A 'NORMAL' PAIN RESPONSE: IMAGES
For all the variation of response discussed in this chapter, it might seem reasonable to believe that somewhere in the head there is a big flashing light that says, 'PAIN', and that this would be in the same location in everyone in pain. This is an allegory but in actual recorded data one might expect a certain group of nerve cells to become more active, and to be the coded signal associated with our feelings and doings that we call pain as a shorthand expression for the associated activity. In the last chapter, we looked at examples of activity in various parts of the brain associated with pain states. They were obtained by the PET scanning method, which produces fascinating results but has weaknesses. To protect the subject from radiation, it is necessary to inject only small amounts and so the detected signal is weak. To raise the signals out of the noise, it has been necessary to add the results from many individuals. Obviously, this has not permitted an answer to the question of individual differences.
I mentioned a new method, fMRI, which does not involve radioactivity, has a better time and spatial resolution, and permits the analysis of the amounts of neural activity in individuals. The first results are just appearing and they are a shock. Twelve normal individuals were scanned while they were receiving brief, hot, painful stimuli to one finger. As we have come to expect, the amount of pain reported by the subjects varied widely (from 1.5 to 8 out of 10), despite the fact that the stimuli were identical. If all 11 individuals are pooled, they show increased activity in the same group of structures that had been shown to be active in the PET studies. However, if the individuals are examined one by one, no two individuals have the same pattern of activity. Of course, this is a serious shock to the traditional thinkers who expected specific, specialized brain structures that could be named as the location of the flashing sign.
The result is not as hopeless as traditional thinkers might feel. They may be a sign of each individual adopting a personal tactic for dealing with the stimulus. We have looked in this chapter at the variety of options that are apparent in a group of individuals. In the same individual with repeated identical stimuli, the tactics may change. Everyone has had that experience at the dentist. I was a guinea-pig for an antimalarial drug trial which involved my giving daily blood samples for weeks. I grew to hate the needles and used every trick I could invent to cope with the brief pain of the intravenous needle. The traditional expectation is that neural codes exist in the rate and amount of activity of groups of cells. The more the cells fire, the bigger will be the signal. Of course, there are many other possible codes and there is evidence for some of them. For example, there might be a time code where first one group fires and then another. The time resolution of the contemporary imaging methods is not yet quite good enough to achieve such a search.
There is another more subtle version of the time code in which assemblies of cells could group together in such a way that only synchronous impulses count as letters in the code. Much of modern computer software is concerned with codes, and each variation suggests ways in which the brain might operate. A nightmare for the neuroscientist would be the existence of shifting codes because these are hard to crack. We should not be depressed that the most advanced modern techniques fail to show a single simple focus of brain activity associated with pain. Pain may be described as a single simple word but it implies a class of responses involving many areas of our brains and bodies. The pattern of response varies from person to person, and within an individual it varies from one painful episode to another.
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Pain Relief