Saturday, February 21, 2015

Where Does it Hurt?

The mystifying cognitive biology of pain.

Pain is perhaps the bedrock of consciousness. If we want to know we are awake, we pinch ourselves. And torture is the use of inflicted, involuntary consciousness of pain against the person's higher faculties, interests, and desires. Before there was cogito ergo sum, there was morsus ergo sum. But that hasn't made pain any easier to locate and analyze as an activity of the brain and greater nervous system. There is no "pain center" in the brain, for instance.

But the brain is central to pain. Sure, we have local reflexes that respond involuntarily and unconsciously. But if pain is consciously experienced, it comes through the brain, no matter how far away in the toes it might be mapped. It is remarkable, indeed, that our brain can tell us to feel something in our toes based on signals that the toe sends from several feet away. For all our complex knowledge of the pain receptors and their circuitry on the way to the brain, the conscious experience remains mysterious.

The best approximation to date is something called a "neurologic pain signature" (NPS), which comes from neuro-imaging and consists of a variety of locations in the brain that associate specifically with the most immediate experience of pain. It is not a point, but a large network of activities and locations, all of which are often involved in other things as well. Just like consciousness in general, there is no little incubus in there, but a specific pattern of activity, that must be the experience of pain.

A recent paper and review about our ability to manipulate our pain experience through distraction, placebo effects, and other high-level cognitive mechanisms outlined these issues. The paper basically divides the pain system into two parts- a basic reception part that gets signals from the body, (and corresponds to the NPS mentioned above), and a cognitive part that happens when those signals hit yet other parts of the brain, especially in the frontal system. This latter part, which can be engaged successfully to dampen a subject's pain experience, upon appropriate instruction, had big effects on the subjective experience of pain, but no effect on the activity of the NPS, as observed in brain scans.
"This means that whatever information is used in the NPS decoding, it doesn't simply represent the subjective experience of pain. Instead, the authors found that the influence of modulation is reflected in different brain regions—notably the nucleus accumbens and ventromedial prefrontal cortex (in brief, greater activity reflects less pain)."
"Thus, our NAc-vmPFC [frontal area] pathway findings may reflect evaluative processes that play an important role in the construction of pain experience and in shaping long-term motivated behaviors and outcomes."

Diagram from the paper, showing the subjective pain ratings (left, bottom) compared to the neurologic pain signature by brain scanning (right, bottom). Subjective ratings were modestly affected by conscious training. Part A shows the overall schematic, that the NPS, a brain core network, doesn't budge when one tries to consciously either dampen to increase the pain, while certain locations in the frontal areas (blue) are more active when trying to dampen the subjective pain experience.

So subjective pain is something else again, not really the NPS alone, though clearly derived from it somehow. Are these small frontal areas that light up caused by the effort involved in modulating the pain, or are they more directly associated with the subjective pain itself? Likely the former. The brain is full of mechanisms to shut out distracting stimuli, since only a small portion of all the sensory modes and data coming in ever make it to consciousness. The frontal areas are frequently a big part of this filtering and habituating process. Thus the NPS may still constitute pain consciousness, whose access or presentation is regulated, much as vision or hearing is regulated, by other parts of the brain in a (slightly) top-down way.

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