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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



reeling the heat to an area of the forehead thoroughly 

 blackened with India ink and obtaining values which 

 all fell within ± 1 2 per cent of the mean. They describe 

 the subject's experience as follows: "The sensation is 

 one of warmth, heat and burning which seems to 

 "swell' and then to "draw together' into a prick at the 

 end of the third second. Minimal after-sensations of 

 heat and burning pain are common." This they call 

 the pricking pain threshold. Bigelovv et al. (26) also 

 identified another threshold — that for "burning pain.' 

 This on the forehead is, they say, about 20 to 30 meal, 

 lower than the pricking pain threshold. When these 

 workers extended their measurements to 1 50 individu- 

 als about the same mean intensity of stimulus, 0.21 gm 

 cal. per sec. per cm-, evoked ' pricking pain' with a 

 maximal variation of ± 1 5 per cent. 



This uniformity is said to persist throughout a 24 hr. 

 period of enforced wakefulness (241) in women before, 

 during and after labor (135), and over much of the 

 body surface (i 18). But Wolfl & Goodell (304) also 

 state that, if the subject is unable to concentrate on the 

 testing procedure or "to maintain a detached, un- 

 prejudiced attitude" because of fatigue, lethargy, sug- 

 gestibility or other reason, then the pain threshold 

 varies greatly and is unpredictable. 



A number of workers have been unable to confirm 

 the described uniformity of pain threshold in these 

 painfully collected data. Chapman & Jones (40) 

 found the theshold of 200 normal subjects to vary 

 much more widely — from —40 per cent to -I-50 per 

 cent. Others who have reported inconstancy of 

 thresholds by this technique include Clausen & King 

 (44), Leduc & Slaughter (160), Schamp & Schamp 

 (237) and Slaughter & Wright (252). Benjamin (20) 

 agreed that the tactic of painting the skin with India 

 ink resulted in absorption of nearly all (94 per cent) 

 of the incident heat and it^ transmission into the skin 

 by conduction, but found pain sensitivits' in the palm 

 less than in the forearm. Whyte (297} has advanced 

 this cogent criticism: the validity of the method de- 

 pends on the contention of Oppel & Hardy (203) that 

 the rise in skin temperature produced by radiant heat 

 is proportional to thermal intensity. If this were true, 

 then at the increa.sed radiant heat thresholds for 

 morphine described by Wolff ?< al. (306) extrapolation 

 of their curves would indicate that the skin tempera- 

 tures would have reached about 54°C. The actual fore- 

 head temperatures of Whyte's subjects at the pricking 

 pain threshold were about the same before and after 

 morphine, ranging from 46.2 to 47.5°C. From this 

 Whyte logically concludes that an increase in sweating 



or in blood flow to the skin may have occurred follow- 

 ing the morphine rather than the presumed increase 

 in nervous threshold. Beecher and associates have also 

 been sharp critics of the contention that consistent 

 thresholds are obtainable by the method. Denton & 

 Beecher (63) found that an operator widely experi- 

 enced in the radiant heat method, who was called in 

 to correct their failure to get consistent data, was able 

 to do so as long as he knew what drug had been ad- 

 ministered; he failed when he did not know. They 

 interpreted this to be a consequence of unconscious 

 guidance by the operator of the subjective response he 

 was seeking. Much space has been devoted to con- 

 sideration of this single technique in order to point out 

 the difhculties of precise measurement of a sensation 

 as threatening to the individual as pain. 



Electricity 



Electrical stimulation has also been used to test 

 cutaneous sensation. The threshold feeling, according 

 to Bishop (27), varies depending upon the special 

 sensitivity of the skin spot tested. Using a condenser 

 discharge deli\'ering a spark when the point of the 

 stimulating needle was about 0.5 mm from the .skin, 

 he was able to stimulate without mechanical contact. 

 With this device he found spots "mediating ordinary 

 touch or light pressure" and others inducing the sensa- 

 tion of prick "which becomes pricking pain on a 

 stronger stimulation." But he says that a "single 

 stimulus applied to a single prick spot at the threshold 

 is not painful but elicits a tactile experience usually 

 accompanied by a faint aura of itch. This tactile 

 sensation is not associated with a feeling of pressure. 

 . . . On the other hand a single threshold stimulus 

 applied to a touch ending is experienced as a slight 

 tap." Distinctions between a tactile sensation with and 

 one without pressure, and between a prick and prick- 

 ing pain, certainly do not lend themselves to quantita- 

 tive analysis. Mueller et al. (ii^i), seeking to develop 

 an electrical method for the testing of pain threshold, 

 found the most clear-cut end point to be the sensation 

 of prick but, in critical electrical measurements, they 

 found the prick to occur during breakdown of skin 

 impedance .so that the electrical quantity was not 

 purely an index of threshold pain but was dependent 

 on the dielectric properties of the skin. Beecher (16) 

 after a thorough re\ iew of the whole proijlein of the 

 measurement of pain of both 'experimental' and 

 'pathological' types concludes that "no con\incing 

 demonstration has yet been given that the pain 



