Experimental changes in pain threshold and severe pain threshold for electrically induced pain

Pain and severe pain thresholds were measured in groups of 15 (Experiment 1) and 20 (Experiment 2) normal volunteers. In Experiment 1, the subjects underwent a series of ten constant level, painful stimuli each of 10 sec duration at an arbitrarily chosen level between the thresholds. This level was recorded. It ranged from 14% to 81% of the difference between thresholds. The thresholds were then remeasured. For the second experiment the experimental stimulus commenced at a non-painful level and increased over a period of 5 sec to a level midway between thresholds for a series of three stimuli and again thresholds were remeasured. An overall effect was demonstrable only in Experiment 2, where the severe pain threshold was significantly reduced (p less than 0.001). There was a marked individual variation with increases and decreases in both thresholds occurring in different individuals at significance levels varying from p less than 0.05 to p less than 0.001. No subject changed the two thresholds in opposite directions. Examination of the responses in Experiment 1 suggested that for any increase in either threshold to occur it was necessary that the repeated painful stimulus should be at a level below the mid-point between the two thresholds, but that this was not a sufficient condition.