Optimal electrotactile stimulation waveforms for human information display

Electrotactile (electrocutaneous) stimulation can deliver visual, auditory, and remote tactile information to the skin by directly stimulating afferent touch nerve fibers in a localized region. We review sensory physiology, the mechanism of electrotactile stimulation and its limitations, and several applications. A 16-channel custom electrotactile stimulation system produces rectangular current pulses with current 0 - 50 mA, interphase interval 0 - 1000 μs, number of pulses/burst 1 - 100, pulse repetition rate 0.1 - 25 kHz, phase width 2 - 1000 μs, and functionally-monophasic (zero dc current) or balanced-biphasic pulse type. Controlled by a command file and by analog inputs, the system automatically delivers the specified current waveform to each electrode through a high-performance transconductance amplifier, prompts the subject, and logs subject responses. A new method to measure the electrotactile dynamic range determines the waveform variables that maximize the subjective magnitude (intensity) of the electrotactile percept at the maximal current without discomfort. The magnitude dynamic range is maximized with number of pulses/burst= 6, pulse repetition rate within a burst = 350 Hz, and phase width = 150 μs. Six pulses/burst doubles the magnitude-based dynamic range compared with one pulse/burst but has little effect on the traditional dynamic range measure - the ratio of pain threshold to sensation threshold.National Institutes of Health, grant NS26328. Wisconsin Center for Space Automation and Robotics (Madison, WI). VA Rehabilitation Research and Development, grant to J. Wertsch