STUDIES ON PAIN. A NEW METHOD FOR MEASURING PAIN THRESHOLD: OBSERVATIONS ON SPATIAL SUMMATION OF PAIN

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Abstract
A new method is described for quantitatively measuring the pain threshold. The light from a 1000 Watt bulb, the intensity of which was regulated by a rheostat, was focussed by means of a lens and allowed to fall for exactly 3 secs. on 3.5 cm.2 of blackened forehead of the subject. The intensity of this non-penetrating thermal radiation which barely evoked pain was denoted as the threshold intensity and was measured by a Hardy radiometer and expressed in absolute units of g. calories per sec. per cm.2 Individual threshold measurements for 3 subjects were 0529, 0.231, and 0.233 g. cal./sec./cm.2 and all measurements were within [plus or minus] 12% of their respective avg. values. The standard deviation for a single measurement was calculated to be [plus or minus]2%. Pain thresholds measured in this way did not vary consistently with the time of day, the general effectiveness, or the emotional state of the subjects. The intensity of radiation which produced blistering in 3 secs. was observed to be only twice that which barely elicited pain. The peripheral structures responsible for pain sense were distinguished from those of temp. and touch by demonstrating that occluding the blood for 25 mins. by means of a pressure cuff on the forearm did not directly affect the pain threshold in the ischemic hand, whereas other sensations could hardly be elicited. Also, the senses of pain and heat which were always stimulated together were shown to be separate sensations through the action of acetylsalicylic acid, which lowered the heat threshold, and raised the pain threshold. Intense pain in any part of the body raised the pain threshold in the skin in other parts 35%. Pain sense had no spatial summation in the sense that the pain threshold for many end organs was no lower than that for a few. This was the case for minimal stimuli and for supra-minimal stimuli after morphine adm.