I once got hit just above my eye by a cricket ball, which is much like a baseball only harder. One instant, the missile was safely cupped in the wiry fists of a fellow nicknamed The Wooloomooloo Whippet, and the next instant it was rattling my braincage. An hour later, my eye feeling very swollen, I strolled around the Ladies’ Stand awaiting congratulatory warrior-worship type comments. None. Not even one. Nobody commented on my heroics or my brutal injury. I sulked off to the bathroom, the mirror of which revealed that my eye was not in the least bit swollen. I can guarantee it had felt swollen. I had even been able see the lump protruding into my peripheral vision. In fact, as soon as I saw myself in the mirror, the feeling that it was swollen, and the bit of it I could “see,” vanished. How does that work? Well, how our body feels—the awareness we have of our physical self—is constructed by the brain. It depends on the maps of the body that are held within our brain and emerges as a conscious output.
These body maps become altered in people with pathological pain. For example, in phantom limb pain, which involves feeling pain in a limb after it has been amputated, the altered maps may in fact contribute to the pain. One way to treat such pain is by directly training the brain to correct the distorted maps. Another way to treat such pains is by instructing the patient to imagine making certain movements with the phantom limb. Although we don’t know how such motor imagery works in the brain, one possibility is that it, too, corrects the distorted maps.
The Map Moves
In a lovely study by neuroscientist Kate MacIver and colleagues from the University of Liverpool, 13 arm amputees with phantom limb pain underwent brain scans before and after a training program in which they imagined movements of their phantom limb during daily periods of relaxation. The key measures from imaging were brain activity evoked by: pursing the lips, opening and closing the intact hand, and opening and closing the phantom hand. Why scan the brain of people with upper limb pain while they purse their lips? There is very good evidence that in amputees with phantom limb pain, the brain maps reorganize so that the representation of the lip (the “virtual” lip) shifts to where the missing hand should be—about four millimeters away. In amputees without phantom limb pain, there is no, or very little, shift. A shift of that size may seem trivial, but considering that the sensory cortex has about 20,000 brain cells per cubic millimeter, it actually represents a monumental change in the response profile of brain cells.
Here is what the team found before training: When the healthy controls pursed their lips, they activated their virtual lips. When they imagined moving their hand, they activated their virtual hand. No surprises there. In contrast, when the amputees pursed their lips or moved their phantom hand, they activated both their virtual lips and their virtual hand. They also activated parts of the sensory cortex that normally represents the other side of the body—the virtual opposite hand if you like. These results are interesting enough but not altogether surprising. They corroborate a growing body of literature that demonstrates that people with pathological pain have distorted maps of the body, or a generalized disinhibition of parts of the brain (reduction of the normal inhibitory control that keeps brain activations in check).
The real punch of this study lies in the changes that were imparted by training. Here is what they found after six to 12 weeks of the training program: nine of the 13 reported that the intensity of their pain had been halved, the amputees started to show the same activation pattern during lip pursing and phantom hand movements as the healthy controls do, and the extent of pain relief and the extent to which brain activations returned to normal were correlated.
One obvious limitation is that, although this study had healthy controls in training for comparison, it did not have a control condition of patients and healthy subjects who did not get the training. So we don’t actually know for sure whether the training program was important in imparting the effects. That said, anyone in the know would be absolutely gobsmacked if it didn’t. Perhaps a more interesting issue is what contribution the two main components of the training might have made to the effect. The obvious aspect is the imagined movements: we know that imagined movements involve the same brain mechanisms as executed movements and we know that practising movements refine those mechanisms. This study clearly suggests that the same processes apply to phantom limbs as well as intact limbs. The less obvious facet of the finding is the relaxation/body scanning part, which actually comprised the bulk of the training sessions. Simply thinking about body parts activates their virtual counterpart—one can’t feel one’s body without using neurons that represent it. Honing in on a particular body part requires inhibitory processes, the loss of which might underpin the extravagant activation patterns that were observed in the pre-training scans. I hope this research group teases out these components in its next study!
The Mutable Brain
Regardless of the active component, this study gives insight into a possible mechanism of pain relief for people with phantom limb pain. In itself, this finding is terrific, because phantom limb pain is common in amputees, it is resistant to drugs and it can be at least as debilitating as the absence of the limb.
The study also corroborates a growing literature on the lability of the human brain. Although brain plasticity might underpin the remapping that contributes to phantom limb pain in the first place, the very same plasticity can also be exploited to return the brain to normal and reduce phantom limb pain. Psychiatrist and psychoanalyst of the Columbia University Center for Psychoanalytic Training and Research Norman Doidge refers to this effect as “the dark side of plasticity.”