AIM:Limb loss is a dramatic event with a devastating impact on a person's quality of life. Prostheses have been used to restore lost motor abilities and cosmetic appearance. Closing the loop between the prosthesis and the amputee by providing somatosensory feedback to the user might improve the performance, confidence of the amputee, and embodiment of the prosthesis. Recently, a minimally invasive method, in which the electrodes are placed subdermally, was presented and psychometrically evaluated. The present study aimed to assess the quality of online control with subdermal stimulation and compare it to that achieved using surface stimulation (common benchmark) as well as to investigate the impact of training on the two modalities.METHODS:Ten able-bodied subjects performed a PC-based compensatory tracking task. The subjects employed a joystick to track a predefined pseudorandom trajectory using feedback on the momentary tracking error, which was conveyed via surface and subdermal electrotactile stimulation. The tracking performance was evaluated using the correlation coefficient (CORR), root mean square error (RMSE), and time delay between reference and generated trajectories.RESULTS:Both stimulation modalities resulted in good closed-loop control, and surface stimulation outperformed the subdermal approach. There was significant difference in CORR (86 vs 77%) and RMSE (0.23 vs 0.31) between surface and subdermal stimulation (all p < 0.05). The RMSE of the subdermal stimulation decreased significantly in the first few trials.CONCLUSION:Subdermal stimulation is a viable method to provide tactile feedback. The quality of online control is, however, somewhat worse compared to that achieved using surface stimulation. Nevertheless, due to minimal invasiveness, compactness, and power efficiency, the subdermal interface could be an attractive solution for the functional application in sensate prostheses.

Modern hand prostheses are used to restore the motor functions lost due to an amputation. However, the lack of sensory feedback remains a major challenge. Subdermal stimulation is a promising technique to restore tactile sensations when using prostheses, since it may overcome the disadvantages of surface electrodes without resorting to surgery that is required for a direct nerve interface. The present study evaluated the short-term repeatability of the perceptual properties of subdermal electrical stimulation over eight hours in healthy subjects and compared them to those of surface stimulation. Specifically, the detection threshold, pain threshold, dynamic range, just noticeable difference, resolution and quality of evoked sensations were tested and used for short-term repeatability evaluation. The results demonstrated that the detection threshold was more stable under subdermal stimulation, whereas the short-term repeatability of the pain threshold and just noticeable difference was better under surface stimulation. On the other hand, several psychometric parameters (dynamic range, resolution, sensation quality, intensity, and comfort) were equally stable and did not change significantly across sessions in either surface or subdermal stimulation. The subdermal stimulation was better localized and elicited fewer unwanted sensation modalities (p <; 0.05), whereas surface stimulation was characterized by a higher resolution (p <; 0.05). The results suggest that subdermal stimulation could be a viable alternative for the implementation of electro-tactile feedback as it generates sensations that are equally stable as in surface stimulation, and yet it has some important advantages for the practical applications (e.g., compact interface, permanent placement).

Use of electrical stimulation through implanted neural interfaces makes it possible to selectively activate afferent neurons and provide natural sensory feedback. However, effective stimulation patterns for creating natural sensory feedback are not uniquely characterized in the literature. Thus, there is often a need to evaluate various combinations of different stimulation parameters for specific applications, which can produce a wide range of possible stimulation patterns in a multichannel stimulation system. It makes the use of sensory feedback an impracticable and time-consuming task. We therefore designed and implemented a computerized tool referred to as a “Psychophysical Testing Platform” to easily control multichannel stimulation and characterize the evoked sensations. The tool was tested in a clinical trial including one amputee with the aim to relieve his phantom limb pain (PLP) by manipulation

Electrotactile stimulation has been suggested as a modality for providing sensory feedback in upper limb prostheses. This study investigates the multiday variability of subdermal and surface stimulation. Electrical stimulation was delivered using either surface or fine wire electrodes placed right under the skin in eight amputees for seven consecutive days. The variability of psychophysical measurements, including detection threshold (DT), pain threshold (PT), dynamic range (DR), just noticeable difference (JND), Weber fraction (WF) and quality of evoked sensations, was evaluated using the coefficient of variation (CoV). In addition, the systematic change in the mean of the parameters across days was assessed in both stimulation modalities. In the case of DT, PT, DR, and perceived intensity at 100 Hz, the CoV of surface stimulation was significantly smaller than that of subdermal stimulation. Only PT showed a significant systematic change in the mean value across days for both modalities. The outcome of this study has implications for the choice of modality in delivering sensory feedback, though the significance of the quantified variability needs to be evaluated using usability tests with user feedback.

