Saturday, June 29, 2013

Melzack&Katz, Pain. Part 11c: Older brain models just don't cut it.

The paper, Pain

Most recent blogpost: Part 11We need a new conceptual brain model! Part 11b: Intro to a new conceptual nervous system


OK, here we are, back from lunch and ready to hoist our canoes for the trek through brush, uphill, so we can find a place to enter the river once again, continue our paddling up to headwaters. 

We left off just as Melzack evoked Hebb in his 1989 Hebb Memorial Lecture, to explain why he thought we needed a new conceptual model of the nervous system. Thirty-four years had sailed by. For 34 years, Melzack had been away from Hebb's lab, out in the world, working with Scottish terriers, working with Wall, getting the gate theory together. 

My favorite bit from Hebb's 1955 reference is this: 
"About 1930 it began to be evident that the nerve cell is not physiologically inert, does not have to be excited from outside in order to discharge (19, p. 8). The nervous system is alive, and living things by their nature are active. With the demonstration of spontaneous activity in c.n.s. it seemed to me that the conception of a drive system or systems was supererogation.For reasons I shall come to later, this now appears to me to have been an oversimplification; but in 1945 the only problem of motivation, I thought, was to account for the direction taken by behavior. From this point of view, hunger or pain might be peculiarly effective in guiding or channeling activity but not needed for its arousal. It was not surprising, from this point of view, to see human beings liking intellectual work, nor to find evidence that an animal might learn something without pressure of pain or hunger.The energy of response is not in the stimulus. It comes from the food, water, and oxygen ingested by the animal; and the violence of an epileptic convulsion, when brain cells for whatever reason decide to fire in synchrony, bears witness to what the nervous system can do when it likes. This is like a whole powder magazine exploding at once. Ordinary behavior can be thought of as produced by an organized series of much smaller explosions, and so a "self-motivating" c.n.s. might still be a very powerfully motivated one." - Don Hebb 1955
Hebb is the famous Canuck whose Hebbian theory about learning is remembered most easily as "Neurons that fire together wire together." Reference 19 goes to The Organization of Behaviour, a book he wrote in 1949. Melzack was one of his students

Picking up Melzack 1989 (cont.):
"Our present knowledge of the number of brain cells and their connections makes the imagination reel (Hoyenga & Hoyenga 1988). There are, it is estimated, 100 billion nerve cells in the brain (a 10-fold increase over the number in most textbooks only 5 to 10 years ago) and the number continues to rise."
Actually I think at last count it was 86 billion. Still a heck of a lot. Weird to think, though, that earlier people thought there were so many fewer. And remember, we still aren't counting glia, which outnumber neurons 10-1, and do a whole pile of important computational activity of their own, plus run all the neuronal synapses.

"If we recognize that many neurons in sensory projection systems may have synaptic connections with tens or hundreds of thousands of other neurons, and if we carry our computation through the cortical layers and their projections to deeper levels which may, in turn, project to the cortex again, the number is astronomical."
Yeah. Something like more possible connections than there are particles in the universe or something... Here's Ramachandran - I'll let him explain this in his 10 minute TED talk.

"To think that we are even close to an understanding of brain functions and their relation to experience and behaviour is absurd. At this stage, as Don Hebb so wisely told us, we can only speculate and try to build reasonable neural models - in short, a "conceptual nervous system." Our old concepts of the brain are totally inadequate in their ability to explain phantom limb phenomena and it is time to sketch out a new one. First, however, let us look at earlier conceptual nervous systems to explain phantom limb behaviour."
[Can you feel the strain of carrying that canoe, and everything you had stowed in it, up the hill? taking care to avoid snagging it on tree branches? Having that happen anyway? taking care to avoid stumbling on tree roots and rocks? Having that happen anyway? How are your mental knee caps doing? Isn't thinking fun? It's hard work, but isn't it also exhilarating? Are you sweating yet?] 


