Patrick Wall is a neuroscientist who is best known for the very influential gate theory of pain which he described in collaboration with Ronald Melzack. He was a co-inventor of transcutaneous nerve stimulation, which has proved a valuable method of pain relief for many patients. He is one of the foremost authorities on pain, and has helped to bring about a radical restructuring of our understanding of its neurophysiological basis. In this book he presents his latest thoughts on the subject for a general audience without specialist medical or scientific knowledge.
By way of introduction he adduces some puzzling facts about pain. There is no simple correlation between the severity of an injury and the degree of pain experienced by the patient; indeed, some severe injuries produce no pain at all at the time. The complexity of the phenomenon is illustrated by a number of case histories involving humans and also animals. The implications of these apparent paradoxes form the basis of the discussion in the rest of the book.
Pain, Wall insists, is not a simple matter of transmitting impulses along nervous pathways to some putative pain centre in the brain. There is no such centre. Modern techniques of imaging have shown that many brain areas are involved, including the frontal lobes, motor and sensory areas, anterior cingulate cortex, hypothalamus, midbrain, and cerebellum. Thus pain perception is a response of the whole brain, not of any one area.
The involvement of motor areas is particularly remarkable. Neuroscientists who adhere to classic views of pain think that this can be explained by saying that sensation occurs first, and is then followed by motor planning and the beginnings of movement. Wall, however, believes that this sequential idea may be a fundamental error. He suggests that there is no separation between the sensory brain and the motor-planning brain, and he goes on to describe a new hypothesis about pain based on these recent findings.
On Wall's hypothesis, attention is critical in pain perception. This explains the paradox that the degree of injury is unrelated to the severity of pain; everything depends on where we focus our attention. It also explains how hypnosis can prevent pain, because in the hypnotic state the attention is directed elsewhere. It is now possible to visualize the process of directing attention by imaging the brain. When pain is perceived, the anterior cingulate cortex becomes activated first, which is exactly what would be expected if attention is the key.
The classic theory says that the brain analyses the sensory input to determine what has happened and presents the pain as a pure sensation, after which what one might call an executive decision is taken regarding what to do about it. Wall's theory is that the entire brain, and indeed the whole body, are involved from the outset. There is no such thing as a pure pain; pain is always part of a motor response, or at least a planned motor response (changes occur in the motor system of the brain even when no overt muscular contractions ensue). This is a radical but illuminating idea, with far-reaching implications for those of us who are involved in trying to relieve pain in patients. It also has implications for philosophical problems such as the nature of consciousness itself.
On the practical level, Wall makes well-justified criticisms of the pseudo-diagnoses that patients suffering from pain are often offered. The cause of back pain is unknown in 85 per cent of cases, yet there is no lack of therapists who will give confident explanations ('slipped disc', or 'arthritis') to every patient they see. (To be fair to the therapists, patients often demand explanations of this kind and may feel insecure if the therapist professes ignorance.) 'Trapped nerves' is another pseudo-diagnosis; it is intuitively appealing, largely because it is conceptually simple, but although freeing nerves surgically sometimes gives relief, there is little evidence for nerve damage in such cases. In the case of Morton's metatarsalgia, a painful disorder of the foot which is supposed to be due to trapping of nerves between the ends of the bones in the forefoot, the surgeon removes the "trapped nerve" and this may help, yet when the tissue that is removed is examined it is rare to find any nerve material at all.
So why does treatment sometimes work in such cases? Perhaps the benefit comes simply from disturbing the tissues; this may be true for many kinds of surgical intervention for pain, including disc surgery and the operations used to treat trigeminal neuralgia. Wall is surely right to say that just because removing some tissue relieves pain, it does not follow that the tissue in question was causing the pain originally.
Pain is a fascinating subject theoretically and also of the utmost importance to us all, since we all experience it at some time and some of us suffer it constantly and severely. This book presents a lot of facts and interpretations which will be new to many people, health professionals and patients alike, and it is therefore to be welcomed. Unfortunately, however, Wall has not been well served by his editors. Minor and not so minor typos are plentiful. Some are obvious, such as 'complimentary' for 'complementary', but others are more subtle. It took me a couple of minutes to work out that the 'body hole' between each pair of vertebrae, containing the dorsal root ganglion, was really the 'bony hole'; would a reader without anatomical knowledge ever understand what was meant? And the date of the arrival of acupuncture in England is given as 1685 which is said to be in the Elizabethan age.
But it isn't only typos and historical errors that are the problem. There are a number of places where the sense is hard to follow, not because it it is over-technical, but because the editing has been careless. Reading the chapter on the philosophy of pain I gained the impression at first that Wall was defending dualism. This was obviously out of keeping with the rest of his approach and I did in the end manage to disentangle his meaning, but I was left with the feeling that the issue hadn't been given enough space to allow it to be properly aired. I had the same feeling about the discussion of the placebo phenomenon and how it relates to Wall's pain theory, which is a pity, because it is important and deserved a fuller treatment than that allowed to it here.
Much of the difficulty I found with this book stems, I think, from the fact that it is pitched at the wrong level. It touches on numerous fascinating ideas but they needed more elaboration than they get here. Probably the editorial policy was to avoid putting off readers with no knowledge of science or medicine, but I think it would have been better to have at least included some appendixes in which more detailed information was given.
As it is, no references are provided for the studies mentioned in the text, so that even if one wishes to look up the sources it is difficult to do so. Indeed, in the case of one really remarkable piece of research it's impossible. In a recent study it was found that stimulation of the vestibular system of the inner ear could temporarily restore normal body perception to patients who were neglecting one side of their bodies after a stroke. This is indeed astounding, as Wall says, and I'm sure that many readers would wish to read the original paper; but all we are told about the authors is that they were Italian doctors in Milan.
In his preface, Wall says that this is not supposed to be a textbook, and he refers readers seeking for more detail to his 1996 book 'The Challenge of Pain', written in conjunction with Melzack. However, the present book touches tantalizingly on a lot of new material, which deserves a fuller discussion than it gets here.