Pain: an unpleasant topic.

This essay is an attempt to clarify the construct of unpleasantness in the context of the psychophysics of pain. The first critical point is that one aspect of unpleasantness is tightly coupled to stimulus intensity and is therefore a sensory discrimination. Pain has this quality, but so do other somatic sensations such as itch and dysesthesias that are not recognized as painful by most people. A corollary of this is that pain must have a quality other than unpleasantness that allows it to be unequivocally identified. I use the term algosity for that quality. In addition to stimulus bound (primary) unpleasantness, there is an unpleasant experience that reflects a higher level process which has a highly variable relationship to stimulus intensity and is largely determined by memories and contextual features. I have termed this experience secondary unpleasantness. I suggest that the sensory-discriminative/affective-motivational dichotomy has outlived its usefulness and is currently more of an impediment than a guide to neurobiological explanations of pain. In order to increase our understanding of pain we need psychophysical tools designed specifically to differentiate primary unpleasantness from both algosity and secondary unpleasantness. These tools can then be used to determine the neural mechanisms of pain.