Monday, June 03, 2013

Melzack and Katz: Part 6d. Ref 4. Final takedown

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”?

Part 6c: History of pain, Ref 4, cont.. : There is no pain matrix, only a neuromatrix

Continuing on from where we left off yesterday in "Reference 4", which is the history of pain section in Melzack and Wall's book, Challenge of Pain, first published in 1982:

Does any psychological evidence exist?

They point out there is no evidence, not one scrap, for there being any one-to-one correspondence between pain perception and stimulus intensity.
"Instead, the evidence suggests that the amount and quality of perceived pain are determined by many psychological variables in addition to sensory input."

Pavlov and his dogs. Think about those for a minute (Pavlov 1927). Pavlov fed the dogs and rang a bell; he conditioned the dogs to salivate when he only rang a bell. 

He did a lot more than that, apparently - he provided them with shocks, burns, cuts. As long as he gave them food too, the dogs didn't care - they "eventually responded to these stimuli as signals for food and failed to show 'even the tiniest and most subtle' signs of pain."

Wow. Think about that for a moment. The dogs learned to uncouple nociceptive input from disagreeable motivational affective input processing. Holy moly. 

The authors continue:
"If these dogs felt pain sensation, then it must have been non-painful pain (Nafe 1931) or the dogs were out to fool Pavlov and simply refused to reveal that they were feeling pain. Both possibilities, of course, are absurd. The inescapable conclusion from these observations is that intense noxious stimulation can be prevented from producing pain, or may be modified to provide the signal for eating behaviour." (p 156)
Relationship of nociception to pain is that of a fish to a bicycle.
Just imaginary (albeit pretty, carefully sustained) wishful thinking.

Seems so obvious, the way they describe it. They get even more blunt about this:
"The concept of four rigid modalities of cutaneous experience is wrong." (p. 156)
Hard to see how they could be any more blunt.
But they aren't done yet - not by a long shot.

Is there any clinical evidence?

They move over to a heading labelled "Clinical Evidence." Apparently there is none to find. Instead, just a wasteland of failure: here they raise the topic of how phantom limb pain, causalgia (yes, they were still calling it causalgia in 1996), and a category they refer to as 'the neuralgias', "provide a dramatic refutation of the concept of a fixed, direct-line nervous system." They note how surgical ablation of parts of the peripheral or central nervous system didn't succeed in abolishing these kinds of pain permanently. They point out that even gentle touch, vibration, non-noxious stimuli in general, can elicit excruciating pain in some people; furthermore, pain can occur spontaneously for hours with no stimulus whatsoever. New pains can arise in new areas of the body, spread unpredictably into areas where there is no pathology.

Some of the downright painful positions people can sense their phantom limbs to be stuck in

For these reasons, entertaining the idea of a "rigid, straight-through specific pain system" makes no sense whatsoever. None. Nada. Zip.

But they aren't done yet. Oh no. Not done. 

Surely there must be physiological evidence...
"Oh.. NO!!!!"

Sorry, labelled line people. The answer is... no.
  • no evidence exists to show that stimulation of one type of receptor fibre, or spinal pathway elicits sensation in only a single psychological modality
  • specialized fibres exist, and respond only to intense stimulation - but that doesn't mean they are 'pain fibres' - "that they must always produce pain and only pain when they are stimulated."
  • neurography studies show there is no simple relationship between fibre types and quality of sensation
  • furthermore, central cells that respond to noxious stimuli are not 'pain cells' - 
"There is no evidence to suggest that they are more important for pain perception and response than all the remaining somaesthetic cells that signal characteristic firing patterns asbout multiple properties of the stimulus, including noxious intensity. The view that only the cells that respond exclusively to noxious stimuli subserve pain and that the outputs of all other cells are no more than background noise is purely a psychological assumption and has no physiological basis. Physiological specialization is a fact that can be retained without acceptance of the psychological assumption that pain is determined entirely by impulses in a straight-through transmission system from the skin to a pain centre in the brain." (p 157)

Next in the history of pain, now that we've covered all the dearth of evidence for a labelled line model for pain, Melzack and Wall discuss all the pattern theories that arose.

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