2 (right column). For both subjects, restoring both forms of feedback resulted in a significant increase in success (a) and (b) and a significant decrease in failures (c) and (d) compared to no feedback. When using the prosthetic hand while blindfolded, audibly occluded, and without additional sensory feedback, S1 and S2 removed 3.1 ± 0.8 and 2.1 ± 1.9 blocks, with 7 ± 3 and 6 ± 2 errors, and 1 ± 1 and 0 ± 0 corrections, respectively. With both pressure and aperture feedback, S1 and S2 removed 6.4 ± 2.6 (p = 0.007) and 5.2 ± 1.0 (p < 0.001) of the blocks, respectively. S1 averaged only 1 ± 1 error (p < 0.001) and S2 a nearly perfect 0.4 ± 0.7 (p < 0.001) error rate. There was no significant difference in the number of failures between the two subjects (p = 0.610). While S1 did not significantly vary his corrective actions (e), averaging 2 ± 1, S2 had 4 ± 1 corrections (f), which was significantly higher (p < 0.001). As more feedback was provided, both subjects trended toward a perfect success rate without failures. ***(p ≤ 0.001), **(p ≤ 0.01), *(p ≤ 0.05).

Figure 4

Embodiment improved when sensory feedback was provided during functional, sighted tasks. ***(p ≤ 0.001), **(p ≤ 0.01), *(p ≤ 0.05).