A ‘NORMAL’ PAIN RESPONSE: PAIN THRESHOLDS IN NORMAL VOLUNTEERS
When faced with an uncomfortable puzzle such as pain, where stimulus and response seem so strangely connected, we have learnt over the past two centuries to mobilize science in the belief that it will reveal order in chaos. In psychophysics, the relevant branch of science, a measured controlled pain-producing stimulus is given as heat, cold, pressure or electricity. The pain response of a normal volunteer is measured on some scale or by their physiological changes in a precisely controlled environment.
This sounds like 'real' science and it is true that the results are reproducible. There are, however, three caveats which we must consider if these results are to be applicable to pains that occur in ourselves or that we observe in the clinic. First, it is obvious that the experimenter and the subject are certain that the stimulus will not produce any prolonged injury or pain. This is a necessary restriction but makes it an artificial pain outside normal experience, where pain comes together with a packet of worry and doubt. Second, the subjects are assured by the scientist that they are able to stop the pain at any instant when they decide it is not tolerable. What is being measured is pain without suffering that can be instantly abolished. We should all be so lucky if that was the type of pain we wish to understand. Finally, there is a problem that applies equally to patients and to volunteers: they are both individuals and they are members of a culture.
Despite these caveats, the measurement of pain in these circumstances has been carried out in thousands of trials.
Inherent in these trials is the concept of a pure sensation of pain liberated from perceptions and meanings. Many believe such a sensation exists. I do not. I think it an artifice generated in part by our dualistic culture and confirmed by the experimental design in which the subject has agreed to play the Cartesian game. What are the results?
A disciplined, attentive, well-trained group of volunteers report on the amount of pain they feel gradually rising from barely perceived to the intolerable limit. The curve of their rising pain is consistent and repeatable within the group. The onset of pain is the same between groups. The problem starts with the upper limit. A group of Harvard students set their upper limit of tolerable stimulus far below a group in Munich. In spite of identical experiments and the same instructions, the group had collectively decided on the upper limit which must presumably have depended on the society within which they lived. These experiments have been repeated many times and each seems to give a lawful relation between the stimulus and the response, but toleration shifts between groups. The person conducting the experiment has an unintentioned effect on the tolerance. The results differ depending on whether the person applying the stimulus is male or female, a professor, a technician or a fellow student.
In some experiments, a deliberate attempt is made to encourage the subject to go beyond their chosen limit. 'What! You ask me to stop increasing the stimulus? Most people take much more than that!' Some individuals refuse to give their permission but most allow a higher limit. If the instructions to the subject do not mention the word 'pain' and they are asked only to estimate how big the 'stimulus' is, they give quite different responses on its size. They also do this if asked to estimate the weight or brightness of a stimulus. This emphasizes a crucial difference of pain from neutral events. You may estimate weight, blueness or warmth without any necessity to do anything about it. For such neutral stimuli, the test can be a purely intellectual game. Pain always implies an impending threat that will eventually engulf you if it is allowed to escalate. This type of science is not useless, because it can even be used to assess analgesics, but its utility is limited to the highly unusual context in which it is necessarily performed.
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