Explaining pain: Comment on Robinson, Staud and Price (2013)

Here, I briefly respond to Robinson, Staud and Price6 concerning what constitutes the ‘neural signature’ of pain (p. 325), note a logical mistake in their article, and highlight a reason why explaining pain is difficult. It is probable that conscious pain may be subserved by an unconscious physical base with a specific neurophysiological signature. Explaining pain in this direct way aims first to describe the base as a correlate of pain, then ultimately to achieve a reductive neurophysiological explanation of pain. Multiple evidential lines demonstrate that the neurophysiological base of pain need not be limited to one physical location, as Robinson, Staud and Price rightly note (p. 325). Since the hypothetical pain base is probably distributed, and therefore is more akin to the immune system than the liver, it is mistaken to expect that if it is not confined to a single neural region, or a single pattern of functional interaction, then there cannot be a physical signature of pain, as Robinson, Staud and Price appear to think (p. 325). Instead of a region-based view of the hypothetical pain base, it may be more accurate to think of it as a distributed mechanism.5, 8

The mechanism of pain could involve any number of neurophysiological systems (nervous, endocrine, immune), or reciprocal interactions between them, or any number of neurophysiological levels (pathway, network, single cell, molecular), or reciprocal interactions between them.1, 7, 8 The probability of a distributed mechanism, combined with the open-ended probability concerning the systems and level at which the mechanism exists, explains why current hypotheses and theories of pain in the literature, including those made in the article by Robinson, Staud and Price, are relatively unconstrained. However, the absence of constraints is not indicative of the likely truth of Cartesian dualism, the futility of searching for neurophysiological pain correlates, or the unreliability of verbal pain self-report. Rather, it indicates that pain science has much to do.

Neurophysiological mechanism and pain experiences can be correlated for a variety of reasons: the mechanism is part of the cause of pain; the mechanism is part of the effect of pain; the mechanism indirectly parallels pain; the mechanism is what pain can be identified with.2, 8 Discovering the neurophysiological signature of pain requires the identification of some neurophysiological mechanism with pain. The correlation of mechanism x with pain is informative because x may be the one for identifying pain. Correspondingly, mechanism y that does not correlate with pain indicates that y may not be the one. If there is a pain mechanism with a neurophysiological signature identifiable with pain experiences, the scientific and clinical benefits could be huge. Thus, investigating pain directly is worth a try.

Now, it is quite possible that a scientist may be looking at an instance of the pain signature without comprehending that it is an instance. This will occur if the physical base of pain does not possess an identifying property that is obvious to naïve researchers, but is comprehensible only through the availability of a more complete general theory of brain function.2, 3, 4, 8 The limitations in explaining pain are not simply technological. After all, how would a person know, independently of Antoine Lavoisier’s studies on oxygen, that metabolizing, burning and rusting are identical with the same mechanism, but that lightning and sunlight are not? Thus, Robinson, Staud and Price are right in asserting that it is misconceived to replace pain ratings with neuroimaging data, especially at this early stage of pain investigations.

References

Chapman CR, Tuckett RP, & Song CW: Pain and stress in a systems perspective: reciprocal neural, endocrine, and immune interactions. J Pain 9: 122-145, 2008.

Churchland PS: A neurophilosophical slant on consciousness research. Progress in brain research 149: 285-293, 2005.

Frith CD, Perry R, Lumer E: The neural correlates of conscious experience: an experimental framework. Trends in Cognitive Science 3: 105-114, 1999.

Northoff, G: Philosophy of the brain: The brain problem (Vol. 52). Amsterdam, John Benjamins Publishing Company, 2004.

Northoff, G: Region-Based Approach versus Mechanism-Based Approach to the Brain. Neuropsychoanalysis: An Interdisciplinary Journal for Psychoanalysis and the Neurosciences 12: 167-170, 2010.

Robinson ME, Staud R, & Price DD: Pain Measurement and Brain Activity: Will Neuroimages Replace Pain Ratings? J Pain 14: 323-327, 2013.

