Abstract

This review covers 106 years of experimental work. Hardly an item has been mentioned for which there have not been opposing data to be considered. This fact has required a rather formidable length of presentation of data. Every effort has been made to give opposing views fairly. But where it is possible to do so, conclusions must be drawn if progress is to be made. The reviewer has set down the conclusions he believes are warranted by the data, but references to the text and, in the text, numerous references to original sources are given, so that the reader can consult the basis for the conclusions stated and arrive at better ones if he can.

1) Pain cannot be satisfactorily defined, except as every man defines it introspectively for himself (II).

2) Pain sensations and pain perceptions are identical. Neither represents the "original sensation" alone but represents also an indefinite amount of psychic processing or reaction component (IX).

3) No convincing demonstration has yet been given that the pain threshold is a constant from man to man, or from one time to another in a given man (VIII).

4) More than a score of factors are ssid to produce variations in the pain threshold. Not a single experimental study has controlled even the majority of these factors. Conclusions concerning pain threshold must therefore be tentative (X).

5) "Experimental" pain and "pathological" pain are both composed of "original sensation" and the psychic processing of the original sensation (XII, B). The results of this processing are synonymous with the psychic reaction component (XII). The two components have not yet been satisfactorily separated experimentally (XII, B). Pain from the two origins differs greatly in the quantitative representation of the two components (XII, B). It is essential that these diffrences be taken into account when scientific study of pain or pain relief is undertaken.

6) The experimental pain techniques at present generally employed in man, while useful for some purposes, are probably useless for the appraisal of the analgesic agents (XI, D, E, F). The same techniques in animals have definite usefulness with the powerful narcotics (XI, G), but none apparently with the acetylsalicylic acid class of compounds (XI, F).

7) Assay of analgesic power can be carried on with less than a 10% error when pathological pain is employed in man provided one works in the steep part of the dose response curve (V, B, 2, a, 3).

8) Techniques for the appraisal of side action liability in sick individuals have not yet been satisfactorily developed and established (V, B, 2, g).

9) No dependable relationship has been established between the action of analgesic agents and the experimental pain threshold in man (XI). The record is better for animals but still far from perfect (XI, G). Uncritical acceptance of the view that a dependable relationship exists in man has done much to confuse and mislead work on pain.

10) Analgesic agents appear to exert their principal, if not entire, effect on the "reaction component" rather than on the "original sensation" (XII). This is perhaps at once the most striking and most surprising concept to come out of this long study. If this view can be further substantiated and if it applies also to other subjective reaponses as well as to pain, and this appears to be the case, then acceptance of this concept will require a wide shift in therapeutic planning. Heretofore the goal has been to dull the "original sensation." Strong evidence has been presented to direct future therapeutic research to modification of the psychic reaction to the original sensation. Here is a promising area for further experimental attack.

11) Quantitative work with pain is possible and rewarding. Experience in this area has already as a prototype to guide work with other subjective responses. Quantitative study of the psychological effects of drugs is an urgent need; such work is properly a part of pharmacology. The possibility of accurate quantitative work in this field has been demonstrated; but even so, accomplishments to data constitute no more than a beginning in what promises to be a great development in pharmacology. Successful pursuit of studies in this field is basic to the sound growth of the behavioral sciences.