Surface Electrical Stimulation to Evoke Referred Sensation

INTRODUCTION Surface electrical stimulation (SES) is a noninvasive method to interface with the nervous system. When used to target the nerve trunk, SES can activate muscles and/or sensations in locations other than the point of stimulation. For example, stimulation of the median nerve at the elbow could produce muscle activation and/or sensation in the hand. Sensations obtained using SES are often described as tingling or prickling (paresthesias). In an individual with an amputation, these paresthesias can be felt in the missing limb and are being investigated as a treatment for phantom limb pain [1]. An improvement in the SES technique to produce natural sensations such as touch may improve the treatment for phantom limb pain as well as provide a noninvasive method to deliver sensory feedback. There are multiple methods under investigation to restore sensation to prosthetic users. Sensory substitution methods use electrocutaneous stimulation [2-3] or vibrotactile stimulation [4] to translate information such as grip force to the residual limb of the user. Haptic methods employ mechanisms such as force applicators [5] or pneumatic bladders [6] to provide a more realistic sensation of touch or pressure, but still on the residual limb. To obtain sensation that is referred to the missing limb, force applicators have been used on skin surgically reinnervated by nerves of the hand [7]. Electrical stimulation of the residual nerves has also been used to elicit referred sensation in the missing limb. The first implanted system consisted of an electrode placed near the median nerve and successfully provided sensations as if they were coming from the hand [8]. Improvements in technology have made an implanted system more feasible, and several groups have been re-evaluating the use of implanted nerve stimulation to communicate sensory information. Using wires inserted into the nerve, tactile-like sensations have been produced that change in magnitude and location based on the stimulation [9-11]. In addition, Tan et al. used nerve cuff electrodes to produce sensations at different locations in the hand through extraneural stimulation and found that nonstandard waveforms can elicit touch-like sensations without paresthesias [12]. The goal of this research is to develop a noninvasive method to elicit natural, referred sensation. This article reports preliminary work focused on defining the parameter space. Threshold values for electrical stimulation can be described using a stimulus amplitude and duration relationship called a strength-duration curve. Prior studies investigating the strength-duration curve for SES of sensory fibers used compound action potentials to quantify the response [13-14] rather than reported referred sensation. Establishing this baseline relationship for referred sensation was an important first step to looking at the ability of electrical stimulation to produce natural sensations. In this article, we report the testing of three hypotheses. First, SES at the elbow can produce referred sensation in the hand without causing discomfort in other locations. Second, the strength-duration curve can be found using reported sensation as a measure of threshold rather than a physiological measurement. Third, it is possible to produce a natural sensation using SES. METHODS Experimental Setup Subjects between the ages of 18 and 65 were recruited from the Hope College campus community. The protocol was approved by the Hope College Human Subject Review Board, and all subjects gave written informed consent. Each study session lasted from 45 to 90 min, and subjects were compensated for their time. To look at variability between sessions, we tested 10 of the subjects at a second session, at least a week later than their first visit. Prior to their scheduled session, subjects were instructed to drink plenty of fluids to reduce their skin impedance. They were also given a bottle of water to drink during the trial. …