When the volume on a radio or TV is set too low, our brains interpret voices as background noise – we are unable to understand what is actually being said. In the same way, there is a threshold of stimulation below which nociceptors do not report pain to the brain.
As we have seen, sensitization can lead to extreme, widespread pain. However, the opposite outcome may also occur. If the prolonged or repeated irritation of pain receptors is only at a mild to moderate level, a process known as habituation might begin. With habituation, the reporting of pain to the brain actually reduces, rather than increasing as in central sensitization. More than just “getting used to the pain”, habituation is a process in which the brain interprets nonthreatening pain messages less strongly.
It seems that our perception of pain is the result of an interaction between pain-producing and pain-reducing mechanisms. Understanding how the nervous system responds to pain offers an opportunity to develop better treatment strategies. One study7 has shown that when a group of healthy volunteers were given repeated painful experiences – for example, by having one of their arms exposed to 20 minutes of extreme cold, for eight consecutive days – the degree of pain reported (see page 14) reduced day by day, even though the same level of cold exposure was applied. The other arm and the legs also showed reduced sensitivity to the pain stimulus (in other words, they had also developed a higher tolerance to pain) – although not to the same extent as the originally tested arm. Interestingly, the higher pain tolerance persisted after the experiments were completed.
TOP TIP By making conscious relaxation a part of your daily routine, you can encourage a process of habituation to chronic pain that will help you manage your experience of it.
The researchers suggest that these changes in pain perception, which take place in the brain, probably represent a “defensive strategy against pain” by the brain. It is important to understand that the studies involved healthy adults and that similar habituation changes may not take place in the same way in individuals with existing chronic pain conditions – therefore research continues.
What we can learn from sensitization and habituation is that the amount of pain we feel, and how well we tolerate it, depends to a large extent on the way the brain interprets pain messages. The habituation response suggests that the brain and nervous system may be able to recognize when pain messages are of lesser importance. One objective in managing chronic pain might therefore be to encourage habituation, so that the brain can ignore unimportant pain messages. Methods such as relaxation and visualization, as well as recognizing pain for what it is without giving it excessive importance, appear to be ways of encouraging habituation. Whatever else we do about pain and its causes, we also need to try to reduce our overall stress levels, as these can have considerable influence on the pain load, and on sensitization to it.