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Can one have pain and not know it?

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By Flavia Di Pietro

I think about this a lot.  It leads me to ponder the distinction between pain and nociception.  We found a chapter on exactly this in a great book we are slowly reviewing at BiM – The Science of Pain.  The chapter’s title grabbed me: Conciousness and Pain.  It’s really got me thinking about both, and in particular that the latter can’t happen without the former.  If you were struggling to explain to someone the difference between pain and nociception, perhaps an easy way of doing so might be to point out that nociception may occur when someone is unconscious, whereas pain by definition cannot [1].

A nociceptive, tissue-threatening stimulus evokes adaptive behavioural responses which aren’t necessarily intentional or reasoned, called nocifensive responses. They are to minimise or escape from noxious stimulilike the flexor withdrawal response (muscle groups in the leg automatically bending the hip, knee and ankle after stepping on a spiky something).  We don’t need to be conscious for nociception to occur.

Pain however is an experience of the conscious brain, a sensory and emotional percept.  The most potent way we have of suppressing someone’s pain is through general anaesthetic, a process which globally eliminates sensory experience.  It’s possible that even in an unconscious, ‘pain-free’ state that nocifensive responses, like changes in cardiovascular rhythm, persist.

So the key ingredient for pain is the conscious brain.  Pain= nociception + consciousness.  That’s enough maths. Let’s talk about consciousness.  We don’t really understand how it happens.  There can’t be anything supernatural about it—there has to be an underlying neural activity and circuitry which, albeit complex, accounts for the emergence of conscious experience and willed activity [2], but we just can’t seem to quantify it.

We can use what we know about pain to learn more about the neurobiology of consciousness.  Pain is likely to tell us more about consciousness than other systems will.  Why?  Firstly, there is a highly variable relationship between stimulus and percept in the pain system.  In the absence of anything wrong with the visual system itself, light of a certain wavelength evokes the colour blue.  But a stubbed toe hurts more for some people than it does for others.  Secondly, emotion and affect play such a key role in pain as compared with other systems.  Happy or sad, I’ll see the colour blue (no pun intended).  My stubbed toe however may not bother me if I happen to be running away from something scary as I stub it.

Since I can’t help myself, let’s talk about this from a neuroimaging perspective.  Pain is subjective, and it cannot be ‘seen’ by anyone other than the person experiencing it.  Even with neuroimaging we cannot assess it objectively.  Imaging experiments tell us that there is a pattern, albeit variable, of spots in the brain that are active when someone is in pain.  But no image can tell us that the pain experienced is rated by that brain as a 5 out of 10 pain, around L4/5, and can escalate to 9 out of 10 when the mother-in-law comes to stay.  The ‘pain neuromatrix’ is too widely distributed for that, and there’s immeasurable capacity for functional reorganisation and redundancy in the system.

For the time being there are limits to the extent that we can ‘measure’ pain, and say with confidence that we fully understand pain.  Perhaps the neurobiology of consciousness is a good start?

About Flavia

Flavia Di PietroFlavia Di Pietro is a PhD student in the Moseley Group investigating the development of Complex Regional Pain Syndrome (CRPS) after wrist fracture. Flavia’s PhD focuses on the early detection of brain changes in CRPS using fMRI.  But get this – Flavia did Physiotherapy Honours degree at Notre Dame and completely cleaned up – Brian Edwards Memorial Award, Physio Research Foundation Award, Dean’s Award. Now, these things mean that she is not only ticking the academic boxes but all the other fluffy stuff too. No surprises that the NHMRC of Australia jumped to support her PhD.  So she has come over from Perth where she has been working as a physiotherapist.  All her achievements, however, pale in comparison to her celebrated status as the best Shoe Salesperson south of Milano, as evidenced by her taking out the 2006 and 2008 Diana Ferrari Golden Boot Award.  Clearly, she did not write this bio.

* One of the awards in Flavia’s bio is fictitious.

References

ResearchBlogging.org [1] Merksey, H. (1986). Pain terms: a current list with definitions and notes on usage. Pain Suppl. 3 S215-S221

[2] Chalmers, D. (1995). Facing up to the problem of consciousness. J. Consciousness Studies, 2, 200-219.

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