An investigation of the gate control theory of pain using the experimental pain stimulus of potassium iontophoresis.

This study investigated a prediction derived from gate control theory-that there would be a pulse of pain as a pain stimulus was being ramped off due to the rapidly transmitting, inhibitory large fiber activity falling away sooner at the spinal level than the excitatory activity of the slow-transmitting, small nociceptive afferents. A further prediction was that the more distant the peripheral stimulus was from the spine, the greater the pain pulse would be. Fourteen subjects had the pain stimulus of iontophoretically applied potassium ions (K+) applied to an upper and a lower site on the dominant arm. In a threshold detection task using the double random staircase method, subjects were asked to indicate whether they could detect a pulse of additional pain during this ramp-off phase. The average rate of stimulus ramp-off in order to detect a pain pulse was statistically greater for the upper-arm site (14.3 micrograms K+/sec) than for the lower-arm site (9.4 micrograms K+/sec). These results were consistent with gate control theory. Alternative explanations, including intrinsic differences in nociceptive responding for different dermatomes and anode break, were considered. It was concluded that the detection of a pain pulse during the ramping off of a peripheral pain stimulus potentially provides a quantitative measure of the spinal modulation of pain.