Evoking Apparent Moving Sensation in the Hand via Transcutaneous Electrical Nerve Stimulation

Abstract

The restoration of sensory feedback in amputees plays a fundamental role in the prosthesis control and in the communication on the afferent channel between hand and brain. The literature shows that transcutaneous electrical nerve stimulation (TENS) can be a promising non-invasive technique to elicit sensory feedback in amputees, especially in the lower limb through the phenomenon of apparent moving sensation (AMS). It consists of delivering a sensation that moves along a specific part of the body. This study proposes to use TENS to elicit tactile sensations and adopt AMS to reproduce moving sensations on the hand, such as those related to an object moving in the hand or slipping upward or downward. To this purpose, the developed experimental protocol consists of two phases: (i) the mapping of the evoked sensations and (ii) the generation of the AMS. In the latter phase, the pulse amplitude variation (PAV), the pulse width variation (PWV), and the interstimulus delay modulation (ISDM) methods were compared. For the comparative analysis, the Wilcoxon-Mann-Whitney test with Bonferroni correction (P < 0.016) was carried out on the success rate and on the ranking of methods expressed by the subjects. Results from the mapping protocol show that the delivered sensations were mostly described by the subjects as almost natural and superficial tingling. Results from the AMS protocol show that, for each movement direction, the success rate of ISDM method is higher than that of PWV and PAV and significantly higher than that of PAV for the ulnar-median direction. It recreates an AMS in the hand that effectively allows discriminating the type of sensation and distinguishing the movement direction. Moreover, ISDM was ranked by the subjects as the favorite method for recreating a well-defined and comfortable moving sensation only in the median-ulnar direction. For the ranking results, there was not a statistically significant difference among the methods. The experiments confirmed the good potential of recreating an AMS in the hand through TENS. This encourages to push forward this study on amputees and integrate it in the closed-loop control of a prosthetic system, in order to enable full control of grasp stability and prevent the objects from slippage.

Keywords: apparent moving sensation; sensory feedback restoration; slippage; transcutaneous electrical nerve stimulation; upper limb prostheses.

FIGURE 1

Scheme of the experimental setup. The control software sends the features of the stimulation waveform to the electrical stimulator. The stimulation waveform used in the study is the symmetric biphasic square wave, whose parameters are the pulse amplitude (PA), the pulse width (PW), and the pulse frequency (PF). The current is applied through the use of superficial electrodes to the user’s skin. The red dots indicate the electrodes for the median stimulation, while the blue ones are for the ulnar stimulation. The light color is used for the active electrodes and the shaded color for the passive electrodes. The custom-developed graphic user interface is used by the subjects to indicate the main characteristics of the evoked sensations.

FIGURE 2

Scheme of the experimental protocol, which is composed of the (A) mapping and the (B) apparent moving sensation (AMS) protocols (PAV, pulse amplitude variation; PWV, the pulse width variation; and ISDM, interstimulus delay modulation methods).

FIGURE 3

Regions reported by the subjects after the median (red), ulnar (blue), and concurrent (green) stimulations. The regions indicated by the nine subjects have been overlapped in a hand map, which is divided into minimum, medium, and maximum intensity of the electrical stimuli. The areas depicted with a more vivid color indicate the regions with a higher number of occurrences than the others more reported by the subjects. The levels of intensity are the range of values of pulse width (PW) and pulse frequency (PF) provided during the stimuli. Minimum intensity for the median nerve is PW = [220;300] μs and PF = [50;100] Hz and for the ulnar nerve is PW = [380;420] μs and PF = [50;100] Hz. Medium intensity for the median nerve is PW = [340;420] μs and PF = [150;300] Hz and for the ulnar nerve is PW = [460;500] μs and PF = [150;300] Hz. Maximum intensity for the median nerve is PW = [460;500] μs and PF = [400;500] Hz and for the ulnar nerve is PW = [540;580] μs and PF = [400;600] Hz.

FIGURE 4

The red boxes represent the median values of the injected charge necessary to induce a specific type of quality during the median stimulation. The blue and the green boxes represent the same results obtained, respectively, during the ulnar and concurrent stimulations. The + signs represents the outliers of the median for each box.

FIGURE 5

Linear regressions of the median value (±SD, standard deviation) of the injected charge for the referred intensities reported by the subjects during charge modulation of the (A) median, (B) ulnar, and (C) concurrent stimulations. R² is the coefficient of determination, which describes the goodness of the linear regression.

FIGURE 6

Linear regressions of the median value (±SD, standard deviation) of the referred intensities reported by the subjects for each pulse frequency (PF) of frequency modulation of the (A) median and (B) ulnar stimulations. R² is the coefficient of determination, which describes the goodness of the linear regression.

FIGURE 7

(A) Success rate (±SD, standard deviation) of the discrimination of the movement direction of each subject for each method for both median–ulnar (MU) and ulnar–median (UM) directions. The comparative analysis (Wilcoxon–Mann–Whitney test with Bonferroni correction) reports a statistically significant difference in terms of success rate (SR) between pulse amplitude variation (PAV) and ISDM for the UM direction (P_PAV–ISDM = 0.0089). No statistically significant differences were reported for the other comparisons: for the MU direction P_PAV–ISDM = 0.0367, P_PAV–PWV = 0.6536, and P_PWV–ISDM = 0.1432; for the UM direction, P_PAV–PWV = 0.3912 and P_PWV–ISDM = 0.1432. (B) Mean of the ranking position (± SD, standard deviation) for each method for both MU and UM direction. Low mean values indicate that the subjects ranked the method in high positions like 1 or 2. The comparative analysis (Wilcoxon–Mann–Whitney test with Bonferroni correction) reports no statistically significant differences in terms of ranking preference among PAV, PWV, and ISDM: for the MU direction, P_PAV–ISDM = 0.0340, P_PAV–PWV = 0.8858, and P_PWV–ISDM = 0.0297; for the UM direction, P_PAV–ISDM = 0.7630, P_PAV–PWV = 0.9992, and P_PWV–ISDM = 0.9184. Statistically significant differences (P < 0.0016) are depicted by asterisks.