Referred Sensation Using Surface Electrical Stimulation
Dr. Katherine Polasek
Phantom limb pain is a post-amputation phenomenon resulting in pain and/or extreme discomfort in the missing limb of amputees. Since it is hypothesized that new or strengthened connections from adjacent sensory regions in the brain may prompt neurons to fire pain signals1, the stimulation of these target sensory regions may reduce or eliminate the painful sensations. When electrical stimulation is used threshold values can be described using a stimulus amplitude and duration relationship called a strength-duration curve. This study examined the ability of the strength-duration curve to describe threshold values for referred sensation through surface electrical stimulation. The median and ulnar nerves in the left arm of able-bodied adults were stimulated. An optically isolated biostimulator and MATLAB software were used to provide voltage controlled non-symmetric square wave pulse trains. Threshold values for hand sensation were determined using the adaptive procedure Parameter Estimation by Sequential Testing (PEST)2. Strength-duration curves were determined using Weiss’s formula3. Curve characteristics were compared across and between subjects. All 25 participants achieved distal sensation as a result of surface electrical stimulation at the elbow. Variability found between subjects did not allow for generalizations to be made. Location and type of sensation achieved also varied based on stimulation parameters and arm position. However, a subject specific strength-duration curve provided reliable predictions regarding stimulation parameters that will elicit distal sensation. An analysis of the decrease in error with each additional point suggested two repetitions of five points would allow for adequate recreation of the strength-duration curve in less than five minutes. Activation threshold for referred sensation can be found quickly and reliably for each subject using ten points. These results will be used in a future study to determine the location and specific type of sensation that can be obtained using surface stimulation with the overall goal aiming to reduce the occurrence of phantom limb pain.
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