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Electrocutaneous stimulation via bipolar current pulses : models and experiments Bütikofer, Rudolf

Abstract

Mathematical models have been used to study the effects of various electrical stimuli on nerve. The applicability of the findings to electrocutaneous stimulation in man has been investigated experimentally. With the model of a nerve membrane the influence of variations in the stimulus parameters have been investigated. This was done for multiple bipolar pulses by considering selected cases and for single bipolar pulses with a systematic investigation. The main findings were i) that the threshold charge for a single bipolar pulses changes only slightly for different pulsewidths; ii) that the threshold charge monotonically decreases with pulsewidth and threshold charge also decreases monotonically with increasing delay of the symmetric negative pulse; iii) that threshold amplitude for multiple bipolar pulses was only slightly lower than the amplitude for a single bipolar pulse. The influence of different components involved in cutaneous stimulation, such as skin, electrode, and neuroanatomy, have been examined. Corresponding models for the passive components involved were selected. From these models the following limiting conditions for the stimulus were derived: i) the stimulus has to be current regulated; ii) it must be bipolar (no net charge transfer); and iii) the electrode voltage must remain below the skin break-down voltage. The aspect of the conversion of stimulus' energy into heat in the skin has been examined in detail. A review of mathematical models of the active nerve membrane is presented and the applicability of a nerve model to the stimulation of peripheral nerve fibres in man is discussed. Numerical methods were used to solve the model's differential equations. The effects on the solution of different integration methods and of different integration step sizes has been assessed. Experiments with electrocutaneous stimulation have been performed using single bipolar current stimuli. The duration of a pulse was less than 100 microseconds. For the experiments, an electrically isolated stimulator has been designed and built. It operated under the control of a PDP-12 computer. The sensations produced were slightly suprathreshold and painless. The thenar region of the hand was stimulated using a concentric electrode. The results of the experiments supported the theoretical predictions and indicated the possibility of using models to investigate the optimization of stimulus parameters within the range tested. The close correspondence between the experimental results and the nerve model calculations seems to provide some evidence for the hypothesis that in electrocutaneous stimulation the nerve fibres are stimulated directly.

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