Available evidence suggests that the sensation of itching is closely related to that of pain (1). Titchener (2) observed that when the skin was explored with a fine hair, well-defined points were found which gave rise to itching when the intensity of stimulation was low, and to pain on stronger stimulation. Bishop (3) found that itching resulted from repetitive low intensity electrical stimulation of pain spots in the skin. Lewis, Grant and Marvin (4) pointed out that noxious stimuli, if their intensity be decreased, can be made to produce itching instead of pain. Forster (5) and Bickford (6) reported that in patients who had undergone section of the lateral spinothalamic tracts, thus abolishing perception of cutaneous pain but not of touch, itch was also abolished. Bickford also observed this combination of sensory changes in two patients with syringomyelia. Ehrenwald and K6nigstein (7), however, stated that they had found two cases of syringomyelia with loss of pain but preservation of touch and itch, and one in which pain was intact but touch and itch absent. McMurray (8) and Kunkle and Chapman (9) found that individuals who were constitutionally insensitive to pain were likewise insensitive to itching, but they experienced touch sensation. Bickford observed that itching and pain disappeared at the same time when a cutaneous nerve was anesthetized. Hardy, Wolff and Goodell (10) observed that in addition to feelings of warmth, occasional itching resulted from sustained thermal irradiation at an intensity not quite strong enough to induce pain. Th6le (11) found during spinal anaesthesia, as the sensitivity of the skin to painful stimuli decreased, that stimuli which initially induced pain later induced itching and finally no sensation at all.In the present investigation many of the experiments described by Goldscheider (12), Bickford, and Lewis and associates have been repeated. Additional experiments were designed to illumi-37 nate further the neural mechanisms involved in itch, "itchy skin" and tickle sensation.
ITCH Subjects and methodsThe subjects were healthy adults, chiefly the authors, although other individuals participated from time to time. Itching was elicited by the application of cowhage to an area of sldn approximately 1 cm. in diameter. Cowhage is the familiar "itch powder," consisting of the fine fibers or spicules of the plant Mucuna pruriens. It was found by trial that the itch so produced was indistinguishable from that following the bites of mosquitoes, or the intracutaneous injection of histamine. When an area of sldn on the arm, hand, leg or back was used for experimental procedures the corresponding areas on the opposite side of the body served as control.Description of sensation induced following application of cowhage to the skin Cowhage applied to an area of skin approximately 1 cm. in diameter induced, usually within 10-20 seconds, an intense itching sensation which had both burning and pricking qualities. It was localized but seemed to spread for a few millimeters beyond the borders...
