This is part three of my review of the Lorimer Moseley and Paul Hodges conference on pain and motor control. You can read parts one and two here and here. This section will describe Lorimer’s discussion of the body maps and their connection to pain, motor control and immune responses.
The brain’s map of the body
Like pain, body perception is an output of the brain, not an input. It is something that the brain constructs based on information from the body, not something that is given. Various names have been assigned to the neural structures or neurotags which create this output - body maps, cortical body matrix, virtual bodies or body schema.
Moseley discussed a great deal of research showing how the cortical body matrix is intimately connected to various other outputs of the brain that basically comprise everything we care about in relation to manual therapy or corrective exercise - pain, coordination, and immune responses such as inflammation. Prepare to have your mind blown if you haven't read up on this stuff before.
Phantom movement maps
Body perception and movement ability are fundamentally linked. We can only do what we can imagine doing. Here’s an interesting example. Moseley asked phantom arm owners to try to move their phantom hands into physically impossible configurations. First they were unable to do so, but with practice some of them succeeded, presumably due to changes in the way the arm was represented in the brain. Their newfound ability coincided with a subjective inability to perform normal movements with the phantom limb. The lesson - we can only move in accordance with our maps. If there is no map for the movement, it cannot even be imagined, much less executed.
The sense of ownership
One fundamental aspect of perceiving the body is the sense of ownership over body parts. We take it for granted that our brain knows which physical objects in its vicinity are part of the body and which parts are not. But the sense of ownership is not so simple. Moseley has shown that it is fairly easy to encourage the brain to take “ownership” over inanimate objects, and “neglect” actual body parts. One of the best ways to cause this confusion is with the rubber hand illusion.
Rubber hands and neglect
I have previously discussed the rubber hand illusion. This involves placing one hand on a table, the other out of sight behind a screen, and a rubber hand to the side of the hand in sight. The subject’s hand behind the screen is then stroked with a brush while he watches the rubber hand stroked in the same way. Pretty soon he will get an uncanny sense that the rubber hand is part of his body, and he will even flinch when it is threatened.
What has happened here is that the brain has basically taken ownership of the rubber hand. This is amazing enough, but what I find particularly incredible is that the brain also disowns or “neglects” the hand that is out of sight. How do we know this? There is a change in blood flow to the neglected hand. Yep, the hand behind the screen actually gets colder. And it just gets weirder. It turns out that coldness and neglect are a two-way street, so that it is easier to experimentally induce neglect in a hand that is already cold, than in one that is warm.
Here’s another cool experiment. A hand which has been disowned through the use of the rubber hand illusion will suffer less inflammation in response to a physical insult then a normal hand. Wow.
But what relevance does this have to situations not involving deliberately constructed illusions? Lots. Research shows that similar forms of neglect and map confusion are involved in many common forms of pain.
Chronic pain and fuzzy back maps
Try to imagine the outline of your back. Some parts will seem clear and others might seem fuzzy, harder to define. Research shows that your the ability to form a clear outline of the back will decrease in the area of chronic pain.
Moseley discussed a great deal of other research showing map disturbance in cases of chronic pain. For example, subjects with one sided back pain tend to locate their vertebra closer to the painful side than the non-painful side. Two point discrimination (the ability to know whether you are being poked with two needles or one) is less sensitive in areas where there is chronic pain. There are many other studies showing that pain is associated with disturbances in the cortical body matrix, and that body parts in pain tend to get neglected
Why the neglect?
So why does the brain neglect body parts that are in chronic pain? I have previously speculated that this is due to sensory gating - more nociception means less attention to proprioception, which means less information necessary to fill out the details of the body map. But Moseley introduced another speculation to account for poor body awareness in areas of pain: that neglect serves a useful protective purpose. To describe this idea Moseley used the example of a patient with CRPS (complex regional pain syndrome). Such patients can experience pain and inflammation of their affected limb simply by thinking about it. Perhaps neglect is a way to avoid the overactivation of pain and immune response neurotags that have become too facilitated or imprecise. (Recall that neglected hands suffer less inflammation in response to a wheal.)
And consider this study that Moseley also discussed. Subjects with CRPS who view their affected hand with magnifying glasses experience increases in their pain, while wearing “minimizing” glasses will reduce pain. It seems that greater awareness of the hand creates more pain. Perhaps neglect is a way to protect the hand from too much awareness. Interesting idea, and a caution that body awareness, like movement, can be a double-edged sword, capable of causing harm if it is to threatening.
Maps of space
Apparently the brain’s outputs in regard to a certain body part, including the sense of ownership, extend not just to that body part, but to the area of space associated with that body part.
In other words, if you have a painful or neglected right hand, you might also have a painful or neglected right “space” around the hand. Patients with CRPS tend to neglect their painful limb and it is consequently usually colder than the unaffected limb. Guess what happens when you have a person with CRPS sit at a table and swap the positions of their hands for 10 minutes? Their unaffected hand gets colder, showing that the patient not only has a cold right hand, but a cold right side.
Other research shows that if you move a neglected hand close to a certain part of your back, then your two point discrimination in that part of your pack is reduced. In other words the brain will continue to neglect the space of the painful right hand wherever it goes.
I think it was at about this point in the presentation when Lorimer asked his audience - “Now how hard is your job?” The hard reality is that the body is not a simple machine, and mechanical factors are just a small part of the whole pain story. Unfortunately, we can't fix back pain by just stretching the hip flexors and strengthening the abs. It ain't easy folks!
Coming up next
That's it for now. In the next part I will try to wrap up the Moseley talk with his discussion of nociception, the variety of ways it can be modified, and the ways we can classify the different kinds of pain.
Read part four here.
If you want to read more about pain and some of Lorimer Moseley's research, check out some of the posts below: