YOUR PAIN: THE SENSATION OF PAIN ITSELF

Jun
16
2011

We are used to discussing sensation as the consequence of stimulation in a series of boxes: first, injury generates an announcement of its presence in sensory nerves; second, the attention mechanism selects the incoming message as worthy of entry,- and third, the brain generates the sensation of pain. But we have to ask how the brain interprets the input. The classical theory is that the brain analyzes the sensory input to determine what has happened and presents the answer as a pure sensation. I propose an alternative theory: that the brain analyzes the input in terms of what action would be appropriate.Let us explore these alternative theories because they have practical consequences for pain therapy. If the classical theory were true, the first action of the brain is to identify the nature of the events that generated the sensory input. This should produce the first sensation of injury as pure pain. The next stage of the classical theory is that different parts of the brain perceive the pure sensation and generate an assessment of affect: ‘is the pure pain miserable, dangerous, frightening and so on?’. My first reaction, on introspection, is that I have never felt a pure pain. Pain for me arrives as a complete package. A particular pain is at the same time painful, miserable, disturbing and so on. I have never heard a patient speak of pain isolated from its companion affect.Because classical theory assigns different parts of the brain to the task of the primary sensory analysis and the task of adding affect, one would expect some disease to separate pain from misery. No such disease is known. During neurosurgical operations, very small areas of brain can be stimulated, some of which cause pain. There has never been a report of pain evoked that was not accompanied by fear, misery or other strong affects. Finally, there are parts of brain, such as the primary sensory cortex, which have been classically assigned the role of primary sensory analysis studies and yet in the imagine studies, these are often reported as silent when the subject reports pain. Even for the sympathetic pain on hearing of the death of a friend, the sensation is inseparable from the sadness and loneliness.Instead, let us examine the alternative, which is that the brain analyzes its sensory input in terms of the possible action that would be appropriate to the event which triggered the whole process. There is in this absolutely no suggestion that any action need take place. Trained subjects and stoics may receive a clearly painful stimulus with no overt movement, even though they can later report the nature of the pain they felt. There are elaborate and extensive areas of our brain concerned with motor planning as distinct from motor movement itself. It is precisely these areas that are most obviously active when the brain is imaged in subjects who are in pain but who are quite stationary with no movement.The most astonishing example of the involvement of motor bias in sensory interpretation is seen in people who have suffered a stroke which has destroyed their inferior parietal cortex. This part of the brain lies on the side of the brain just above the pinna of the ear. If the stroke has occurred on the right side of the brain, these people appear completely unaware of anything on the left side of their world. They appear blind and deaf to anything occurring on the left and, most bizarre of all, when shown their own left hand they deny that it is part of them. When asked to draw the numbers on a clock face, they fill in the numbers from one to six and then stop.Italian doctors in Milan showed that this neglect of the left half of the world even applied to the memory of a scene. They asked their patients to imagine that they were walking into the cathedral square in Milan by a road that enters the square opposite the cathedral on the south side. They were then asked to describe the buildings in the square. These citizens of Milan could recite the famous buildings on the east side of the square but were quite unable to recall any on the west. After a rest, they were asked to imagine that they were entering the square by a road on the north side. Now, imagining that they are facing south, they can recite the names of buildings to their right on the west side of the square but are quite unable to list the buildings on the other side, which they had been able to describe when they imagined they were facing the other way. This all sounds like complete madness, and it is true that the patients are poorly from their recent stroke and usually paralyzed on the left side. However, this precise condition of one-sided neglect has been observed repeatedly in patients in many countries.Classical theory explained this condition by proposing that there was a complete sensory map of the outside world and of the body in the brain, and that the stroke had destroyed the left side of the map. Now comes the really astounding fact. Italian doctors, whose results were confirmed by many others, discovered that stimulation of the vestibular system in the ear completely restored all sensation on the left side. It disappeared again as soon as the stimulation stopped. What could be going on? The vestibules in the ear continually inform the motor system about the body’s position in the up-down and sideways directions. It is our major organ of balance. It is obvious that the map had not been destroyed in the patients but that they did not have the ability to refer to the entire left side of the map. How could that be? Disturbed messages from the vestibular system, which controls sensory-motor posture, had slammed the frame of reference for the whole brain so far to the right that it was unable to perform both its sensory and motor tasks on the left side. It is apparent that we can sense only those events to which we can make an appropriate motor response.What would be the consequences of following the hypothesis that sensory events are analyzed in terms of the appropriate motor responses? It would provide a more satisfactory explanation of the paradoxes produced by the classical hypothesis and would help us begin to understand the facts just described. What are the appropriate motor responses to the arrival of injury signals? They attempt: first, to remove the stimulus; second, to adopt a posture to limit further injury and optimize recovery,- and third, to seek safety, relief and cure. The youngest, most inexperienced animal may attempt a series of these responses triggered by built-in mechanisms. As the animal grows in experience, the reactions will become more subtle, elaborate and sophisticated. If the sequence is frustrated at any stage, the sensation and posture remain.Humans develop and elaborate the three-stage responses from the moment of birth. Until some ten years ago, pain in newborn babies was neglected and even denied by professionals for two reasons. The first was that the human brain was seen as a hierarchy of levels: the spinal cord, the brainstem and the cortex. This view had been introduced by Hughlings Jackson in the nineteenth century. Each level was believed to dominate and control the level below. The hierarchy of levels was believed to be an evolutionary development and to be repeated in the development of each individual. The ability to feel pain, misery and suffering was assigned as a property unique to the cortex. All reactions to injury in the absence of cortex were called simple reflexes and were thought mechanical and free of sensation or emotion. This was the view that led Descartes to deny mind to lower creatures, and was perpetuated in post-Darwinian neurology, which assigned sensation and emotion to recently evolved structures such as the forebrain and cortex. It is true that we have a poorly developed cortex at birth. It takes two years for the major motor outflow from the cortex to establish control over the spinal cord. The second line of reasoning was that, because babies could not feel pain, there was no point in giving them potentially dangerous analgesic drugs.Fortunately, thinking has changed such that pain in babies and children has become a major focus of attention. Of the many pioneers who brought about this revolution, I will mention two: K. J. S. Anand, a paediatric anaesthetist of Sikh origin now living in the United States, and Maria Fitzgerald, a neuroscientist in London. Turning away from endless inconsequential philosophy on whether a baby feels pain, they and others turned to practical objective measures. The first question was whether a baby who must be operated on soon after birth would prosper better if treated with a full battery of analgesics, as would be given to an adult. The answer was a powerful yes, and the result has been a marked change in neonatal anaesthesia and in survival. The second question was whether the injuries commonly suffered by babies, especially premature ones, produce a long-term shift of behaviour. Again the answer is yes. Fitzgerald showed that even the act of taking a blood sample without anaesthesia changed the motor behaviour of premature babies.In the child and the adult, there is a continuous development of the way in which the victim moves through the three stages of reaction. Experience teaches skills. Society adds its methods of help and its prohibitions. Expectation becomes tuned.*75\219\2*