This paper evaluated the psychophysical properties of subdermal electrical stimulation to investigate its feasibility in providing sensory feedback for limb prostheses. The detection threshold (DT), pain threshold (PT), just noticeable difference (JND), as well as the elicited sensation quality, comfort, intensity, and location were assessed in 16 healthy volunteers during stimulation of the ventral and dorsal forearm with subdermal electrodes. Moreover, the results were compared with those obtained from transcutaneous electrical stimulation. Despite a lower DT and PT, subdermal stimulation attained a greater relative dynamic range (i.e., PT/DT) and significantly smaller JNDs for stimulation amplitude. Muscle twitches and movements were more commonly elicited by surface stimulation, especially at the higher stimulation frequencies, whereas the pinprick sensation was more often reported with subdermal stimulation. Less comfort was perceived in subdermal stimulation of the ventral forearm at the highest tested stimulation frequency of 100 Hz. In summary, subdermal electrical stimulation was demonstrated to be able to produce similar sensation quality as transcutaneous stimulation and outperformed the latter in terms of energy efficiency and sensitivity. These results suggest that stimulation through implantable subdermal electrodes may lead to an efficient and compact sensory feedback system for substituting the lost sense in amputees.

This paper describes the implementation of two virtual reality (VR) games based on the traditional mirror box therapy (MBT) for lower limb amputees. The games are displayed in VR using a head mounted display (HMD) and two motion controllers attached to the intact leg. Two games were developed which both include a sitting and lying version with the purpose of testing the possibilities for enabling more freedom of leg movement. The games were tested on 8 healthy subjects to compare the sitting and lying versions. The results indicated that the lying position was preferred in one game, whereas the sitting position was preferred in the other game. This suggests that VR can enable more freedom of movement.

Phantom Limb Pain (PLP) affects many amputees and many conventional treatment methods have been proven unreliable. This paper describes the implementation of a PLP treatment system for upper-limb amputees, using Virtual Reality (VR) to create an illusion that they regain and can control both hands again, which may suppress PLP. The system uses motion capture sensors to track the movement of the intact hand and arm, and mirrors the hand movement to the amputated side in the Virtual Environment (VE). Electrical stimulation was used to provide haptic feedback. The system consists of three games: (1) a bending game where the user has to grab, move, bend and release a 'nunchuck' to encourage hand rotations, (2) a frequency discrimination game, where the user has to distinguish between haptic frequencies, by pressing different buttons, and (3) a location discrimination game, where the user has to distinguish between stimulation locations. The system was tested on three amputees with PLP over the span of three weeks. For two participants, the games encouraged the use of both intact and amputated hand while playing. These two participants also felt an increased control of the amputated limb.

This study aimed to investigate the human ability in discrimination of spatial and temporal parameters in electrocutaneous stimulation.Three surface electrodes were positioned on the ventral forearm of 14 able-bodied subjects.The subjects were instructed to discriminate between: (1) six different stimulation sites or site pairs, or (2) five different stimulation frequencies, or (3) hybrid parameters including both stimulation site and frequency, in three respective experiments.The results showed that two-site discrimination had a significantly lower success rate than one-site discrimination with a mean difference up to 12.1% (p<0.01).Temporal (frequency) discrimination appeared more challenging compared to spatial (site) discrimination.Moreover, the female subjects' performance was noticeably better than the males in all the three discrimination tasks with the mean difference up to 11.9% (p<0.01),15.4% (p<0.01), and 16.7% (p<0.001),respectively.The findings may provide an insight into building an effective sensory feedback strategy in relation to development of functional hand prostheses and treatment of phantom limb pain.

This paper describes the implementation of an affordable phantom limb pain (PLP) home treatment system using virtual reality (VR) and a motion sensor to immerse the users in a virtual environment (VE). The work is inspired by mirror therapy (MT) which is used to treat patients with PLP. This project focuses on phantom pain in amputees. Using a motion sensor, the system tracks the movement of a user's hand and translates it onto the virtual hand. The system consists of exercises focusing on opening and closing the hand, rotating the hand, and finer finger movements. These exercises are conveyed in the VR as three games: (1) A bending game, where the patients have to bend a rod, (2) a box game where the patients pick up and place boxes with their hands, (3) and a button memory game where the patients have to push buttons in a given sequence. These games were tested on twelve healthy participants to see if the games encouraged similar movements as in MT. Prior to the experiment a preliminary test was conducted on an amputee with PLP to gather qualitative feedback from an end-user. The results indicated that the games did convey the exercises from the MT, although further testing is needed.

