Elevating pain thresholds in humans using depolarizing prepulses

Electrocutaneous stimulation is a potentially useful communication tool for applications in virtual reality, sensory substitution, and sensory augmentation. Many of these applications require the use of arrays of small electrodes. Stimulation through small electrodes is often painful, however, limiting the practicality of such arrays. The purpose of this study was to test a method for elevating the pain threshold to electrocutaneous stimulation through small (1-mm diameter) electrodes on the fingertip. We hypothesized that long, subthreshold, depolarizing prepulses (PP) would elevate the pain threshold so that a subsequent stimulus pulse (SP) would be less likely to be painful. We used psychophysical methods to measure the probability that an SP would be perceived as painful both by itself and when preceded by a PP that was 2, 4, 6, 8, or 10 dB lower in amplitude than the SP. We found that the PPs significantly increased the pain threshold, reducing the likelihood that the SP was painful (p < .0001). The dose effect of PP amplitude was also highly significant (p < .0001), with larger PPs elevating pain thresholds more. To our knowledge, this is the first report of PPs being used to elevate electrical stimulation thresholds in humans. PPs may be useful for selective inactivation of neural subpopulations in many human neuroprosthetic applications.