Saturday, August 6, 2016

No, I do Not Really Feel Your Pain

Where is pain in the brain, and is empathic pain in the same domain?

Our culture has gone through some interesting evolutions in the experience of pain. From antiquity through medieval times, inflicting pain was entertainment and justice, in forms such as gladiatorial fighting, bull-fighting, burning at the stake, trial by torture, and plain old torture. In modern times we seem to have softened to the point of respecting human rights, one of which is to not inflict gratuitous pain, even on deserving criminals. Perhaps this is due to the subjugation of pain in medical and dental settings, which has raised our expectations for an untroubled existence. Or perhaps it is due to dominance of secularism which continues to guide moral innovation and refuses to countenance twisted theistic rationalizations for our painful existence, let alone explicit theological torture. With the current election, we seem to be taking a step backward in the excruciating-ness department, but still one may hope for the best!

Despite such variation in cultural tolerance of pain, (especially that of others), our intrinsic capacities are naturally long-standing and common to most animals. In particular, pain is perhaps the most immediate and insistant instance of consciousness, and thus a particularly interesting case for investigating the nature of consciousness and its presumptive basis in the brain. One thing we have learned is that pain comes in many forms, from vague itches to excruciating burns. And in addition, we experience various abstract pains, such as bereavement, the sudden horror of devastating news, and the empathy we feel for the pain of others- at least some people, in some cutural settings(!)

So it is not a simple field. A recent paper made some new observations about empathic pain. Prior work has shown substantial overlap of self-pain and empathic pain as represented in the brain, leading to theories that it is involuntary and almost as vivid as one's own. But we know that that is not quite true- that spectators at the colosseum could, though social training and construction, view the torture and dismemberment of unfortunate gladiators with enthusiastic bloodlust rather than squeamish sympathy.

So where is it, and how do we feel pain of both types? These authors discuss various defects with the current model, particularly that brain locations that have been noted as activated during both own-pain and empathic pain are not really pain centers per se, but are activated by many non-pain events, to the point of being some of the most frequently activated areas in the brain across all fMRI studies.
"Only a small minority of dACC neurons are pain-related, and the dACC encodes emotional events, including rejection and general negative emotion, in a way that is distinct from how it encodes pain."

Indeed, the best anatomical correlate of pain perception in the brain is not found at a single anatomical location, but in a network (the Neurologic Pain Signature or NPS) that is something of an abstract, deduced entity, built up out of statistical measures of where the hurt shows up, specifically and reliably, in fMRI. The authors try to refine these anatomical claims with their own fMRI studies.
"The NPS has over 90% sensitivity and specificity in predicting somatic pain relative to several other salient states, including non-painful warmth, anticipated pain, pain recall, social rejection, and general negative emotion."

Active sites from actual, somatic heat-induced pain on the subject's arms or legs.

Their hapless subjects where seared on their forearms or feet with about 10 seconds of high temperature (46, 47, pr 48˚C, which is 115, 117, 118˚F). This doesn't sound too bad, really, perhaps just noticeable. The vicarious / empathic pain was more interesting: being shown pictures of a person putting a shovel through their foot, or cutting a finger with a knife, and asked to imagine this happenening to themselves. Trained actors were used, I assume. What they showed certainly causes a chill to go down your spine, but it does not share specific aspects with the experienced pain. The vicarious pain was not heat- better to show someone taking a blowtorch to their extremeties. Interestingly, however, the researchers claim to be able to differentiate between pain vicariously induced at the arm vs the leg. Even by this very indirect means, there are site/anatomy-specific signatures evident in the brain signals.

For exprienced pain, they validated the NPS network, seeing it reliably show up in their scans. But the vicarious / empathic pain was a different story. In fact, they found that the NPS areas were negatively correlated with empathic pain, and devised a newly calculated network to describe what was activated, which they call the vicarious pain signature, or VPS. Helpfully, however, they compared their VPS pattern with those prompted by negative affect pictures, and by romantic/social rejection and found no correlation. So evidently, fMRI is getting good enough to distinguish between several negative emotions.

Active sites from vicarious pain, i.e. from pictures of other people doing painful things to themselves. Note the quite different patterns of activation vs above.

Global correlations between experimental fMRI patterns and the modeled networks of the NPS or VPS. Note the complete lack of correlation of the somatic pain with the VPS, and conversely the vicarious pain with the NPS.

One question is how high-level the NPS network is. Does it represent the actual physical pain but not the consciousness of it, or does it represent both? In prior work, the same lab showed that they also saw complete distinction between NPS and efforts the subject might make to cognitively control that pain, seen as dampening from the frontal cortex. Insofar as the subjects of these studies could manage their pain, it did not affect the NPS intensity at all, but worked at another, higher level. So no, while the NPS correlates with some aspects of pain and its cognition, it does not seem to encompass the highest level processes of conscious pain management, and perhaps even perception.

But back to the main study. The authors observed the NPS quite consistently, and saw its intensity increasing with increasing amounts of applied pain. And as noted, the VPS was quite different. But it did overlap in very few areas- the anterior insula (aINS) and the dorsal anterior cingulate cortex (dACC). These have been previously cited as the points of overlap, thus as sites showing that we feel the pain of others. The aINS is a profound part of our brain, functioning in awareness, consciousness, sense of self, and sociality and other emotions, like the sense of importance of things- meaning. Thus it is natural that it might be activated in both these experimental regimes, without "meaning" quite the same thing.

Likewise, the dACC also functions in emotion and consciousness, as well as reward evaluation, internal conflict, and impulse control. Again, functions that might come up in both experimental scenarios without meaning quite the same thing. Whether a pain is on your own body or elsewhere, at some very high level it is just another input to evaluate for what we are going to decide to do next. In any case, the researchers give additional evidence from lack of correlation on the finest voxel-level scales within both regions to suggest that what is going on in these locations is quite different in the two pain scenarios.

So, despite our mirror neurons and the best intentions, pain is not readily shared, and counts quite a bit less when it happens in others rather than in ourselves. Which is good, since being incapacitated by someone else's pain would probably help neither person. It also points to the importance and privacy of personal consciousness. While we keep coming up with technologies to share / immerse our mental selves in other worlds (telling stories, books, movies, VR), we do so because our capacity to do so unaided is quite limited. We are naturally wired first and foremost to feel ourselves.

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  • We are not a healthy country.

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