The International Association for the Study of Pain (IASP) has defined pain as an unpleasant sensory and emotional experience associated with noxious stimuli or described in such terms (1). Implicit in this definition are two important features of pain. First and foremost is that pain is a perception that occurs in a conscious brain, requiring activation of multiple cortical areas to produce an "experience." In contrast, "nociception" is the term used to describe activity in either the peripheral or the central nervous system (CNS) evoked by noxious stimuli. Importantly, nociception may or may not result in the perception of pain. The implication of this distinction is that pain not only requires consciousness, but also an intact nervous system and a nervous system that has developed sufficiently such that activity in subcortical nocicep-tive circuits is able to influence activity in the appropriate cortical circuits (2). Second, pain has both sensory and emotional content. This notion is supported by data from brain imaging studies as well as deficits observed in patients following specific brain injuries. Imaging data indicate that noxious stimuli result in the activation of SI and SII sensory cortices, brain areas critical for sensory discrimination (3-5). Noxious stimuli also result in the activation of brain areas critical for processing of emotion, such as the amygdala and the anterior cingulate cortex (4,6-8). The relative contribution of each of these areas can be manipulated experimentally, resulting in differences in perception (9). Furthermore, patients suffering unilateral damage to SI and SII cortex, which would eliminate sensory, but not affective components of pain, report vague unpleasantness in response to noxious stimulation of body regions contralateral to the site of brain injury (10,11). A third feature of pain, not implied in the IASP definition, is that it involves a cognitive component. In other words, pain has meaning and its meaning can impact both the sensory and the emotional experience. For example, a little abdominal discomfort following a bowl of chili in a person prone to intestinal gas may mean something very different, and will likely be perceived very differently, from the abdominal discomfort experienced by a person recently hospitalized for a bleeding ulcer.
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