It hurts less if you bang yourself than if someone else bangs you!

In a recently published study (Wang, Wang, & Luo, 2011) the researchers wrapped 4 polyhedral crystal bead strings around a ring. In one condition, participants were asked to squeeze the ring in their left hands with their right hand (as in the first picture below). In another condition, the experimenter squeezed their left hand (as in the second picture below). The researchers have labeled the first condition "active pain" (because people administered the pain using self-propelled movement) and the second condition "passive pain" (because the movement was driven by someone else).

After each squeeze, participants rated how much it hurt and how unpleasant the sensation was. Each squeeze was a trial and participants received a series of these trials. A subset of participants also had their brains scanned while performing the squeeze trials.

The first thing they found was that both pain intensity and unpleasantness of self-induced pain were rated significantly less painful than externally induced pain. The second thing they found was that areas in the brain that have been linked to pain processing (particularly the primary somatosensory cortex, anterior cingulate cortex, and the thalamus) were inhibited during self-induced pain. In other words, brain activity during self- vs. other-inflicted pain was distinct.

Now, the researchers interpret their findings in terms of motion. That is, they conclude that active movement (where one uses one's own muscles to move one's own hand) inhibits mechanical pain occurring at the same time as the movement and leads to inhibitory action in brain-associated areas in the cortex. This may be true but it also seems possible that the observed inhibition was the result of enhanced feelings of control. When you do something to yourself, you are in control. You can stop at any time. The perception of high levels of control have been shown to be associated with lower pain.

The upshot is that I'm not sure whether the motion or the control explanation is correct. Perhaps it's both. Perhaps its neither. It would be interesting to replicate this study but to include conditions where the self-induced pain is accomplished with and without movement. Then we will have a better idea whether it's control, movement, or both.

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Wang, Y., Wang, J.-Y., & Luo, F. (2011). Why Self-Induced Pain Feels Less Painful than Externally Generated Pain: Distinct Brain Activation Patterns in Self- and Externally Generated Pain. (A. Serino, Ed.)PLoS ONE, 6(8), e23536. doi:10.1371/journal.pone.0023536.t001

TENS and the remarkable power of suggestion

In clinical trials of a new drug it's easy to implement a placebo (comparison) group. Just give people a pill that looks the same as the active pill yet is absent the active ingredient. Many participants will try to determine to which group they had been assigned, but it will usually be difficult to do. Now some individuals will try to figure it out by whether they have any side effects (patients will have been told of potential side effects as part of the risks of participation during informed consent), but this is not a reliable because some people who are in fact in the placebo group (and receive nothing but starch) will exhibit side effects, perhaps because they are anticipating them (this is known as the placebo effect's dark side counterpart: nocebo effects). But in the case of non-drug treatments it can often be very tricky to develop a useful comparison group.

For example, say you want to investigate the efficacy of transcutaneous electrical nerve stimulation (TENS). TENS involves the delivery of electrical currents through the skin to the nerves. When a TENS unit is operating you feel it as a tingling sensation. People know this and they will get suspicious if that tingle is not there! So you can compare a treatment session with real TENS against a "sham" treatment in which all the equipment is there and used as it would be used during a real TENS session except that the electricity is turned off. But this is not likely to fool people and this will tend to lead to overestimates of TENS efficacy because suspicious people will have lowered their expectation for improvement, thereby widening the gap between improvement due to the specific action of TENS and improvement due to placebo effects.

Well I came across a study (Langley, Sheppard, Johnson, & Wigley, 1984) that was actually done many years ago but the findings were very intriguing I thought. They randomly assigned rheumatoid arthritis (RA) outpatients to one of 3 groups: Groups A and B received two different forms of electrical stimulation and Group C (the placebo group) received no electrical stimuli but instead watched a representation of TENS electrical stimulation on an oscilloscope. The researchers found no difference between on any of the groups on changes in resting pain and grip pain from pre- to post-treatment in these RA patients. This means that even in the absence of any tingle sensation they accepted the visual feedback as sufficient evidence that they were receiving TENS treatment, which was enough to boost placebo effects to those achieved by the TENS treatment, with the tingle and all.

The power of suggestion really is almost magical. We want to believe. We really do. Especially people who are in pain (as these RA patients were) and are looking desperately for relief. We will latch onto the evidence is support of our beliefs while ignoring even strong evidence to the contrary.

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Langley, G. B., Sheppeard, H., Johnson, M., & Wigley, R. D. (1984). The analgesic effects of transcutaneous electrical nerve stimulation and placebo in chronic pain patients. A double-blind non-crossover comparison Rheumatology international, 4(3), 119–123.

Childrens' pain catastrophizing is positively related to their parents' pain catastrophizing

Pain catastrophizing (PC) refers to a kind of exaggerated response to pain. According to Michael Sullivan (at the Center for Research on Pain, Disability and Social Integration, at McGill University) PC has 3 components: rumination, magnification, and helplessness. Thus people who would rank high in measures of PC would tend to spend more time thinking about their pain, worrying about it and its consequences, and feeling less control over their pain than those who would rank low in PC.

A recent study administered the Pain Catastrophizing Scale (PCS; Sullivan, Bishop, & Pivik, 1995; a very commonly used measure of PC) to chronic pain patients, their spouses and adult children.

They found that there was a moderate positive correlation between both parents' PCS scores and their children's' scores (r=.37) and that parents' PCS scores explained 20% of the variance in their children's' scores. Of course this means that 80% of the variance in children's scores is attributable to other factors, this is not not trivial amount either.

