Specificity Theory

The search for a working model, or theory, of how pain works has probably been going on as long as the concept of pain has been in existence. Descriptions of pain treatment were recorded as early as 4000 B.C. in ancient Egypt. Throughout history, several theories of pain have dominated current thinking. With advances in our understanding of biology, psychology, and with the advent of new medical technologies, older theories have been replaced or updated.
Descartes specificity theory


Descartes’ (1664) concept of the pain system was of a ‘straight-through’ channel running from the skin to the brain, which transmits the sensation of pain from one place to another in the way that ‘a rope pulled in one room causes a bell to ring in another’. Specificity theory suggests the existence of a specific pain system which carries messages from pain receptors in the skin to a pain area in the brain. Specificity theory may sound reasonable to those of us who have only experienced pain following tissue damage. However, there are scientific findings that specificity theory cannot account for. For example, the phantom limb pain felt by amputees – if the sensation of pain is generated at the level of the pain receptor and transmitted faithfully to the brain, how can pain exist when the receptors are no longer there and the nerve fibres to the brain are cut? Specificity theory also cannot explain what is happening in patients who feel pain in the absence of any stimulus.


Descartes rationist epistemology


The way which Descartes arrived at his theory was through his own deductive reasoning. Descartes would have known that nerve fibres existed, but his description of how they functioned was based purely on his own thoughts and was not linked to scientific evidence. Although this method sounds unusual to us in the present day – where hypotheses must be rigorously tested – during Descartes’ time it would have been quite acceptable to arrive at conclusions simply through reasoning. This is typical of the rationalist epistemology popular at that time, where the source of knowledge and truth was believed to be intellectual and deductive, rather than sensory. The modern scientific method is based in empiricist epistemology, which asserts that true knowledge can only be based on experience and evidence. Despite the shift in scientific thinking towards empiricism in the 17th Century, Descartes theory persisted in some form until the mid 19th Century.


Sources:
Descartes, R., & Hall, T. S. (2003). Treatise of Man (Vol. 8). Prometheus Books.
Gatchel, R. J. (1999). Perspective on Pain: A historical Overview. In R. J. Gatchel & D. C. Turk (Eds.), Psychosocial Factors in Pain: Critical Perspectives (pp. 3–17). NY: Guilford Press.





The Definition of Pain

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The International Association for the Study of Pain (IASP) published this definition in 1979:

“PAIN: An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (IASP 1979, p.249).

what is pain?


This definition seems brief, given the obvious complexity of pain. Upon closer inspection, we can appreciate that is does indeed cover a great deal of what is known about pain, and it does so simply and concisely.

It tells us that pain is an unpleasant, multidimensional experience. It recognises that pain can be felt in the absence of tissue damage, yet is described in terms of such damage. It also takes into account that pain is subjective. It is an experience that exists in the mind of the individual in pain.

The subjective and individual nature of pain is explained further by a note which accompanies the definition:

Note:
The inability to communicate verbally does not negate the possibility that an individual is experiencing pain and is in need of appropriate pain-relieving treatment. Pain is always subjective. Each individual learns the application of the word through experiences related to injury in early life. Biologists recognize that those stimuli which cause pain are liable to damage tissue. Accordingly, pain is that experience we associate with actual or potential tissue damage. It is unquestionably a sensation in a part or parts of the body, but it is also always unpleasant and therefore also an emotional experience. Experiences which resemble pain but are not unpleasant, e.g., pricking, should not be called pain. Unpleasant abnormal experiences (dysesthesias) may also be pain but are not necessarily so because, subjectively, they may not have the usual sensory qualities of pain.

Many people report pain in the absence of tissue damage or any likely pathophysiological cause; usually this happens for psychological reasons. There is usually no way to distinguish their experience from that due to tissue damage if we take the subjective report. If they regard their experience as pain, and if they report it in the same ways as pain caused by tissue damage, it should be accepted as pain. This definition avoids tying pain to the stimulus. Activity induced in the nociceptor and nociceptive pathways by a noxious stimulus is not pain, which is always a psychological state, even though we may well appreciate that pain most often has a proximate physical cause.

Sources:
International Association for the Study of Pain. (1979). Pain terms: a list with definitions and notes on usage. Recommended by the IASP Subcommittee on Taxonomy. Pain, 6(3), 249–252.
Merskey & Bogduk 1994, IASP Task Force on Taxonomy Part III: Pain Terms, A Current List with Definitions and Notes on Usage, p.2-3





Philosophy of Pain

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What is meant by the term ‘pain’?

All forms of animal life on Earth have evolved into highly sophisticated survival machines which are able to escape and avoid harmful situations. At a very basic level of life, even simple multi-cellular organisms have the capacity to move away from harmful stimuli. For example work by Wittenburg & Baumeister (1999) demonstrated that the nematode worm Caenorhabditis elegans is able to detect and move away from a noxious heat source. Such detection of harmful stimuli is termed nociception, and triggers a reflex causing muscle contraction, moving the animal out of harm’s way. This type of avoidance behaviour has obvious benefits for survival, but does it bear any relation to what humans would describe as pain?
what is pain?


Humans recognise nociception as a component of the pain experience, but there is more to pain than just simple physical response to stimuli. The human feeling of pain only begins with the sensation of actual or potential tissue damage. The state of being ‘in pain’ is an abstract idea that we are able to communicate to ourselves and others. Pain is associated with behaviour changes such as guarding and resting the affected area to allow healing to take place, emotions such as fear and anxiety permit learning to avoid the harmful stimulus in the future, and we report to others the extent of the harm, using words to describe our pain experience.

In order to demonstrate that pain is more than just simple nociception, we need look no further than the many words with which we communicate about the richness and complexity of the human pain experience. One count, by Melzack & Torgerson (1971), put the number of words his patients commonly used to describe pain at 66, which could be sorted into three categories. The sensory words such as ‘pressing’ or ‘burning’ are used to describe the stimulus itself; the affective words describe what the pain is doing to a person, for example ‘sickening’, ‘fearful’; the evaluative words express the extent of the suffering, from ‘mild’ to ‘excruciating’.

We are also able to experience the pain of others in the absence of verbal communication. A number of neuroimaging studies reviewed by Jackson and colleagues (2006) have shown that not only do people report feeling pain when viewing an image of an injured person, but also there is activity in the same parts of the brain that are activated during nociception.

Being in pain therefore must be thought of as an experience that is simultaneously:
  • Biological – as a mechanism to signal bodily harm
  • Psychological – as an abstract concept that has meaning in and of itself
  • Social – with a reliable set of observable behaviours and its own language by which it is described.
We will see in future posts that this bio-psycho-social model has emerged as the most useful approach to understanding and researching pain.


Sources:
Melzack, R., & Torgerson, W. (1971). On the language of pain. Anesthesiology, 34(1), 50–59.
Wittenburg, N., & Baumeister, R. (1999). Thermal avoidance in Caenorhabditis elegans: an approach to the study of nociception. Proceedings of the National Academy of Sciences of the United States of America, 96(18), 10477–10482.
Jackson, P. L., Rainville, P., & Decety, J. (2006). To what extent do we share the pain of others? Insight from the neural bases of pain empathy. Pain, 125(1-2), 5–9. doi:10.1016/j.pain.2006.09.013.