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Can pain be conditioned? The plausibility and the evidence

 •  • by Paul Ingraham
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Weekly nuggets of pain science news and insight, usually 100-300 words, with the occasional longer post. The blog is the “director’s commentary” on the core content of a library of major articles and books about common painful problems and popular treatments. See the blog archives or updates for the whole site.

Can chronic pain be a “learned response” to things that shouldn’t hurt, like Pavlov’s dogs salivating to the ring of a bell? “Classical conditioning,” that is. It’s an interesting idea, with obviously optimistic implications — because what is learned might also be un-learned.

If that works, it would be a cool brain hack, a clever and surprising solution to one of the hardest problems there is.

Kind of a big “if” though.

This is an excerpt from a major update to my full article on this topic, focusing the plausibility and the evidence. Spoiler alert: in addition to conditioned pain being quite implausible, there’s also very little science to back it up. So it’s amazing how many professionals believe that pain can be conditioned — most of them, at least 80% (Madden et al).

But low plausibility and a lack of evidence never seems to stop people from believing things! Nor is it going to stop me from writing about it, because it’s also so interesting that it’s worth discussing and exploring.

A B&W photograph of Ivan Pavlov, the famous Russian neurologist and physiologist, seated with his hand resting on his chin in a contemplative pose. To the left of the image is a humorous caption reading: “Pavlov probably thought about feeding his dogs every time he heard someone ring a bell.”

Just a little classical conditioning humour. I laughed on cue.

The plausibility of conditioned pain

Despite all the mysteries of life, we do know this much: conditioned responses are behavioural, physiological, and emotional — hunger, fear and anxiety, or excitement — and not sensations.

That is, we can’t be conditioned to sense things that do not exist, cannot learn to hallucinate on cue: smell a pie baking, see a walrus, hear a fart. So why would it be possible to learn to feel pain?

There might be some answers to that question — pain is quite complex, and there is plenty of controversy about how it works. For instance, the key to the phenomenon, if it exists, might be related to the fact that pain is interoceptive (sensing things inside the body), and interoception might be fundamentally more susceptible to conditioning than exteroception (sensing the outside).

But all such possible answers are still just speculative for now. The fact that conditioning doesn’t involve tinkering with sensations at all is a large conceptual hurdle — so large that it should make anyone hesitate to “believe” in conditioned pain without really good evidence. It is an extraordinary claim about how humans work … and most such ideas turn out to be wrong.

Most ideas are wrong? Dorky epistemology detour

Isn’t “most ideas are wrong” spectacularly cynical? Oh, yes, it is! And that cynicism is wisely baked right into science with the weird “null hypothesis” thing, a formal way of saying that most ideas about complex things turn out to be wrong, and a rigorous (scientific) test will probably find nothing (null).

“The null” is usually confirmed in science, because there are just too many things we still don’t know, and human ideas about how things work are very, very badly polluted with all kinds of biases and foolishness. But biology and health? That’s next level complexity, a “black box” that makes other black boxes feel downright pale. Our ideas about how bodies work are particularly sketchy.

And so most medical trials of virtually any tricky idea are either negative outright, or (more common today than ever) “positive” results that are just doomed to be overturned by more rigorous studies.

Understanding that it’s unwise to bet against the null is like knowing that “the house always wins.”

That’s important context for an idea like “we can learn to hurt,” which 80% of professionals believe without the benefit of clear supporting evidence.

The evidence for pain conditioning, such as it is

A 2023 trial by Kang et al is one of the few direct tests to date. It concluded that “conditioned pain may exist, albeit most likely in rare cases or under specific situations” (Kang et al). And that “may exist” conclusion — damning with faint praise? if it can be trusted at all? — is at least consistent with a 2016 review of what little other direct evidence exists (Madden et al).

But it’s really not enough to go on, and Kang et al. has been strongly criticized by some experts, arguing that the results are meaningless because the test was “rigged” (Cohen et al). Asaf Weisman:

“I would change the name of the study to: ‘In search of conditioned pain: A monumental waste of time.’ The discussion of the authors who performed 3 different experiments is quite honest in admitting that they were not really successful and that the risk of reporting bias is too big. Additionally, they did not attempt to control demand characteristics (DC). Hence, the supposedly positive results in 2 experiments are suspected to be artifact of lack of control for DC.

So, even in light of “positive findings” in 2 out of three experiments, those results do not support the hypothesis. The idea of conditioned pain is nonsense.

In a follow-up comment, he suggests that more research is…

“…a waste of time because an experience cannot be conditioned. Do you think I can condition your experience of the color red to be green?”

I’m quoting Mr. Weisman for the sake of acknowledging “who disagrees and why.”) This is a strong disagreement between experts, and non-scientists can’t pick a winner here.

Indirect evidence?

There is plenty of indirect evidence of conditioned pain, which can keep hope alive for those who both believe in it and follow the science — so not many.

For instance, McCarberg et al is a review of the literature on the neurobiology of the relationship between memory and pain, concluding that pain can be modulated substantially by cognitive and emotional inputs, and thus “chronic pain can be seen as persistence of the memory of pain and/or the inability to extinguish painful memories.” These conclusions lend some support to the hypothesis that classically conditioned pain is possible.

I go over a few other examples in my full article (this post being just an excerpt). But there’s a mighty gap between research that shows that conditioned pain might be possible and research that shows that it actually is.

“Conclusions,” ha!

That’s the evidence situation. Such as it is. So it’s all a bit squishy, mostly just an absence of evidence, and not evidence of absence.

But 80% of professionals “believe” it. Alrighty then!

For the foreseeable future, the existence of the phenomenon of conditioned pain will remain a matter for speculation, debate, belief, opinion, and (of course) scoffing at people.

If you’d like to learn more about the conditioned pain idea, there’s about 3x as much to read in the full article:

Chronic Pain as a Conditioned Behaviour: If pain can be learned, perhaps it can be unlearned

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