Pain is essentially a subjective sensation. It can therefore only be described in terms of an individual's expression of it and is subject to that individual's previous experience. For the experimenter this poses a number of problems. A subjective experience can be quantified in man but not in experimental animals, because the animal is not able to express its experience in terms that are intelligible to us. However, we can measure the various effects of and reactions to a stimulus which can itself be described as painful on the basis of the effect of that stimulus administered to man. If the observation made is itself an objective observation such as a reaction time or the frequency of firing of a neurone to the noxious stimulus there is no problem until attempts are made to extrapolate the objective observation to the subjective phenomenon of pain. Particularly difficult to evaluate are experiments on animal behaviour in which the behaviour is expressed in anthropomorphic terms. Furthermore, the objective tests, even in man, do not usually assess the quality (degree of unpleasantness), as distinct from the perception or intensity of the pain.

In contrast, nociception is the perception of a painful stimulus. The ability of a neurone or an animal to detect the stimulus, or the threshold stimulus intensity to produce a response can be accurately measured. Provided that we bear in mind the difficulties in relating nociception to the quality of pain sensation, then experimental studies in man and animals can and have greatly advanced our understanding of the neuronal control mechanisms involved in the control of nociception by physiological neuronal systems and by drugs.