Neurostimulation Perception Obeys Strength-Duration Curves and is Primarily Driven by Pulse Amplitude

Stimulation of peripheral nerves can elicit sensations that are felt on distal or amputated portions of the limb, and thus is a promising technique to provide sensory feedback for prosthetic limbs. Sensory feedback provided in this way can confer a sense of proportionality by modulating the frequency, amplitude, and duration of stimulation pulses, however the relationship between stimulation amplitude and pulse duration has not been characterized. In this study, we demonstrate that neurostimulation perception closely follows strength-duration curve models and are generally constant over the course of up to 24 months, with a median rheobasic current of 113 $\mu \mathbf{A}$ and chronaxie of 193 $\mu\mathrm{s}$. Monotonicity and concavity of data are also demonstrated to significantly predict the confidence interval size for rheobase and chronaxie estimates. Goodness of fit for the strength-duration curve model was high for data which showed significant monotonicity. Furthermore, modeling the psychometric response of stimulation amplitude and duration modulation revealed that amplitude modulation has just-noticeable difference of 7.7%, less than half that of duration modulation at 18.3%. The results taken together suggest that the strength-duration curve framework describes both nerve excitation and perception threshold relationships, and that neurostimulation pulse amplitude primarily drives discrimination for modulating sensory feedback.