Tracey I, Mantyh PW: The Cerebral Signature for Pain Perception and Its Modulation. Neuron 55: 377-391, 2007.

van Rysewyk S: Pain is Mechanism. PhD Thesis, University of Tasmania, 2013.

Why are pain patients all unique? A type-token identity theory answer

Variations in response to pain have been reported in clinical settings (e.g., Bates et al. 1996; Cherkin et al. 1994; Jensen et al. 1986; Unruh, 1996; Wormslev et al. 1994). Patients with similar types and degrees of wounds vary from showing no pain to showing severe and disabling pain. Many chronic pain patients show disabling chronic pain despite showing no observable wound. Other patients show severe wounds but do not show pain. Why is it that two persons with identical lesions do not show the same pain or no pain at all? Why are all pain patients unique?

I propose that mind-brain identity theory may offer an answer to this difficult question. There are two main versions of identity theory: type and token identity. A sample type identical property is to identify “Being in pain” (X) with “Being the operation of the nervous-endocrine-immune mechanism” (Y) (i.e., X iff Y) (Chapman et al. 2008; van Rysewyk, 2013). For any person in pain the nervous-endocrine-immune mechanism (NEIM) must be active, and when NEIM is active in a person, he or she is in pain. Thus, type identity theory strongly limits the pattern of covariation across persons. According to token identity theory, for a person in mental state X at time t, X is identical to some neurophysiological state Y. However, in the same person at time t1, the same mental state X may be identical to a different neurophysiological state Y2. Token identity theory doesn’t limit the pattern of covariation across persons; it only claims that, at any given time, some mind-brain identity must be true.

In response to the topic question, I propose a hybrid version of identity theory – ‘type-token mind-brain identity theory’. Accordingly, for every person, there is a type identity between a mental state X and some neurophysiological state Y. So, when I am in pain, I am in NEIM state Y (and vice versa), but this NEIM state Y may be quite different across persons. Type-token identity theory therefore proposes a type identity model at the level of every person (i.e., it may vary across persons). A type-token identity theory implies that group-level type identities (i.e., type-type) cannot fully explain the pattern of covariation in pain responses across persons. Measuring changes of a pattern of psychological and neurophysiological indicators over time may then support a unidimensional model of chronic pain for each pain patient. Thus, being in chronic pain for me is identical with a specific pattern of NEIM activity (Chapman et al. 2008; van Rysewyk, 2013), but for a different patient, the same state of pain may be identical to a different pattern of NEIM activity. In preventing and alleviating chronic pain, it is therefore essential to best fit the intervention to the type-token pain identity profile of the patient.

References

Bates, M. S., Edwards, W. T., & Anderson, K. O. (1993). Ethnocultural influences on variation in chronic pain perception. Pain, 52(1), 101-112.

Chapman, C. R., Tuckett, R. P., & Song, C. W. (2008). Pain and stress in a systems perspective: reciprocal neural, endocrine, and immune interactions. Journal of Pain 9: 122-145.

Cherkin, D. C., Deyo, R. A., Wheeler, K., & Ciol, M. A. (1994). Physician variation in diagnostic testing for low back pain. Who you see is what you get. Arthritis & Rheumatism, 37(1), 15-22.

Jensen, M. P., Karoly, P., & Braver, S. (1986). The measurement of clinical pain intensity: a comparison of six methods. Pain, 27(1), 117-126.

Unruh, A. M. (1996). Gender variations in clinical pain experience. Pain, 65(2), 123-167.

van Rysewyk, S. (2013). Pain is Mechanism. Unpublished PhD Thesis. University of Tasmania.

Wormslev, M., Juul, A. M., Marques, B., Minck, H., Bentzen, L., & Hansen, T. M. (1994). Clinical examination of pelvic insufficiency during pregnancy: an evaluation of the interobserver variation, the relation between clinical signs and pain and the relation between clinical signs and physical disability. Scandinavian journal of rheumatology, 23(2), 96-102.

