558 Sensory Masking 



of which were published in 1935. Akhough both sets of experiments were 

 essentially similar (with respect to the points under discussion), we were in- 

 terested in the concomitant tingling sensation, Lewis in the subsequent hyper- 

 algesia. We did not observe such hyperalgesia, perhaps because of the kind 

 of electric stimulus that we used, but more probably because we were not 

 looking for it. Lewis ascribes this hyperalgesia to liberation of a chemical in 

 the skin supplied by the nerve stimulated, supporting this by an appeal to a 

 statement of Foerster's that stimulation of the distal segment of a divided 

 cutaneous nerve produced pain, relieved by section of overlapping nerves. 

 So far as I know, this statement of Foerster's awaits confirmation. It would 

 lead one to expect sensation upon stimulation distal to a block, but neither 

 in our experiments nor iji Lewis's was this observed, with the following trivial 

 exceptions. (1) Lewis observed a little sense of burning accompanying the 

 /;o5^stimldatory hyperalgesia described by him. (2) We sometimes found that 

 when the stimulus applied distal to a procaine block was very strong, tingling 

 was felt everywhere distally except in the area supplied exclusively by the 

 blocked nerve under stimulation. We call this a "negative picture" of tingling; 

 but, since this tingling was widespread, it may be ascribed simply to spread of 

 the strong current to the other nerve trunks. In my opinion, Foerster's state- 

 ment should be ignored in this connection, at least for the present. Whatever 

 may be true of the subsequent hyperalgesia, it seems clear that the tingling 

 sensation accompanying electrical stimulation of cutaneous nerves does not 

 depend upon the peripheral liberation of a chemical, since it occurred upon 

 stimulation proximal to a complete block. Experiments under the condition 

 of partial block were interesting. If cutaneous tests showed that only the 

 (small) temperature and pain fibers were blocked, distal stimulation evoked 

 tingling indistinguishable from that accompanying stimulation of normal 

 nerve; hence the sensation may be attributed principally to stimulation of 

 the unanesthetized (large) touch and pressure fibers. The idea that the tingling 

 upon electric stimulation of nerve fibers does not depend upon end-organs 

 is further siq^ported by observations made in 1934 by two other assistants of 

 mine (R. C. Combs and H. S. Kimball): they found that normal throbbing 

 could be felt in a cutaneous area completely anesthetized by freezing with an 

 ether spray (radial area, once in each of five subjects). 



It seems clear that the tingling sensation accompanying stimulation of a 

 cutaneous nerve depends upon bombardment of the sensorium by impulses 

 ascending from the electrode, the sensation being "projected" by the brain 

 into the cutaneous distribution of the nerve stimidated; in other words, this 

 form of tingling is a central phenomenon, resulting from impingement of 

 nerve impulses upon the sensorium. To some this may seem so obvious as 

 scarcely to require proof; but in view of our finding that the type of masking 

 under consideration is invariably associated with tingling, it has seemed de- 

 sirable thus to prove to the hilt that this sensation depends upon a mechanism 



