Monday, October 31, 2011

Real Pain, Social Pain

Test Tubes

We often talk about emotional trouble as painful. We feel hurt by rejection, we smart from hurtful remarks, We get burned by a bad relationship, our hearts ache for company and so on. There's lots of similar expressions in other languages too; bitterness can be expressed as a form of pain in Japanese (苦痛) and your ears will hurt (耳が痛い) from hearing a painful truth. In Swedish, too, rejection and other negative emotions are painful, and experiencing others misfortune can be heart-cutting (hjärtskärande).

Emotional distress as pain is a good, productive metaphor. But — what if it's more than a metaphor? Our experience of pain is a function of our brains after all, just like emotions are. Bodily pain starts with receptors on our skin and elsewhere, but the experience of painfulness definitely happens in the brain itself.

Amputees can experience phantom pain, where the brain is led to believe there's pain in a body part that no longer exists. On the other hand, many pain relievers like morphine or codeine act on the brain pain centers rather than at the source of the pain; I've heard one person describe the effect of a similar drug as "I knew it still hurt a lot; I just no longer cared."

Do Pain Relievers Help with Social Pain?

Now, if emotional pain is real pain — if, in other words, "painful" emotions actually use some of the same circuits as physical pain in the brain — then central nervous system pain relievers, such as acetaminophen — commonly known as paracetamol — should work for emotional pain as well. And this is what a group led by Naomi Eisenberger set out to test recently. They published a paper, Acetaminophen reduces social pain: behavioral and neural evidence about this last year.

Unfortunately, the paper is heavily paywalled and I can't get it from the publisher. By current standards of science journalism we'd be going well above and beyond our duty simply by reading the abstract. But fortunately the authors have put up the paper on their own website: you can download the PDF right here1. And there's another, earlier paper from Eisenberger, Why rejection hurts: a common neural alarm system for physical and social pain2, that lays out a lot of the evidence for a common mechanism between physical and social pain.

They recruited two groups of participants — 62 university undergraduates in total. One group took paracetamol twice a day for three weeks, and the other one took a sugar pill, or placebo. The participants rated the amount of social pain they experienced every day. Social pain dropped significantly3 over time among those who took the pain reliever, while the people with the placebo showed no change. The pain reliever seems to lessen the pain of bad social events.

They also did a brain scanning experiment with two smaller groups, 25 people in total. The groups got either paracetamol or a placebo for three weeks. Then they got to play a simple computer ball-tossing game while lying in an fMRI scanner. They thought they played with two other people, but the game was really pre-programmed. The computer "players" gradually ignored the player and refused to toss their ball to them, making them feel rejected and left out.

They found that those who had taken paracetamol for three weeks had much less activity in two brain areas (the anterior insula and anterior cingulate) that we know are involved in the emotional aspect of pain. But there was no difference between the two groups in how painful that ball-game rejection was to them.

So it does seem that long-term doses — note that the effect took a couple of weeks to appear — of some pain relievers really can lessen the pain of social rejection. But the effect is not big, and it doesn't seem consistent. So don't go eat paracetamol on a daily basis to feel socially better — acetaminophen is not good for your liver, and especially so if you also like to drink alcohol.

fMRI scanner. That's me lying there getting ready for a scan. I wasn't ill or anything; I just volunteered as participant in an experiment. It was a fun experience, though difficult to stay awake for the entire experiment. And as a bonus you got confirmation that there's nothing obviously wrong with your brain.

…But There's More!

As it happens, another group led by Tor Wager did a similar experiment just this year (it's Open Access; anyone can read it). They recruited a group of people that had recently been dumped by their partners and stuck them in an fMRI scanner to find out what areas are involved with social rejection. They ran two sets of scans, one to find areas for social pain, and one for physical pain.

To test social pain they showed the volunteers either a picture of their ex-partner and asked them to think about their rejection; or a picture of a friend and asked them to recall a pleasant experience they've had with them. This was repeated multiple times while their brain activity was scanned. This way you can compare the brain activity with and without the bad experience, and the areas that are active only for the bad experience are probably connected to their rejection in some way.

They did the same kind of thing for physical pain: the volunteers either got burned on the arm4, or just pleasantly warmed on the same spot. Again, you look for differences between the painful and the non-painful heating in the brain scans. That should show you what brain areas that react specifically to physical pain, rather than to heat or things touching your arm and so on.

