Comparison of rectangular and exponential current pulses for evoking sensation.

There exists a paucity of quantitative information comparing the stimulating currents for different waveforms. Thus, the objectives of this study was to compare the threshold peak current (I) for sensation using rectangular and exponential; that is, capacitor-discharge, cathodal pulses of equivalent pulse duration (d). In 10 human subjects, stimuli were applied to the skin of the forearm, and I was determined alternately for each current waveform at each of several pulse durations (d). Strength-duration curves for sensation were obtained using d of 0.01-50 ms. The threshold peak current (Ir) for a rectangular pulse of duration d was compared to the threshold peak current (Icd) for a capacitor-discharge pulse of duration d, where d was the time constant; that is, the time required for the current to decrease to 1/e, or 37% of its peak value. Chronaxie, the pulse duration at which I is twice the infinite-duration current asymptote (i.e., the rheobase), was calculated for each waveform and subject using the Weiss-Lapicque expression for excitability. Icd was found to be always higher than Ir of equivalent duration. Chronaxie for the capacitor-discharge pulse was, on the average, twice that for the rectangular pulse (p less than 0.01). Moreover, the ratio Icd/Ir increased with decreasing d. These results indicate that these two waveforms are not equivalent on the basis of an equal-charge requirement for excitation, particularly at the short pulse durations. Furthermore, they suggest the need of a better expression to describe the excitability characteristics of tissues.