Prick Threshold Stimulation with Square-Wave Current

Threshold sensory stimulation of the skin has become the object of increasingly intensive investigation since the classic report by Hardy, Wolff and Goodell' on the pain threshold of thermal radiation. The literature on the subject is rapidly becoming voluminous with interesting ramifications appearing in many fields of medicine. More recently Cormia3 reported on the itch threshold in experimental histamine pruritus and still more recently Benjamin and Ivy' have described the flare threshold associated with the pain of thermal radiation. The purpose of this communication is to report studies on the prick threshold of the skin and its measurement with squarewave stimulation. In previous studies I have found' that threshold cutaneous sensory stimulation could be achieved with square-wave current, a type which is essentially interrupted, unidirectional, and produces repetitive impulses. The frequency, duration, and strength of the impulses are regulated by the square-wave stimulator to provide threshold stimulation. It was also found that the size of the active electrode must be adequate (about two inches square) to cover a sufficient number of sensory units in order to produce consistent responses; and in person-to-person comparisons the same body site should be stimulated, since sensory responses are not uniform over the body. Bishop2 has shown that the prick fibers of the skin are highly versatile, mediating sensations which vary in quality from tactile and pressure sensations to frank pain, depending upon the intensity of the electrical stimulation; also that prick fibers have a lower threshold to electrical stimulation than those of touch fibers. It is thus clear that minimal electrical stimulation of the skin is necessarily prick threshold stimulation and that the resulting responses are the weakest mediated by the prick fibers. In attempting to measure and standardize the prick threshold of the skin the chief problems were: