Before answering the question, what is feedback, it is necessary to understand what a system is. A system is a collection of elements that work together to achieve a common function or objective, where the elements that comprise the system can themselves be other systems. For example, the nervous and immune systems function together as when immune function is altered during times of stress. So the parts of a system work together to achieve some goal. But how does the system know it’s doing the right thing? Put another way, how do systems regulate themselves? How does the endocrine system know when to stop (or begin) releasing a particular hormone? How does the muscular system know how much force to apply to the muscles of the leg? How does the cardiovascular system know what should be the appropriate heart rate and blood pressure? And in a question of particular interest to me, being a researcher in pain, how does the pain system know how much pain is necessary? The answer to all these questions is feedback.
Feedback is, quite simply information that is returned to a system that enables it to regulate further output. Feedback is not just a nice-to-have icing on a system’s cake, but rather is a critical, integral component of all systems. Since almost everything in nature is ultimately hooked into a system, and since feedback is such a crucial component of all systems, it behooves us to understand feedback. Let’s look at some examples to get a feel for how critical feedback is to almost everything.
Try to imagine driving a car without being able to see. You, the driver, are a system. You consist of a collection of elements working together to achieve an objective — driving. But without being able to see where you’re going, you would not be able to function, to drive. You’d try your best to get all the initial conditions correct — you’d get the steering wheel as straight forward as possible, press down with a force that you’d known from the past should produce a particular acceleration. But deprived of feedback you’d soon drive into a wall, or worse.
How do you know you’re a good person, a smart person, a worthwhile person? That’s right: feedback. Over time, you received feedback to consistent with these evaluations. You know you’re worthwhile because you befriended other people who wanted to spend time with you. You know you’re smart because you received good grades in school and you seem to be able to make your way in the world reasonably well. You know you’re a good person because you like to help others whenever possible and you’ve received many “thank-yous” from the recipients of your generosity. Now let’s do a thought experiment. Imagine that you never received any such feedback. Imagine that you went through life without any feedback from the world? How would you know who you are, what you are, why you are? Indeed it is fair to say that your entire identity has been forged through the operation of feedback.
Now, let’s take a look at the problem of pain. Pain is a system like any other system of the body, and like all systems, it relies on feedback. One of the most intriguing demonstrations of this reliance on feedback is the phenomenon of phantom limb pain (PLP). PLP refers to the perception of pain in a limb that has been amputated or deafferented. Notwithstanding its name, PLP is very real, impacts the majority of amputees and can be extremely severe. How could a limb that is no longer present possibly be painful?
Well, it appears that the reason has to do with the fact that there is a one-to-one mapping in the brain between regions of the body and the portion of the brain that represents that region. The hand, fingers, arm, face, legs, etc. are all represented somatotopically; that is, in they are represented by different and specific sections of somatosensory and motor cortical tissue. More than that, the amount of real estate reserved for the various parts of the body in these cortices is proportional to the sensory resolution required. For example, the mouth, lips and fingers take up relatively large proportions of cortical tissue compared to other parts such as the back and arms. The result is that we can distinguish between two points less than 1mm apart in our fingers but two points applied 1 cm apart on the back will feel as if it were a single point. Perhaps you have seen the sensory and motor homunclus (little man) figures depicting the representation of the body in the sensory and motor cortices (see figure below).
Ordinarily, signals (feedback) from the body reach their corresponding portion of the brain and all is good. But what happens if, say an arm is amputated? There are no longer any signals going from the arm to the brain. So what happens to the portion of the cortex that previously received signals from the arm? Well what happens is that the areas of the cortex neighboring the now idle tissue begin to migrate so that the tissue that previously represented the arm is now representing whatever its neighbor represents. As you can see from the sensory homunculus picture, the cortical tissue adjacent to the hand and arm is the face. That means that sensations in the face will begin to be represented in the cortical tissue previously representing the hand and arm. So what’s the problem, you ask? Wouldn’t it just mean that sensations in the face become even better? The answer is no. In fact, this cortical migration is often referred to as “maladaptive plasticity” and is associated with pain. In fact there is a very strong correlation between the magnitude of migration and severity of pain ratings. The exact reason why this remapping of cortical surface is associated with pain is unknown. But a particular treatment, mirror therapy, has been developed that compels a reversal of the migration and with it, a substantial reduction in pain.
Mirror therapy for missing upper body limbs typically entails the use of a device containing two compartments, one of which the individual places his intact hand or entire arm. The mirror reflects the activity in such a way that it conveys the impression that his amputated limb has been returned and it can be moved. To put it in the words of Ramachandran and Rogers-Ramachandran (2000), “When viewing the reflection of the intact hand in the mirror, “the reflection of his own right hand is optically superimposed on the felt location of his phantom limb so that he has the distinct visual illusion that the phantom limb had been resurrected. If he now made mirror symmetric movements while looking in the mirror, he received feedback that the phantom limb was obeying his command.” (Ramachandran & Rogers-Ramachandran, 2000, p. 319). The mirror in this case essentially alters the feedback presented back to individual. As far as his brain is concerned, he has both limbs intact. This feedback is so convincing, in fact, that it gradually leads to the reversal of the problematic cortical migration.
Image credit: http://uc.exteenblog.com/highwind/images/Know/brain_homunculus1.jpg
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