Abstract
Recent studies have shown that direct nerve stimulation can be used to provide sensory feedback to hand amputees. The intensity of the elicited sensations can be modulated using the amplitude or frequency of the injected stimuli. However, a comprehensive comparison of the effects of these two encoding strategies on the amputees’ ability to control a prosthesis has not been performed. In this paper, we assessed the performance of two trans-radial amputees controlling a myoelectric hand prosthesis while receiving grip force sensory feedback encoded using either linear modulation of amplitude (LAM) or linear modulation of frequency (LFM) of direct nerve stimulation (namely, bidirectional prostheses). Both subjects achieved similar and significantly above-chance performance when they were asked to exploit LAM or LFM in different tasks. The feedbacks allowed them to discriminate, during manipulation through the robotic hand, objects of different compliances and shapes or different placements on the prosthesis. Similar high performances were obtained when they were asked to apply different levels of force in a random order on a dynamometer using LAM or LFM. In contrast, only the LAM strategy allowed the subjects to continuously modulate the grip pressure on the dynamometer. Furthermore, when long-lasting trains of stimulation were delivered, LFM strategy generated a very fast adaptation phenomenon in the subjects, which caused them to stop perceiving the restored sensations. Both encoding approaches were perceived as very different from the touch feelings of the healthy limb (natural). These results suggest that the choice of specific sensory feedback encodings can have an effect on user performance while grasping. In addition, our results invite the development of new approaches to provide more natural sensory feelings to the users, which could be addressed by a more biomimetic strategy in the future.
Highlights
In the recent past, a major effort has been dedicated to the use of implantable peripheral interfaces to stimulate the residual nerves of upper limb amputees in order to restore sensations in their phantom hand[1,2,3,4,5,6,7,8,9,10,11,12]
Sensory feedback has mainly relied on two encoding strategies for sensory feedback, which are the linear modulation of either the charge[3,5,7,9] or the frequency[1,3,6,8,10] of the stimulus pulses
When we tested linear frequency modulation (LFM), we found that adaptation time was prolonged only when low frequency pulses were delivered (Fig. 3B): adaptation time for Subject 1 ranged from 460 ± 101 s for low frequencies to 40 ± 12 s for high frequencies, a 91% reduction (Kruskal-Wallis, p < 0.01)
Summary
A major effort has been dedicated to the use of implantable peripheral interfaces to stimulate the residual nerves of upper limb amputees in order to restore sensations in their phantom hand[1,2,3,4,5,6,7,8,9,10,11,12]. A recent study, performed on two trans-radial amputees, showed that both the direct neural modulation of charge (i.e., the amplitude or the pulse duration) and of frequency controlled the intensity of the evoked sensations reported by the subjects[3]. This result is in accordance with the physiology of afferent fibres (according to the population model14,15), which deliver information about the intensity of a sensation to the brain through population recruitment (i.e., more spiking fibres) or changes in firing activity[14,16,17]. We compared the ability of the subjects to control the bidirectional hand prosthesis during the execution of functional tasks
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