This work investigated the gender effect on discrimination of surface electrical stimulation applied on the human forearm. Three experiments were conducted to examine the abilty of discriminating stimulation frequency, location, or both parameters in 14 healthy subjects. The results indicated a statistically significant impact of gender on the discrimination performance in all the three experiments (p <; 0.01, p <; 0.01, and p <; 0.001, respectively). The female group performed noticeably better than the male group (i.e., mean difference 15.4%, 11.9%, and 16.7% in repective experiment). The findings may provide evidence of gender difference in perceiving and interpreting electrical stimulation. Considering the gender difference may improve the efficacy of electrically evoked sensory feedback in applications such as prosthetic use and pain relief.

The need of a sensory feedback system that would improve users’ acceptance in prostheses is generally recognized. Feedback of hand opening and position are among the most important concerns of prosthetic users. To address the two concerns, this study investigated the human capability to identify pulse number and location when electrical stimulation applied on the forearm skin. The pulse number may potentially be used to encode the opening of prosthetic hands and stimulation location to encode finger position.

When applied in the rat model the Transverse Intrafascicular Multi-channel Electrode (TIME) showed selective nerve fascicle recruitment. But results from the larger and poly-fasicular median nerves in pigs indicated that a single TIME could not reach the entire nerve and could only selectively recruit a subset of the nerve fascicles. The use of multiple TIME structures could offer a means to achieve highly selective fascicular stimulation while reaching a larger percentage of the fascicles in the nerve. This work investigates this approach using pairs of TIMEs implanted in the median nerves of anesthesized pigs (n=6). TIME structures were implanted at different angles relative to each other or in parallel with one another. Electrical stimuli was passed through each contact of each TIME and the resulting electromyograms were recorded from seven muscles innervated by the median nerve. The ability to recruit these muscles was used to assess the stimulation selectivity of each contact using a selectivity index comparing the root-mean-square of the the evoked EMG of individual muscles. Results showed a significant increase in the selectivity index, when using two TIMEs compared to one. The optimal improvement was observed when TIMEs were placed in parallel to each other in such a way that they interfaced non-overlapping nerve regions.

Few studies are available in the literature on the sensations artificially created by dual-channel electrocutaneous stimulation. This study assessed the effect of a set of selected stimulation parameters on the sensations evoked by single- or dual-channel electrocutaneous stimulation. The investigated parameters included the stimulation site, the number of pulses, the number of stimulating channels (single- vs dual-channel), and the interleaved time between two channels. The modality, quality, location, and magnitude of the sensations were evaluated when the stimulations were applied on the forearm skin in 16 nondisabled subjects. Tactile perception was found to be induced more easily on the median and ulnar aspect than the dorsal and radial aspect of the forearm. Stimulation site significantly affected the magnitude of the sensation (p < 0.01). Dual-channel stimulation significantly increased the sensation magnitude (p < 0.05) only when the two electrodes were positioned closely. Moreover, a higher number of pulses evoked a movement perception more frequently and the interleaved time showed no significant effect on the magnitude of the sensation. The findings are expected to be useful for sensory substitution and augmentation applications. The results may also help improve users' acceptance of hand prostheses.

Background Consistency is one of the most important concerns to convey stable artificially induced sensory feedback. However, the constancy of perceived sensations cannot be guaranteed, as the artificially evoked sensation is a function of the interaction of stimulation parameters. The hypothesis of this study is that the selected stimulation parameters in multi-electrode cutaneous stimulation have significant impacts on the perception threshold. Methods The investigated parameters included the stimulated location, the number of active electrodes, the number of pulses, and the interleaved time between a pair of electrodes. Biphasic, rectangular pulses were applied via five surface electrodes placed on the forearm of 12 healthy subjects. Results Our main findings were: 1) the perception thresholds at the five stimulated locations were significantly different (p < 0.0001), 2) dual-channel simultaneous stimulation lowered the perception thresholds and led to smaller variance in perception thresholds compared to single-channel stimulation, 3) the perception threshold was inversely related to the number of pulses, and 4) the perception threshold increased with increasing interleaved time when the interleaved time between two electrodes was below 500 μs. Conclusions To maintain a consistent perception threshold, our findings indicate that dual-channel simultaneous stimulation with at least five pulses should be used, and that the interleaved time between two electrodes should be longer than 500 μs. We believe that these findings have implications for design of reliable sensory feedback codes.