Earlier theories included:
  • peripheral mechanisms - nope - See Melzack and Loeser 1978
  • hyperactive deafferented cells in the spinal cord - nope - Dorpat 1971 reported the specific urge to void in patients entirely without bladders, and Jankovic and Glass 1985 reported phantom limb tremor with metoclopramide IV. Hyperexcited spinal cord cells can't account for phantom limbs' seamless coordination with other limbs or the coherence and unity of the experience.
  • body schema? Simmel in 1956 and Weinstein et al in 1964 proposed "body schema" located in the homunculi mapped by Penfield (1937) in the somatosensory cortex, based on Head and Holme's concept back in 1911-12. But Melzack doesn't much care for this idea either.
"First, the idea of "body schema"is too vague to provide an explanation of any of the phenomena. Second, there is no postulate by Head and Holmes on the actual neural mechanisms that comprise their postural "body schema" apart from identifying a tactile schema with the somatosensory cortex. They also deny any relation between postural "body schema" and "body image." The latter, they say, is a visual image while the former is a somatic neural mechanism. Thus, the role of vision in the phantom limb is left unexplained even though we know that the phantom limb experience, in terms of position and other qualities, is powerfully influenced by vision. It is clear that, while phantom limbs obviously have a neural substrate, they are not explained by Head and Holmes conceptual "body schema."
  • identifying phantom limb experience with the post-central somatosensory cortex? Nope. Erroneous. 
"The early reports that a phantom is eradicated by ablating a portion of the post-central gyrus have not been supported; a later evaluation (White and Sweet 1969) of excisions of the somatosensory cortex for phantom limb pain shows that, with time, the phantom limb and pain both return. In addition, if the phantom limb is to be identified with hte somatosensory cortex, we now know that there are at least 7 projections of the body surface at the cerebral cortex (Merzenich and Kaas 1980), which leaves out known additional projections to brainstem areas, the limbic system, and the cerebellum. All of these presumably play a role in the phantom limb experience. But how? It is evident that a new theory is needed." 

Weirdly, the Merzenich and Kaas paper has no link anywhere that I could find. However, it's been cited in many papers - here is the whole citation. 

Merzenich, M.M. and J.H. Kaas (1980) Principles of organization of sensory-perceptual systems in mammals. Progress in Psychobiology and Physiological Psychology 9:1-42.
A lot of the papers it's cited in are about cortical columns, etc. 

Just the other day, a new paper
came out in Science that challenges long held ideas about cortical columns that were based only on neuroanatomy. It used to be thought that all the info from thalamus coming into cortical columns arrived at level 4. Turns out the thalamus sends about a fifth of it to layer 5, separately, for completely different processing. Not sure yet what it all means.. apparently Level 5 is pyramidal cells, according to this Wikipedia entry, anyway.

Christine M Constantinople, Randy M Bruno; Deep Cortical Layers Are Activated Directly by Thalamus. Science, Vol 340, No 6140 28June2013 pp1591-1594

See also:

Basically, what it's saying is, investigators learned the outer part of the brain's rind does something different from the inner part of the brain's rind, fair enough - but that surprisingly the thalamus splits up its projections, sending to both, not just one. The cells in the two layers do different things in the brain, so this news is kind of intriguing. Maybe the "critter brain" is in the inner rind. 



Previous blogposts

Part 1 First two sentences Part 2 Pain is personal Also Pain is Personal addendum., Neurotags! Pain is Personal, Always.

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; Part 6d: History of Pain: Final takedown Part 6e: Pattern theories in the history of pain Part 6f: Evaluation of pain theories Part 6g: History of Pain, the cautionary tale. Part 6h: Gate Control Theory.

Part 7: Gate control theory has stood the test of time: Patrick David Wall;  Part 7bGate control: "The theory was a leap of faith but it was right!"
Part 8: Beyond the gate: Self as mayor Part 8b: 3-ring circus of self Part 8c: Getting objective about subjectivity
Part 9: Phantom pain - in the brain! Part 9b: Dawn of the Neuromatrix model Part 9cNeuromatrix: MORE than just spinal projection areas in thalamus and cortex Part 9d: More about phantom body pain in paraplegics
Part 10: "We don't need a body to feel a body." Part 10b: Conclusion1: The brain generates its own experience of being in a body Part 10c:Conclusion 2: Your brain, not your body, tells you what you're feeling Part 10dConclusion 3: The brain's sense of "Self" can INclude missing parts, or EXclude actual parts, of the biological body Part 10eThe neural network that both comprises and moves "Self" is (only)modified by sensory experience

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