Will science make painfulness disappear?

Some philosophers worry that neuroscience will make painfulness disappear. Broadly, the objection is that if a science reduces a macro phenomenon to a micro phenomenon, then the macro phenomenon is not real or disappears (e.g., Searle, 1992). Using this conception of ‘reduction’, it is then reasoned that because it is observably obvious that a pain is real, it cannot be reduced to neuroscience. This misunderstanding trades on an idiosyncratic understanding of reduction, where it is expected that in science, reductions make macro phenomenon disappear. This expectation is confused.

Temperature was reduced to mean molecular kinetic energy, as recounted above, but no person expects that temperature therefore ceased to be real or became scientifically disrespectable or redundant. Visible light was reduced to electromagnetic radiation, but light did not disappear. Instead, scientists understand more about the real nature of light than they did before 1873. Light is real, no doubt; and so is temperature. Some expectations about the nature of temperature and light did change, and scientific progress does occasionally require rethinking what was believed about phenomenon. In certain instances, previously respectable properties and substances sometimes did prove to be unreal. The caloric theory of heat did not survive rigorous experimental testing; caloric fluid thus proved to be unreal. While no one expects that painfulness will cease to be real or become scientifically disrespectable if it is successfully explained by neuroscience, everyone believes that debilitating chronic pain will be controlled and eventually disappear as a result of scientific reduction. But this belief may turn out to be quite wrong. Simple prudence suggests that we wait and see.

Thus, the reduction of a macro phenomenon means only that there is an explanation of the phenomenon. Scientific explanations of phenomenon do not typically make them disappear. As neuroscience matures, the future of current conceptions of painfulness and sensory experience generally will rely on the empirical facts, and the enduring accuracy of current macro level theories (Churchland, 1993).

Churchland, P.M. (1993). Evaluating our self-conception. Mind and Language, 8, 211-222.
Searle, J.R. (1992). The Rediscovery of Mind. Cambridge, Mass.: MIT Press.

‘The human fetus cannot feel pain’

Fetal pain perception is often modelled on the same neural structures as in the adult.

However –

(1 The neural structures involved in pain processing in early development are unique and different from adults.

(2 Some of these structures and mechanisms are not maintained beyond specific developmental periods.

The immature pain system plays a signalling role during each stage of development, and fulfils this role using different neural resources available at specific developmental times.

Thus, the error here is reading the adult into the fetus.

Does science make things disappear?

If a science reduces a macro phenomenon to a micro phenomenon, then the macro phenomenon either is not real or ‘goes away’. Is this true? Does science make things disappear?

Obstetrics is true, and babies are born every day. Or, are babies born in spite of obstetrics? Does understanding gynecology make women sterile?

At the same time, a science of pain will hopefully reduce – or eliminate – much pain (mammalian and non-mammalian). Science makes pain ‘go away’. Surely a good thing.

How would you explain to a person who cannot experience pain, what pain is?

How would you explain to a person who could never experience pain, what pain is?

Do the following:

1. Get a dairy food that is extremely spoiled.

2. Get the person to fill his or her mouth with this delightful food.

(ESSENTIAL: The food has to remain in the person’s mouth! Under no circumstances can the person spit out the food and clean his her mouth.)

3. Chew the food.

This test is a good model for the nature of pain.

Pain is aversive. We want to avoid pain. Eating spoiled food is the same: we want to avoid at all costs putting, much less chewing and swallowing, bad food.

Having a really bad taste in your mouth is like having a pain in your body.

#SciFund update: video complete!

My #SciFund video is finally complete!

Quite a mission to do (first time), but I am happy with the finished product.

Click on the image:

 

 

 

 

 

 

 

 

Here is a map showing the global distribution of participating scientists in Round 1 (2011) and Round 2 (May, 2012) of the #SciFund Challenge:

SciFunders Standing Tall and Talented