When they compared the two sets of differences, they found that the brain areas that deal with the emotional aspects of pain — the "feeling bad about it" — are activated by both emotional and physical pain. That's the same areas that Eisenbergers group found, and it's exactly what we'd expect. But they also found common activity in areas that deal specifically with physical pain. The emotional rejection activates areas that normally only react to bodily injury, in other words. This is surprising, and previous experiments have not found this.

One major reason, Wager's group notes, may be the level of pain. In Eisenbergers fMRI experiment, people played a simple computer game with strangers who weren't being fair to them. Not nice, but not exactly a major life crisis either.

In this experiment, on the other hand, people that have just been dumped get the picture of their traitorous ex-partner shoved into their face, and are asked to please really think through the whole sordid series of events that ended with them being thrown on the curb like yesterday's garbage. It's a whole different world of hurt, and I can imagine it took a bit of explanation to get this approved by the ethics committee.

So it may simply be that social rejection needs to be strong to actually qualify as pain. Our pain centers don't light up for every touch either; they need a minimum level of hurt to react at all. This could explain the puzzling result from Eisenbergers group, where the pain reliever seemed to have an effect for the students that reported daily social pain, but not when students were scanned. We probably encounter much worse social experiences in our daily lives than the computer ball-game they used for their fMRI scan. The pain reliever would lessen the impact of strong, but not weak, social rejection, just like it has an effect on a real skin bruise or cut but doesn't numb us to touch or slight discomfort.

…As This Is Getting Too Long Already…

The takeaway message is, I think, that the difference between our mental experiences and the physical reality is quite blurred in our brains. Paper cut or hurtful word — by the time it reaches the brain it's all just nerve inputs. There is nothing intrinsically more painful about the signals coming from your skin than from your ears. The difference is only in how our brains treat those signals.

Evolution is ultimately pragmatic. If it is useful to treat strong social rejection as physical pain then it will. It doesn't matter if it doesn't make sense from a design point of view, if it makes for a messy, untidy system, or if it will cause unintended side effects and problems for some distant descendant. The brain is full of opportunistic shortcuts, multiple-use mechanisms and exaptations which makes for a very interesting task trying to untangle it all.


#1 Are they breaking copyright by making their paper available like this? Maybe, and maybe not. A lot of journals do allow authors to make "draft" versions available; the difference to the published version is usually little more than the addition of magazine logos and page numbers. And depending on the legal residence of the researcher and of the journal, and on the exact wording of the contract, the researchers may retain the right — explicitly or though fair-use provisions — to disseminate their own paper.

On a more practical level, any journal that tries to sue its (unpaid, and frequently paying) contributors for passing out their own work is probably going to find themselves in a bad public relations debacle, with far worse consequences than the possibility of having lost fifty or a hundred dollars in revenue.

#2 If you're not familiar with the research world, you may not know why Dr. Eisenberger is the last author in the current paper, but the first in this earlier one. Very simplified, the first author is typically the one who did most of the actual research. The last author is their supervisor, or research leader or PI (principal investigator). They may have done parts of the actual work, but more likely provided guidance, original ideas, money and other resources.

Dr.Eisenberger, we can infer, probably did this earlier paper as a post-doc in Dr. Lieberman's lab, then managed to secure funding for her own lab, where she apparently continues her line of research but now as a leader of a group of young researchers.

#3 That is the science meaning of "unlikely to be due to chance", not the everyday meaning of "sort-of important". If you think about it, though, the meanings do overlap quite a bit.

#4 There's ways of using low heat to create intense burning sensation without any actual damage. Still, pain is not something you'd use lightly in experiments.


canthushme said...

Most there is no real hard evidence that is I get dumped instead of getting hammered or really drunk, I could take acetaminophen instead, and feel...well, less pain?

Guess that explain the whole "lets get drunk"...thus when drun we can feel less pain or just ignore it like the common phrase says "I bet thats going to hurt tomorrow"

Janne Morén said...

I made the alcohol connection as well, at first. But it turns out alcohol does not seem to be a central nervous system analgesic, so it doesn't actually work the same way. Would have been neat if it did.

Alcohol and acetaminophen are probably complementary in effect. But those two are a very bad combination, at least if you like your liver and want to keep it. Also the actual effect of paracetamol is pretty modest, and seems to happen only gradually over weeks, so it's not really a realistic alternative in practice.