Chapter 3. Psychophysical evaluation of pain and olfaction

Psychophysical methods used to evaluate the chemical senses have been instrumental for investigations of pain and pain treatment. The evaluation of chemosensory quality has a direct parallel in the use of instruments that aid diagnosis of pain and choice of treatment by the determination of pain quality, such as pulling, throbbing, radiating and burning. The use of hedonic psychophysics in the chemical senses recognizes that tastes and smells can be described by two principal dimensions. The first is the intensity of the sensation, which is the target of most psychophysical methods. The second is the hedonic valence associated with the sensation, i.e., the degree of pleasantness or unpleasantness. These hedonic dimensions are sensitive to internal state; for example, the pleasantness of sugar solutions increases during hunger and decreases during satiety. The concept of hedonics has been successfully applied to pain, albeit with only a negative valence. The dimensions of pain intensity and unpleasantness have been shown to be independent and changes in internal state can modify unpleasantness without altering sensory intensity. In addition, the amount of unpleasantness for a given intensity has been shown to vary over painful conditions (low in childbirth, high in cancer) with consequences for treatment strategy. Methods used for olfaction have been applied to healthy control subjects using experimentally applied painful stimulation and more recently these methods have been applied directly to patients, providing information about the many unknown mechanisms of pain, and the development of more effective treatments. This chapter will describe similarities in the measurement of pain and olfaction and highlight important differences in the evaluation of these two sensory modalities.