A Bayesian Model for Chronic Pain

I recently read a fascinating article from the UK by Tabor and colleagues (https://doi.org/10.1371/journal.pcbi.1005142). It posits that chronic pain occurs because our brain can’t accurately assess the threat to the body. As this continues, our perception continues with that out-of-date analysis, resulting in the maintenance of pain.

Let’s move back a bit and talk about pain. You all know what pain is. It’s the sensation you feel when we perceive damage to ourselves. Much of it is acute, and once the underlying issue is resolved, the sensation of pain disappears. However, we have the phenomenon of chronic pain, some of which persists even when our bodies are otherwise OK. Is it misfiring neurons? A hidden form of damage?

We’ve begun to recognize that pain results from an inferential understanding by our brain, since the data we perceive are so noisy. Bayes comes in because we understand the barrage of sensory input (say, like getting shot in the gut) based on current information and prior experience. Why pain? Because it’s a protective measure. Avoid the hot pain, dodge the knife attack, watch out for the Lego piece! We are estimating the likely level of harm.

One example the authors mention had participants pair a painful stimulus with a visual cue. The result was that the stimulus was considered more painful if paired with a red visual cue and less painful when paired with a blue one. The point is that we can experience pain differently based on our perceptions. Combine those perceptions with the painful sensation, and you have one of the key principles of Bayes; providing additional information models the response differently. The meaning? Pain doesn’t follow a simple dose response model. It can be excruciatingly painful to get a pizza burn, but not to walk on hot coals.

You can see the same thing in placebo and nocebo effects. When you’re in pain and someone offers you a pill, you expect the pill to relieve your pain, and it does! That is placebo. Nocebo is when you pair pain with a red cue and you feel more pain. Prior expectations and current sensory evidence equals modulation of response.

So, the problem of chronic pain. This is an issue of time. Over time, the brain needs to update our estimates based on the changing environment and does so subconciously. If we lack info, the current pain sensation continues. In fact, we can be conditioned to believe things are more painful as the pain continues (hyperalgesia). Even normal activities are associated with pain (allodynia).

Pretty grim stuff. The brain fools itself into getting into an even worse state. Tabor presents a possible solution. Just as we can fool ourselves into thinking things are getting more terrible, we can fool ourselves into thinking things are going to get better. Unfortunately, we don’t have a valid and reliable technique just yet, at least in modern medicine. Still, the paper is illuminating. It might be “all in our heads”, but we’re starting to learn that our heads can make things reality.

Summary: The brain’s pain response is affected by both the current situation and prior evidence. It may be possible to directly control the experience of pain.

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