Interestingly, the mean PCS scores were higher in the patient group than in their spouses or children. This makes sense. After all, it is to be expected that people who live with pain every day will spend more time thinking and worrying about their pain and feeling less in control than people who do not suffer from chronic pain.

A big weakness of this study is that it's cross-sectional, which means that all measures were taken simultaneously, which can only tell us whether different variables (parent PCS scores and children PCS scores) are related, but can tell us nothing about causation. There is a temptation to conclude that parental behavior somehow influences children's behavior but it seems just as plausible that children's reactions to the pain expressed by their parents influences parents' own reactions. Also the mechanism may be observational learning, genetics and perhaps other factors. Still these correlational studies can help clinicians make predictions. And that's important. From a relatively easy to acquire PCS score clinicians would be able to predict which children are at greater risk of high PC themselves.

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Kraljevic, S., Banozic, A., Maric, A., Cosic, A., Sapunar, D., & Puljak, L. (2011). Parents’ Pain Catastrophizing is Related to Pain Catastrophizing of Their Adult Children. International Journal of Behavioral Medicine. doi:10.1007/s12529-011-9151-z

Making the body appear smaller or larger can alter pain perception

In a recent study, researchers employed a clever experimental setup that used 3 different mirrors to make it appear that a person's hand was twice the size, half the size or the actual size of his/her real hand. To grasp the setup it helps to look at the figure. Looking at the right hand side of the "hand-view condition" panel, you can see that participants sat at a table with left hand behind the mirror and the right hand on the reflective side of the mirror. This setup induced the impression that what they were seeing reflected by the mirror was actually their left hand.

I know this may sound a bit difficult to imagine but I've tried it and believe me the illusion is uncanny -- yeah you know that what you are looking at is really just a reflection of your right hand but what you perceive nonetheless is that the mirror is not a mirror at all but a window and that you're looking at your left hand through the window (the sensation is particularly powerful if you move your right and left synchronously). This illusion has actually been used to successfully treat otherwise stubborn pain conditions such as phantom limb pain and complex regional pain syndrome.

Anyway, the whole reason for this complicated setup was so that the researchers could achieve the illusion of different hand sizes. After an adaptation period to get participants' brains "convinced" that their right hand was actually their left hand, they applied a heat probe that got gradually hotter until participants pushed a pedal to indicate the sensation was painful. They repeated this over a series of trails, swapping the mirror in a random sequence and rated pain threshold under conditions of different apparent hand sizes. One final wrinkle: instead of seeing their hands, some participants saw a block object.

Two interesting results emerged: First, merely observing the hand in the non-distoring mirror increased pain thresholds. That is, just focusing on the body part that was experiencing pain somehow made people less sensitive to pain, enabling them to accept higher heat levels before reporting pain. Second, for people in the hand (but not the object) view condition, visual enlargement of the hand enhanced the analgesic effect occurring when people look at the painful body part, whereas visual reduction decreased the analgesic effect.

Intriguing. If you have a pain in your hand, just stare at it and the pain will diminish. View the painful region under a magnifying glass and the pain will diminish even further!

Perhaps somewhat confusingly, these results are different from those obtained by Moseley et al (2008) who reported that pain and swelling in complex regional pain syndrome (CRPS) patients increased when patients viewed the affected limb enlarged. The authors of the current study suggest that the neurophysiological distinctions between acute and chronic pain may underly the opposite results. For example, they point out that CRPS in fact alters the representation of the affected limb in somatosensory regions of the brain. Such changes might lead to differences in the way the brain perceives the affected regions of the body.

It will remain to future studies to investigate the mechanisms mediating this fascinating finding.

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Mancini, F., Longo, M. R., Kammers, M. P. M., & Haggard, P. (2011). Visual Distortion of Body Size Modulates Pain Perception. Psychological Science, 22(3), 325–330. doi:10.1177/0956797611398496

Assuming a dominant posture makes people less sensitive to pain

Recent studies have found that adopting physical postures associated with power leads to psychological and physiological changes in line with power. For example, in one study (Carney et al, 2010), adopting an expansive posture (taking up more space) led to increased testosterone and decreased cortisol levels, which are hormonal changes linked to dominance behaviors. At the same time, experiencing power can give rise to a greater perception of self-efficacy and being in control, psychological states that are associated with diminished sensitivity to pain. Thus, the authors of this study hypothesized that people who assume power/dominance postures would exhibit lower pain sensitivity compared to those who assume submissive postures. And that's exactly what they found.

The researchers told participants that they were investigating the health benefits of exercise at work and that they would be adopting a series of yoga poses (see below).

So first, participants were given a pre-treatment (baseline) pain test. A blood pressure cuff was placed on the arm and inflated until participants indicated they began to feel pain. The pressure was recorded. Then participants were randomly assigned to one of the three poses below. The arms out pose has been found to be associated with power, and the curved torso with submissiveness. They assumed this pose for 20 secs. Then a post-treatment pain test was administered.

From left to right: dominant, submissive and neutral poses.

They found that participants' post-treatment pain thresholds were significantly higher (were less sensitive to pain) after assuming the dominant pose than either the submissive or neutral poses.

These results are in line with the idea embodied cognition, that the mind is influenced by the form and state of the body. We have long known that the brain can influence the body but there is increasing evidence that the influence works both ways.

These results have important clinical implications. They suggest that people in pain may be able to enhance their feelings of power over their lives and even reduce pain by assuming postures associated with power.

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Bohns, V. K., & Wiltermuth, S. S. (2011). It hurts when I do this (or you do that): Posture and pain tolerance. Journal of Experimental Social Psychology, 1–5. Elsevier Inc. doi:10.1016/j.jesp.2011.05.022