Characterization of multi-channel intraneural stimulation in transradial amputees

Abstract

Although peripheral nerve stimulation using intraneural electrodes has been shown to be an effective and reliable solution to restore sensory feedback after hand loss, there have been no reports on the characterization of multi-channel stimulation. A deeper understanding of how the simultaneous stimulation of multiple electrode channels affects the evoked sensations should help in improving the definition of encoding strategies for bidirectional prostheses. We characterized the sensations evoked by simultaneous stimulation of median and ulnar nerves (multi-channel configuration) in four transradial amputees who had been implanted with four TIMEs (Transverse Intrafascicular Multichannel Electrodes). The results were compared with the characterization of single-channel stimulation. The sensations were characterized in terms of location, extent, type, and intensity. Combining two or more single-channel configurations caused a linear combination of the sensation locations and types perceived with such single-channel stimulations. Interestingly, this was also true when two active sites from the same nerve were stimulated. When stimulating in multi-channel configuration, the charge needed from each electrode channel to evoke a sensation was significantly lower than the one needed in single-channel configuration (sensory facilitation). This result was also supported by electroencephalography (EEG) recordings during nerve stimulation. Somatosensory potentials evoked by multi-channel stimulation confirmed that sensations in the amputated hand were perceived by the subjects and that a perceptual sensory facilitation occurred. Our results should help the future development of more efficient bidirectional prostheses by providing guidelines for the development of more complex stimulation approaches to effectively restore multiple sensations at the same time.

Figure 1

Stimulation set-up. (a) Implanted TIMEs in median (M1-2) and ulnar (U1-2) nerves, are connected to the STIMEP or RIPPLE neurostimulator which sends rectangular, bi-phasic and cathodic first stimuli with changing amplitude but constant pulse-width and frequency. Stimulation of M1, M2 and U2 led to somatotopic sensation feedback located on the amputee’s phantom limb. Each electrode used its own GND. (b) Schemes of multi-channel configurations SCC, DCC and TCC. (c) Insertion of TIME and implanted and fixed TIMEs in ulnar nerve.

Figure 2

SCC and DCC sensation location. Left side: evoked sensation location during single channel, intraneural stimulation (SCC) with TIMEs. Right side: double channel configuration (DCC). (a) Subject 1, (b) Subject 2, (c) Subject 3, (d) Subject 4.

Figure 3

SCC and DCC perceptual threshold. Perceptual threshold for all subjects in respect to injected charge for SCC and DCC. Y-axis shows the average single-contact charge in SCC and DCC configurations, where x indicates subject 1–4. All the data showed significant differences (p < 0.05) between SCC and DCC (N = 30, 162, 30, 33, for Subject 1, 2, 3 and 4, respectively).

Figure 4

TCC characterization. Sensation location for subject 1 (a) and 4 (b) for SCC (blue) and TCC (green). Bottom: Perceptual thresholds for subject 1 (a) and 4 (b) during SCC, DCC and TCC (N = 42, 45, for Subject 1 and 4, respectively). Y-axis: single active site charge level (threshold) needed to elicit a sensation in each stimulation condition. X-axis: ASs which have been used in SCC, DCC and TCCs.

Figure 5

SCC, DCC and TCC cortical recordings. Subject 1 grand-average butterfly SEPs and topographic scalp maps at different latencies for SCC-median, SCC-ulnar, DCC and TCC conditions (respectively N = 412, 349, 427 and 337). Latencies are referred to the onset of the stimulation.

Figure 6

Early latencies zoom on grand-average butterfly SEPs. Early latencies zoom on grand-average butterfly SEPs for Subject 1. As in Fig. 5, topographic scalp maps are shown at different latencies respectively for SCC-median, SCC-ulnar, DCC and TCC conditions (respectively N = 412, 349, 427 and 337). Latencies are referred to the onset of the stimulation.