Sunday, June 02, 2013

Melzack and Katz: Part 6c: History of pain - Ref 4, continued

The paper, Pain.

Part 1 First two sentences

Part 2 Pain is personal

Part 3a Pain is more than sensation: Backdrop

Part 3b Pain is not receptor stimulation

Part 3c: Pain depends on everything ever experienced by an individual

Part 4: Pain is a multidimensional experience across time

Part 5: Pain and purpose

Part 6a: Descartes and his era

Part 6b: History of pain - what’s in “Ref 4”?

Where we left off : Melzack winding up to hit Von Frey's specificity theory of pain out of the park. I'm going to provide the entire build-up here, quote the entire section, so you can get a feel for how charged up the guy is about the topic.
P. 155: 
"Like all psychological theories, von Frey's theory has an implicit conceptual nervous system; and the model is that of a fixed, direct-line communication system from the skin to the brain - of distinct nerves and pathways of four different qualities (analogous to the differently coloured wires of an electrical circuit) running from four kinds of stimulus transducers in the skin to four specific receivers in the brain."
Pain pathway diagrams look a lot like this
The nervous system was studied from the skin in. Pain was studied by poking sore spots on peoples' skin. No one ever knew exactly what went on in the brains of the people who said "Ouch!" It was assumed that there was some "pain center."

But, back to Melzack:
"It is essentially similar to Descartes' concept of pain proposed three hundred years earlier. It depicts a fixed, straight-through conceptual nervous system. It is precisely this facet of the specificity concept, which imputes a direct, invariant relationship between a psychological sensory dimension and a physical stimulus dimension, that has led to attempts at repudiation of the doctrine of specificity in its entirety."
"Consider the proposition that the skin contains 'pain receptors'. To say that a receptor responds only to intense, noxious stimulation of the skin is a physiological statement of fact; it says that the receptor is specialized to respond to a particular kind of stimulus. To call a receptor a 'pain receptor', however, is a psychological assumption: it implies a direct connection from the receptor to a brain center where pain is felt, so that stimulation of the receptor must always elicit pain and only the sensation of pain. It further implies that the abstraction or selection of information concerning the stimulus occurs entirely at the receptor level and that this information is transmitted faithfully to the brain. The crux of the revolt against specificity, then, is against psychological specificity. This distinction between physiological specialization and psychological assumption also applies to peripheral fibres and central projection systems."

I have bolded the bits that stand out for me, the bits I can imagine Melzack being most passionate about.
" The facts of physiological specialization provide the power of specificity theory. Its psychological assumption is its weakness. This assumption will now be examined in the light of the psychological, clinical, and physiological evidence concerning pain (Melzack and Wall, 1962, 1965)."
More to come about that.

Labelled line pain science is still all about poking people (or rats, I guess..) until they say/squeak/behave, "Ouch." It's only quite recently in history that behaviour can be seen inside the brain at the same time as the "Ouch!" is uttered. Some have labelled this [erroneously] the "pain matrix." Worse yet, trundling along erroneous labelled line conceptualizations, people got Melzack's neuromatrix confused with the "pain matrix" that they concocted:

SOURCE: "Fig 1  

The pain matrix mainly consists of the thalamus (Th), the amygdala (Amyg), the insula cortex (Insula), the supplementary motor area (SMA), the posterior parietal cortex (PPC), the prefrontal cortex (PFC), the cingulate cortex (ACC), the periaqueductal grey (PAG), the basal ganglia and cerebellar cortex (not shown) and the primary (S1) and secondary (S2, not shown) sensory cortex. For review see Refs. [19, 20]"

[Note the authoritative tone of the declarative statement beneath this image from SpringerImages. They sound like they know what they're talking about, that the hypothetical "pain matrix" is a given. Like it's a noun.]

The big problem with that, is, the same areas that light up inside the brain with an "Ouch!", concurrent with having one's skin poked, light up with any kind of novel stimulus, through any sensory input, including visual, especially if it seems threatening. 

There is still no such thing, therefore, as a "pain" center, or network, or "matrix" in the brain. There is only the neuromatrix itself, generating pain, when and if it has reason to, from its own perspective.


To be continued.


1. Legrain VIannetti GDPlaghki LMouraux AThe pain matrix reloaded: a salience detection system for the bodyProg Neurobiol. 2011 Jan;93(1):111-24

2. The Neuromatrix of pain: The Brain from Top to Bottom

3. G. D. IannettiA. Mouraux; From the neuromatrix to the pain matrix (and back)Experimental Brain Research Volume 205Issue 1pp 1-12

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